Designing Generator Docking Station Installations

Erika Thorson

Adding a generator docking station to a new or existing facility is a useful solution which requires careful planning to ensure a successful and safe installation.

Learning Objectives

  • Understand the differences between different docking station configurations.
  • Learn special considerations for emergency and service entrance docking station installations.
  • Understand what accessories should be specified for different docking station installations.

Generator Insights

  • Generator docking stations can be used for backup power systems.
  • There are many considerations when specifying these generator docking stations. Depending on the application and if a permanent generator is included, this may include a portable load bank connection.

Generators have become commonplace as a piece of facility infrastructure to support critical building loads. Though these backup power systems are often reliable, they are not perfect. Permanent backup power systems can become unavailable due to planned outages for maintenance or unplanned outages due to component failure.

Engineers must plan for these contingencies, as well as provide systems that comply with the latest NFPA 70: National Electrical Code requirements. One product that is becoming increasingly common as a solution is the generator docking station.

A consistent trend has emerged in modern commercial and industrial power system design: The desire for increased resilience for critical systems. Resilience can be defined as “the capacity to recover quickly from difficulties.” In the context of power systems, it is no surprise the desire to recover quickly from a difficulty such as an unexpected power outage is in high demand.

This increased demand for more resilient power systems has taken on many forms and has resulted in many different project types for consulting engineers to design. Some examples of the project types are:

  • Generator and transfer switch replacements.
  • Generator plant upgrades with Day One paralleling or future paralleling.
  • Redundant utility services.
  • Generator docking stations (retrofits for existing facilities and new builds).
  • Portable Generator and Manual transfer switch additions.
  • Portable Load Bank Connection and Portable Generator Connection for existing Permanent Generators.

Generator docking stations are interesting because of their increasing prevalence in new and existing buildings. Docking stations are not new, but they have significantly evolved from the docking stations of the past.

NEC code requirements

We have seen a significant uptick in the demand for generator docking stations because of this desire for increased resiliency, but also because of new code requirements in NEC. In the 2017 edition of NEC, section 700.3(F) was added to the “Emergency Systems” article. This new section, titled “Temporary Source of Power for Maintenance or Repair of the Alternate Power Source,” requires that emergency systems with a single generator “Include permanent switching means to connect a portable or temporary alternate source of power, which shall be available for the duration of the maintenance or repair.”

This code section also has four exceptions which would permit the omission of this permanent switching means:

  • Exception No. 1: “All processes that rely on the emergency system source are capable of being disabled during maintenance or repair of the emergency source of power.” This exception allows the temporary source to be omitted if the emergency loads can be switched off. This may not be practical for some applications where emergency systems such as fire alarm and egress lighting cannot be disabled.
  • Exception No. 2: “The building or structure is unoccupied and fire protection systems are fully functional and do not require an alternate power source.” It is difficult to see how one could be confident that a building would be unoccupied during the time that an emergency generator would need to be repaired, though there may be some building types where this exception could be applied. Buildings with fire pumps that are required to have an alternate power source would not be permitted to utilize this exception.
  • Exception No. 3: “Other temporary means can be substituted for the emergency system.” It is not clear what temporary means would be permitted to qualify for this exception; therefore this would need to be determined by the authority having jurisdiction.
  • Exception No. 4: “A permanent alternate emergency source, such as, but not limited to, a second on-site standby generator or separate electric utility service connection, capable of supporting the emergency system, exists.” This exception allows paralleled generator systems to be excluded from this code requirement as well as a separate utility service.

While it may be tempting for engineers to fall back on these exceptions to reduce the upfront electrical system cost, it is advisable to discuss the pros and cons of having a generator docking station with the owner. The owner may prefer to install the docking station despite the additional cost to increase reliability and reduce any possible liability should the permanent emergency system experience a failure.

Another requirement of section 700.3(F) is that temporary source switching means contain a contact for indicating the permanent emergency power source is disconnected. This signal must be annunciated at a location remote from the generator or at another facility monitoring system. This is a critical monitoring component that should not be overlooked by engineers and inspectors alike. Successful implementation of this code requirement will ensure the switching means is less likely to be left in the wrong position during the transition back from the temporary to the permanent emergency power source.

Two docking station configurations

There are a few ways where a docking station can be configured to allow a temporary generator to provide power to the permanently installed emergency electrical system.

Common load bus configuration: The first way a docking station can be introduced into an electrical system is by connecting the load side of the docking station to the load side of the generator. In a practical application, this can be accomplished by providing a single feeder from the distribution point for the generator, which may be a panelboard or switchboard (Figure 1). This solution is ideal for retrofit applications, where the generator and emergency distribution system equipment are already installed. This configuration can also be ideal for site configurations where the docking station location is not near the permanent generator location. To comply with NEC 700.3(F), a mechanical or electrical interlock must be installed to prevent the inadvertent connection of two power sources. The simplest way to provide this interlock is with a mechanical key type locking mechanism, which is configured to require the generator circuit breaker to be in the open position before the temporary generator connectors can be accessed.

Figure 1: The common load bus configuration connects the docking station directly to a generator distribution panelboard.

In-line configuration: For this configuration, the docking station is placed between the generator and the distribution equipment (Figure 2). One advantage of this configuration over the common load bus is it does not require a separate feeder from the distribution equipment, providing reduced material and labor costs. This configuration is ideal for situations where the docking station can be placed near the permanent generator location.

Figure 2: The in-line configuration places the docking station between the generator and the generator distribution panelboard.

Particular attention should be paid to the quantity and location of circuit breakers, which are integral to the docking station, as well as the point the permanent generator conductors are terminated. In some single circuit breaker configurations, the conductors on the load side of the permanent generator circuit breaker remain energized when the temporary generator is connected. This creates a potentially dangerous situation, especially if the generator is undergoing maintenance and the technicians are not aware of this condition. The recommended solution is to specify a docking station circuit breaker configuration that fully isolates the generator conductors from the temporary generator source.

Load bank considerations

When a generator docking station is installed, a piece of equipment, which is permanently wired to the emergency power distribution equipment is now accessible at the exterior of the building. For project applications that warrant the frequent use of portable load banks, this presents a great opportunity to make load bank hook-ups quick and efficient. Docking stations are available in configurations designed for load banks only, but it is likely the design engineer or facility owner will also want the capability of connecting a temporary generator. In this situation, it is recommended a dual-purpose docking station be specified. A dual-purpose docking station contains male connectors for temporary generator cables and female connectors for portable load bank cables (Figure 3). The male connectors for the temporary generator can be placed behind a keyed door; this keyed lock can be part of the safety interlocking system designed to prevent the inadvertent energization of multiple power sources at the same time. The female connectors are available to allow a load bank to be connected.

Figure 3: ESL Dual purpose docking station connections for a load bank and temporary generator.

It is highly recommended a circuit breaker be provided for the load bank connections. This circuit breaker provides a convenient way to energize and de-energize the load bank connectors, but it also allows the user to automatically disconnect the load bank should a utility outage cause any of the transfer switches to transfer their load onto the generator bus. To successfully implement this safeguard against overloading the generator during load bank testing the load bank circuit breaker must have a shunt trip coil, which is wired so the coil is energized when any of the transfer switches close their engine start circuit contacts. When designing this shunt trip circuit, it is important to coordinate the voltage source designated for the shunt trip coil with the circuit breaker’s specifications provided by the docking station manufacturer.

Service entrance applications

Docking stations have applications beyond just emergency systems. Many facility owners recognize the value in having the option to bring in a larger temporary generator sized to power the entire facility. For this type of application, a docking station (or more appropriately a Manual Transfer Switch) can be installed between the main power transformer for the building and the main switchboard or distribution panel. This design allows a portable generator to take the place of the utility service in the event of a prolonged outage.

When designing a docking station (MTS) for a service entrance application, the design engineer should consider specifying a docking station with an integral circuit breaker. The addition of this circuit breaker offers multiple advantages such as:

  • Provides a safe and convenient location to disconnect utility power.
  • The temporary generator installer does not have to gain entry to the building to disconnect utility power.
  • The circuit breaker can replace the main overcurrent protective device often found in the main switchboard (Figure 4) (Note: this only applies for installations in which the docking station is located near where the conductors enter the building as required per NEC 225.32).

Figure 4: Insulated-case main circuit breakers with key interlock installed in an ESL service entrance docking station.

It is important to specify a mechanical interlock on the docking station; this mechanism prevents the temporary generator installer from gaining access to the terminations before opening the utility circuit breaker. It is also important to consider this point in the electrical system will often see high fault currents since it is so close to the utility transformer. Most manufacturers will be able to provide a 65kA short-circuit interrupting rating as part of their standard offering. If the available fault current exceeds 65kA, a 100kA rated docking station (MTS) must be specified. A 100kA rated docking station (MTS) may not be available from all manufacturers, so it is important to understand what manufacturers can meet this specification requirement should the application call for it.

When implementing a service entrance docking station, the design engineer and installers should pay particular attention to the grounding system connections. The usual methods of installing a main bonding jumper between the grounded service conductor and the equipment grounding bus still apply when installing a docking station. NEC article 250 requires this bonding jumper be installed at either the service transformer or the enclosure of the first disconnecting means, which would be the generator docking station in this case.

Fire pump installation requirements

Fire pump installations have specific requirements outlined in NEC article 695. When applying a generator docking station solution to a project with a fire pump backed up by a generator, special attention should be paid to the location of the fire pump disconnecting means.

A typical design strategy for serving a fire pump from a generator is to provide a dedicated circuit breaker on the generator set. Very often this is an ideal solution for powering a fire pump from the generator, as it allows the conductors to be kept outside of the building all the way from the generator to the fire pump room, therefore eliminating the need to use expensive two-hour fire rated cabling. However, the addition of a generator docking station in an in-line configuration introduces a new problem: the fire pump’s emergency power feeder will not be energized if a temporary generator is utilized while the permanent generator is switched off for maintenance or repair. This situation is the exact opposite of what the introduction of NEC 700.3(F) is trying to achieve. One possible solution to this problem is installing the fire pump disconnecting means directly adjacent to the docking station with the conductors tapped from the load bus of the docking station.

An alternate method for feeding the fire pump from the generator is providing a feeder from a common generator bus such as a distribution panelboard or switchboard. In this setup, the fire pump feeder overcurrent device will be energized when the docking station is powered from a temporary generator. However, depending on the location of the fire pump room, this situation may require the use of two-hour fire rated cabling if the feeder is to be routed within the building. The implications of putting the fire pump disconnecting means at a convenient location such as generator equipment should be evaluated to determine if the advantages outweigh any additional costs.

Engine start considerations

A code requirement that cannot be overlooked is the engine starting requirements for temporary emergency power sources. NEC 700.3(F)(2) references the same code article (NEC 700.12) that applies to permanently installed generators, which requires the generator to start and transfer the load within 10 seconds. To comply with this requirement, engine start wiring should be provided from the transfer switch generator start terminals to the docking station, as this will provide a convenient point for the temporary generator installer to access the start signal wiring.

Accessories for docking stations

Docking stations have many optional accessories that should be considered for inclusion based on the application requirements. Common available accessories are listed below (Figure 5).

Figure 5: Typical docking station accessories (from left to right): Fuses, ERMS Switches, dehumidistat, and thermostat.

1. 2-wire auto start contacts: A set of posts that provides a convenient and readily accessible set of contacts for connecting the auto start signal from the building transfer switches to the temporary generator.

2. Convenience receptacles/shore power connections: Many temporary generators have separate power connections for jacket water heaters, space heaters, battery chargers and service receptacles. Receptacles can be provided integral to the docking station to keep all the connections in a single convenient location. Most temporary generator optional connections are 120V. These connections are typically only necessary for applications where the temporary generator may be sitting idle for a period of time.

3. Load shed receptacle: For applications where the docking station is serving as a connection point for a load bank, this feature will allow the load bank to automatically be shed if the utility power goes down during a load bank test.

4. Utility indicator lights: Lights which illuminate when the utility voltage is present. This feature can be helpful for the contractors to confirm that utility has been restored before disconnecting the temporary generator (Figure 6).

5. Thermostat and strip heater: Prevents condensation accumulation inside the docking station cabinet.

6. Phase rotation monitor: This device helps the contractor verify the temporary generator phase rotation is correct before energizing the load. It should be noted that NEC 700.3(F) requires this accessory for docking stations serving emergency systems. Even in situations where a phase rotation monitor is not code required, it is recommended that this accessory be provided (Figure 6).

Figure 6: Phase rotation monitor and indicator lights.

How generator docking stations help

Adding a generator docking station can be a very useful solution for complying with current codes and improving system reliability during generator maintenance and repairs. However, the introduction of another system component can increase complexity, so it is critical that docking station installations receive a detailed engineering design to provide a safe and reliable system.

If you’re looking for assistance on specifying or designing a generator docking station, ESL can help! Contact our team now.

Contact ESL!

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NEC requirements for arc energy reduction for overcurrent protective devices 1,200 A or greater

Erika Thorson

Learning Objectives

  • Understand the basic requirements of NEC 240.87.
  • Know the importance of arc energy reduction methods and arcing current.
  • Learn how different arc energy reduction methods are calculated.
 

Arc energy insights

  • Low-voltage electrical systems and technology require electrical engineers to consider arc energy reduction.
  • Zone-selective interlocking systems detect high-level fault conditions, allowing instantaneous tripping in certain situations.
  • NFPA 70: National Electrical Code Article 240.87 is vital to electrical engineers working on these systems, as it discusses arc energy reduction.

Since its inclusion in the 2014 edition of the NFPA 70: National Electrical Code, known as NEC, multiple articles have been written in various publications discussing Article 240.87

While the 2014, 2020 and, to a lesser extent, the 2017 versions of the NEC are discussed in this article, any references to the NEC that do not indicate a specific year or edition refer to the 2023 version only. Essential to understanding how to meet the requirements of 240.87 is IEEE 1584: Guide for Performing Arc-Flash Hazard Calculations. Unless otherwise noted, any references to IEEE 1584 in this article refer to the latest version, 2018, and its subsequent addenda. NFPA 70E: Standard for Electrical Safety in the Workplace provides requirements for safe work practices, including those required by NEC 240.87. Additionally, 240.67 arc energy reduction for fuses will only be tangentially mentioned in this article.

Important edits and additions to the NEC’s arc energy reduction requirements include the following:

NEC 2017

  • Added 240.67, which requires arc energy reduction when fuses are 1,200 A or greater (240.87 is part of a section specific to circuit breakers).
  • Added instantaneous pickup and instantaneous override as options for arc energy reduction.

NEC 2020

  • Added clarification regarding arcing fault currents.
  • Added clarification that temporary instantaneous pickup adjustments are not satisfactory.
  • NEC 2023 included no major changes to 240.87.

240.87 states, “Where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted is 1,200 amperes or higher,” the three sub paragraphs shall apply. These three subparagraphs are (A) documentation, (B) method to reduce clearing time and (C) performance testing.

240.87(A) Documentation has, since its inception, been straightforward in requiring that information about the system be recorded and available to personnel authorized to work on the equipment. The 2020 version of the NEC added a sentence further detailing these requirements, requiring proof that whatever method is used for arc energy reduction works and is in use.

240.87(A) and 240.87(B) note that the arc energy reduction method “be set to operate at less than the available arcing current,” i.e., that the arc energy reduction system will actually operate based upon the specific system’s characteristics. The writer of this piece assumes these requirements were added to prevent the situation where an unscrupulous system operator would circumvent the intent of 240.87 (protecting personnel) by merely installing an arc energy reduction system but not actually having it operational.

It is important to note that the available arcing current is different from the more commonly encountered available short-circuit current. The available short-circuit current, often called the bolted-fault current, can be found at a given bus by reducing the electrical system to its Thevenin equivalent with zero fault impedance. The available arcing current is similar, with the addition of the impedance of a prospective arc included. At face value, this seems a simple calculation, but in practice, IEEE 1584 uses a selection of five different equations based on parameters such as the electrode configuration, nominal system voltage and electrode gap to determine. Arc flash analysis software can be used to accurately determine the arcing fault current.

It is also vital for personnel to understand that arc energy reduction at one overcurrent-protective device or bus is dependent not on that overcurrent protective device, but on the next overcurrent-protective device(s) upstream or on the line-side of that device. For feeder circuit breakers installed on a switchgear, this could be the main circuit breaker; for that same main circuit breaker, protection would need to be provided by the feeder circuit breaker or relay feeding the main. Exceptions to this may include certain types of energy-reducing active arc flash mitigation systems that reduce the fault energy without the assistance of overcurrent protective devices tripping.

240.87(B) provides a list of acceptable means to reduce arc energies, including: zone-selective interlocking, differential relaying, energy-reducing maintenance switching with local status indicator, energy-reducing active arc flash mitigation system, a permanent instantaneous trip setting, an instantaneous override or an approved equivalent means. Each of these methods, including their advantages and disadvantages, is discussed in the following paragraphs. Note that, as stated earlier, all the methods must be set to operate below the available arcing current.

Zone-selective interlocking

ZSI is, at its most simple, a communication system. Overcurrent protective devices are connected such that they communicate with each other when they pick up or “see” a high-level fault condition, allowing instantaneous tripping in certain situations. If both a feeder overcurrent protective device and an upstream main overcurrent protective device pickup on a fault current, the system will operate as usual (i.e., if properly coordinated, the feeder breaker will first attempt to trip the fault). If that same main overcurrent protective device picks up a fault current but none of its associated feeder overcurrent protective device do, the main overcurrent protective device will trip with no intentional delay, regardless of its short-time or instantaneous pickup settings/delays. ZSI systems are generally most common in switchgear systems and can be moderately expensive, especially when interlocking feeder circuit breakers from one switchgear with overcurrent protective devices of a separate switchgear. ZSI is often installed in new switchgear installations. Figure 1 presents a basic visual of a ZSI system for (a) a fault within the ZSI-protected zone and (b) a fault outside the ZSI-protected zone.

Advantages: Fast-acting, allows intelligent zone isolation and selective coordination.

Disadvantages: Moderately expensive, requires interlocking wiring and more advanced circuit breakers, may not be an option between different busses and may not work across multiple manufacturers.

Differential relaying

Differential Relay (ANSI device No. 87) has been used for decades to efficiently detect and isolate faults within a zone of protection, whether that be a bus, cable, transformer or other equipment. As the name implies, if the difference in value between the currents entering and exiting a node is not zero or within a prescribed setting, the relay will trip all such devices. Differential relaying, due to its cost, is usually restricted to large or critical switchgear, transformer, motor or generator systems.

Advantages: Extremely fast-acting, does not impact selective coordination. Modern relay systems provide some security against nuisance tripping.

Disadvantages: Expensive — each circuit requires current transformers and associated wiring; current transformers must be properly matched and/or of sufficient quality and size to prevent through-fault nuisance tripping.

Energy-reducing maintenance switching with local status indicator

Maintenance switching with local status indication generally involves a physical switch and light installed on a piece of equipment. If personnel are going to work on said equipment energized, they would toggle the switch, which would send a signal to the next upstream overcurrent protective device to lower its instantaneous settings to minimum. This allows for a system to maintain coordination under normal operating conditions while also limiting arc hazards to personnel during maintenance or other activities.

It is important to reiterate that to be effective, the maintenance switch must lower the instantaneous setting of the upstream overcurrent protective device, not of the device in the cubicle or equipment associated with the work. Maintenance switching is often found in new switchgear installations, though can be retrofitted into existing systems. Figure 2 presents a basic time-current curve for a circuit breaker system with maintenance switching (a) inactive and (b) active.

Advantages: Relatively cheap compared to the other arc reduction options, excepting instantaneous trip/override; may be installed in new or existing systems; provides both selective coordination and personnel protection.

Disadvantages: Relies on administrative controls to ensure personnel use the system properly for protection — if the system is not engaged when working on equipment, the personnel may not be protected. If the system is not disengaged after work is complete, the system may experience nuisance tripping. In theory, the local status indicator should limit such occurrences, but anecdotally, the latter can be common.

Arc energy-reducing active arc flash mitigation system

Multiple types of energy-reducing active arc flash mitigation systems exist. One of the most common is an arc flash relay system, which normally uses both light sensors and overcurrent pickup to detect an arc flash event and isolate the equipment. Such systems have been around for over a decade and can generally be installed in both new and existing equipment.

Another type of active arc flash mitigation system is the ultrafast earthing switch, which introduces a controlled three-phase line-to-ground fault when sensing arc fault conditions; this fault (of essentially zero impedance) effectively redirects the fault current to an area where it can be contained in a safe manner. UFES systems have advanced greatly in recent years, going from large contraptions that are little more than electrodes fired from shotgun shells into the ground to units that can be installed as parts integral to a switchgear setup.

Arc-quenching is a third type of active arc flash mitigation system that is similar to the arc flash relay system in structure — light sensors and current transformers — and the UFES system in response time. Whereas UFES systems introduce a controlled three-phase bolted fault, arc-quenching systems introduce current-limiting devices to control and redirect the fault current.

Advantages: Regardless the technology, active arc flash mitigation systems provide extremely quick response times to detect and/or isolate an arc. Additionally, while passive arc energy reduction systems — specifically arc-rated equipment — only contain an arc when the exterior doors are closed, active arc energy reduction systems operate irrespective of whether the equipment is open or closed.

Disadvantages: Cost, especially for a UFES system can be high. While an arc flash relay system is little more than point sensors and/or fiber optic cable coupled with standard overcurrent relays (where possible, using CTs and relays already installed) a UFES system is sacrificial in nature. New systems have been developed that contain the introduced fault into a chamber that can be replaced, the replacement equipment can still be expensive.

A permanent instantaneous trip setting or instantaneous override for arc energy

“Permanent” was added by this author to highlight a key aspect of this requirement that “temporary adjustment of the instantaneous trip setting to achieve arc energy reduction shall not be permitted.” While essentially the same in electrical characteristics as the energy-reducing maintenance switching method, temporary adjustment of the instantaneous setting does not provide the same controls that would ensure the personnel adjusted the correct overcurrent protective device or that other personnel may have unknowingly “corrected” the temporarily adjusted setting.

Advantages: As most overcurrent protective devices rated for 1,200 A or greater have an adjustable instantaneous setting, this method (along with the instantaneous override method) likely provides the cheapest and most common means of reducing arc energy.

Disadvantages: As overcurrent protective devices have the twofold and often opposed goals of increasing system coordination and reducing system arc energies, it may not be possible for an instantaneous trip setting to be set low enough to interrupt the arcing fault current.

An instantaneous override is essentially the maximum instantaneous pickup of a circuit breaker and is not an adjustable setting. Refer to the instantaneous trip setting paragraph above for application details.

An approved equivalent means — as the industry’s understanding of arc flash continues to grow and mature, new and novel means of arc energy reduction will likely continue to be developed.

240.87(C) Performance testing requires the arc energy reduction system be tested when first installed to prove its efficacy. Primary current injection (i.e., testing the whole overcurrent detection system, not only the circuit breaker or relay inputs) or another approved method is required. For most arc energy reduction systems, the testing is different from usual overcurrent testing: introduce a current in the primary system and observe how long it takes the system to send a trip signal. Some systems, such as those that use light-sensing, would need additional testing.

Ultimately, the choice of which arc energy reduction system to use is dependent on a number of factors unique to every system, including the type of equipment, the personnel that will operate and maintain it, cost constraints, etc. In many cases, instantaneous settings may provide adequate reduction of arc flash energy while still maintaining coordination. Where that is not possible, the other arc energy reduction options become necessary.

 
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Critical Facilities Technologies: ESL’s Newest MRG

Erika Thorson

Manufacturing Representative Groups (MRGs) have become an increasingly important resource for businesses looking to enhance their customer service and sales support. At ESL, we recognize the value of MRGs and have made it our mission to find the most experienced and capable partners to help serve our customers across the country.

Our MRGs act as a liaison between our customers and our internal customer experience team, providing an expert level of knowledge and support to address any concerns or issues that may arise. By working with MRGs, we aim to increase customer satisfaction and enhance our sales support capabilities, ultimately improving the overall customer experience.

ESL is pleased to announce the latest addition to our expanding ensemble of Manufacturing Representative Groups, Critical Facilities Technologies (CFT). Based in the Midwest, CFT offers a range of services, including design, equipment procurement, project management, and more, to customers in Colorado, Wyoming, Montana, North Dakota, South Dakota, Minnesota, Nebraska, and Kansas. Their team of seasoned professionals possess a wide range of expertise and an impressive wealth of knowledge across diverse industries. This equips them to effectively guide and support projects of any scale, industry, or facility maturity level. Teaming up with CFT empowers ESL to now provide our customers with exceptional levels of support and expertise in these newly covered states. CFT’s extensive regional knowledge is especially valuable when it comes to designing and executing new projects.

ESL proudly provides manufacturer representatives throughout the United States to provide technical and customer service support for our Emergency Power and Entertainment Power Solutions products. To find the right representative for your state, check out or rep map here!

Understanding Hospital Emergency Power Supply Systems

Erika Thorson

Generators and emergency power systems are essential to enabling hospitals and health care facilities to effectively serve their communities

Learning Objectives

  • Gain a basic understanding of the generators and major components of an emergency power system for hospitals.
  • Understand the regulatory requirements for an emergency power system for hospitals.
  • Provide an approach to the design of these systems that accounts for key client and project needs.

Due to constant changes in medical standards of care, technologies and building systems, hospitals have become more reliant on electrical systems to function properly. As such, the reliability of the hospital building’s electrical system is more important than ever.

NFPA 70: National Electrical Code requires every hospital to have two independent power sources that provide a minimum level of reliability: a normal source (i.e., utility) and an alternate source (i.e., generator, fuel cell system or battery system).

Because most health care facilities have traditionally used generators as their alternate source due to runtime and maintenance advantages, this article will focus on generators and essential electrical system (i.e., “emergency power”) design.

For the purposes of this article, the NEC Article 517 term “essential electrical system” and Article 700 term “emergency power system” are synonymous because emergency systems are defined in NEC Article 700, which is applied specifically to hospitals in NEC Article 517.

An emergency system is defined by the NEC as “those systems legally required and classed as emergency by municipal, state, federal and other codes.”

NFPA 110: Standard for Emergency and Standby Power Systems defines the various components that makeup an emergency power system and comprises the emergency power supply and emergency power supply systems.

The EPS is the alternate power source, which in this case is the generator(s). The EPSS consists of the conductors, distribution equipment, overcurrent protective devices, transfer switches and all control, supervisory and support equipment needed for the system to operate between the generator and the transfer switch. Conductors, distribution equipment and overcurrent protective devices on the load side of the transfer switches are not considered part of the EPSS per NFPA 110, but are considered part of the overall emergency power system (see Figure 1).

Figure 1

A generator consists of two major components: the engine that provides the mechanical power via a rotating drive shaft and an alternator, which converts the mechanical energy to electrical energy. A transfer switch is an electrical piece of equipment that is configured to connect two incoming power sources (typically the utility source and the generator source) and one outgoing connection to the load(s) using a switching mechanism to select which of the two incoming sources is connected to the load (see Figure 2).

Figure 2: Typical generator set configuration with major components identified (this example is an indoor installation).

There are other regulatory bodies, codes and organizations that need to be considered depending on where the project is located:

Reviewing the requirements of these regulatory bodies, codes and publications is recommended at the onset of a new project to determine any project specific impacts as the adopted codes vary by state and local jurisdictions.

Emergency power design considerations

Generators are manufactured with two ratings: prime and standby. A prime rated generator is designed to be operated continuously as the primary source of power for the system, typically used where utility power is not available such as extremely rural locations. A standby rated generator is designed to operate intermittently when the main source of power fails or during generator testing. Emergency power systems for hospitals use generators rated for standby use because the generator is functioning as the alternate source of power.

NFPA 110 requires generators and the EPSS to have a Classification, Type and Level. The “Class” defines the minimum run time in hours. The “Type” defines the maximum time, in seconds, to transfer to the alternate source after power loss. The “Level” defines the risk to human life due to the failure of the system.

Hospital emergency power systems typically must be Class 96 (minimum 96 hours of runtime) or have an operational plan to supply 96 hours of fuel to the site, Type 10 (maximum 10 seconds to transfer) and Level 1 (failure of system could result in loss of human life or serious injuries).

The two common fuel types for hospital generators are No. 2 diesel and natural gas. Typically, hospitals opt to install diesel generators for two primary reasons.

  • Hospitals are required to either have 96 hours of fuel stored on-site or an agreement to have the additional fuel delivered to maintain 96 hours of continuous runtime (see the Joint Commission’s Emergency Management 96 Hour Plan for details). Natural gas is delivered to the hospital from the utility via underground distribution piping and cannot be stored on-site in the quantities required. Authorities having jurisdiction do not typically consider an off-site fuel source reliable enough to be the sole fuel source for generators (see NEC 700.12(D)(2)).
  • Emergency generators and the EPSS for hospitals are required to be NFPA 110 Type 10 systems. This requires the system to restore power to the loads in less than 10 seconds. Most natural gas generators are not able to meet this requirement due to the time it takes the generator engine to start.

Generators can be installed indoors or outdoors. Indoor installations have the advantage of being better protected from weather and vehicular traffic and provide ease of maintenance but are typically a higher first cost. The generator room needs to be designed to account for the substantial airflow required to both cool the generators and provide combustion air to the generator. Ideally the air intake is at the back of the room and air discharge is at the front to promote proper airflow over the engine block to facilitate engine cooling. Rooms with air intake or discharge from above or one side of the room may create cooling issues and should be avoided. Design also needs to consider the acoustical impact of the generators at both the air intake and discharge locations. Generators create a lot of noise and sound attenuation within the room may be required to meet local ordinances or hospital requirements (see Figure 3).

Figure 3: Example of an indoor (left) and outdoor (right) generator installation

Outdoor installations typically have a lower first cost but are not as accessible and may be susceptible to degradation of the equipment over time if not properly protected. Typically, a generator installed outdoors will have a weather-proof enclosure with dampers and heating elements to keep the environment within the enclosure controlled to an extent. The enclosure also may have a sub-base tank for fuel storage, sound attenuation or raised personnel platforms depending on the specific requirements of the project. The self-contained nature of an outdoor generator can be advantageous as the issues with ventilation and fuel oil delivery are simplified.

Emergency power distribution equipment

The complete essential electrical system, as defined by NEC Article 517, consists of the EPSS (i.e., everything between the transfer switch and the generator, including the transfer switch) and the switchboards, panels, transformers, feeders and overcurrent protective devices that are connected to the load side of the transfer switch.

In hospitals, the essential electrical system is divided into three separate branches per NEC Article 517: life safety, critical and equipment. Each branch has its own automatic transfer switch, or switches depending on the size of the system, to segregate power distribution in the hospital:

  • The life safety branch is limited to circuits essential to life safety and include illumination of means of egress, exit signs, select alarm and alerting systems, communication systems, generator set accessories, elevators and select automatic doors.
  • The critical branch is primary reserved for systems and equipment that are essential to patient care and safety and include, but is not limited to, task illumination and receptacles patient care spaces, nurse call systems, clinical information technology systems and select power circuits needed for effective hospital operation.
  • The equipment branch primarily consists of mechanical equipment required for effective hospital operation and typically includes air handling units, pumps, boilers, chillers, medical vacuum/compressed air equipment, kitchen equipment and any other optional loads the hospital considers necessary to maintain the facility when utility power is lost.

Transfer switches can be either automatic, nonautomatic or manual. Hospitals primarily use automatic transfer switches, which transfers to generator without personnel input. However, nonautomatic and manual transfer switches are used for optional loads when automatic transfer is not required or desired due to available generator capacity.

The difference between nonautomatic and manual is nonautomatic has an automatic transfer mechanism, but transfer requires personnel to initiate; manual requires personnel to physically move a mechanism by hand from one source to the other.

Automatic transfer switches have three transition types. Open transition is the most common in hospitals and disconnects from the primary source of power (utility) before connecting to the alternate source (generator), also known as “break before make.” Delayed transition is similar to open transition but has a built-in time delay where it is disconnected from both sources for an extended period and is most commonly used for mechanical equipment to allow time for motors to slow down before connecting to another source of power.

Closed transition is less common due to utility company approval needed before installation because closed transition briefly parallels utility with the generator(s). Closed transition will briefly connect to both sources before disconnecting from one source or “make before break.” The advantage is the facility does not experience a brief “blip” in power during monthly generator tests or when transferring from generator back to utility power.

Pictured: 3-Way Manual Transfer Switch includes three breakers which allow the permanent generator to be simultaneously connected to both a load bank (permanent generator testing) and the ATS

Many hospitals require automatic transfer switches to have bypass isolation. Bypass isolation is a switch provided with two switching mechanisms configured so that one switch can be removed and worked on in a safe manner while the other switching mechanism provides power to the loads. The design needs to consider the increased footprint and cost for bypass isolation switches over transfer switches with a single switching mechanism.

Common emergency power system configurations

There are two common system configurations that most hospitals use: standalone and paralleled systems. A standalone system consists of a single generator with transfer switches separating life safety, critical and equipment branch loads. The generator starts when a start signal is received from any of the transfer switches and each transfer switch will transfer to generator power once the switch senses the generator source has reached system voltage and frequency.

The advantage of a standalone system is typically lower first cost in comparison to a similarly sized multi-generator configuration as well as less complicated controls. The disadvantage is failure of the singular generator results in the facility having no backup power to essential loads during the utility outage. In addition, the standalone system has no ability to shed less critical loads if the generator is unable to keep up with the demand load of the facility during the utility outage unless a building automation system interface is provided to monitor real-time load on the generator and shutdown select equipment when it senses the generator is reaching peak capacity. This additional feature will add cost to the project if implemented, which needs to be considered during design.

A paralleled configuration consists of two or more generators connected in parallel to a common bus with multiple transfer switches. Once a start signal is sent by a transfer switch, the first generator to reach rated voltage and frequency will close to the bus. Transfer switches will start transferring to the generator source and subsequent generators will close to the common bus once they reach voltage/frequency and are synchronized with the first generator.

The advantage of paralleled configuration is it provides equipment redundancy in the event a generator fails to start or is offline for repairs. Additionally, the system is able to load shed lower priority transfer switches (i.e., disconnect them from the generator source) if the generators are unable to keep up with the demand load. This prevents a complete outage to the facility and ensures the most critical loads remain operational.

Electrical system redundancy

Hospitals are constantly preparing for the worst-case scenario to ensure they deliver the highest level of care to their patients. Equipment and system redundancy is a priority. It is recommended that designers discuss equipment and system configurations that provide inherent redundancy with the client to ensure the design meets the client’s redundancy requirements and project budget.

For generators, a common configuration is to provide the quantity of generators that provide N+1 redundancy in the event one of the generators fails to start or is offline for repair. For example, if a facility has a peak demand load of 900 kilowatts and the hospital wants N+1 redundancy, providing three 500-kilowatt generators in a paralleled configuration would meet the redundancy goal.

Another strategy to improve the resiliency of the essential electrical system is to separate critical or equipment branches of emergency power into “Critical A” and “Critical B,” each having its own automatic transfer switch. This limits the potential outage to the facility due to a catastrophic failure to a transfer switch or other distribution equipment on that branch of power. It also allows for critical care areas of the hospital to be connected entirely to emergency power while maintaining two separate sources of power which is required by code.

Generator fuel

As previously noted, No. 2 fuel oil is the most common fuel source for hospital emergency generators. Typically, a hospital will have a minimum of 96 hours of fuel on-site, and may have less if complying with the Joint Commission’s Emergency Management 96 Hour Plan or in a local jurisdiction that has a less stringent requirement.

Depending on the utility’s reliability, the generators may only run 15 to 20 hours a year to meet the monthly/yearly testing requirements for each generator. This results in fuel that may sit for extended periods before being used. To avoid degradation of the quality of fuel, most hospitals will install a fuel polishing system to remove water and other particulates from the fuel oil. If a centralized tank is installed to serve multiple generators, a fuel oil pumping system will supply and return fuel to the generator day tanks that are located at each generator.

Modifications, upgrades to existing electrical systems

When designing a generator or emergency power system upgrade for an existing hospital, phasing and outages need to be considered at the outset of the design as they can have a huge impact on hospital operations. Hospitals cannot afford to shut down critical services at any time.

Although outages are unavoidable with a major system upgrade, discussions with hospital administration and key personnel early in the design is crucial as it may require a different design approach to meet the project goals and maintain the facility during construction.

Generators and emergency power systems are an essential system in hospitals to ensure the operational impact of a utility outage is minimal. As health care facilities and staff continue to adapt to the latest standards of care, the need for more robust and reliable emergency power systems will be required.

When initiating the design of a new emergency power system or upgrade to an existing system, owners and design professionals need to be in constant communication to ensure the design aligns with the project goals, budget and hospital’s operational priorities.

Emergency generators and the design of power systems play a crucial role in ensuring reliable power is provided at each facility by providing an alternate source of power in the event the utility source is interrupted.

View the original article and related content on Consulting Specifying Engineer

California Transport Refrigeration Unit Regulations in Development

Erika Thorson

Pending TRU California Regulation

Pending TRU regulations will require that diesel engines be phased out over the next six years in an effort to help improve the environmental impact of the trucking industry. Facility and fleet operators should consider options to reduce emissions at their facilities.

Truck Refrigeration Units, commonly referred to as TRUs, are refrigeration systems that are typically powered by internal combustion engines. TRUs control the environment of temperature-sensitive products that are transported in refrigerated trucks, trailers, railcars and shipping containers. TRUs are used to transport and store many products such as food, pharmaceuticals, plants, medicines, and chemicals.

Some companies use TRUs for extended cold storage. Distribution centers and grocery stores may run out of cold storage space in their buildings and then opt to store overflow goods in TRU-equipped trucks and trailers outside their buildings. Distribution centers, truck stops and other cold storage facilities also attract large volumes of TRUs that contribute to higher localized health risks. New regulations are developing concepts to reduce emissions from facilities with TRU activity by transitioning to zero-emission operation where practical.

According to SCE.com, Southern California Edison is aiding in these pending regulations by offering a program designed to help qualifying SCE commercial customers install the charging/powering infrastructure needed to electrify medium- and heavy-duty fleets. SCE’s Charge Ready Transport Program is providing a funding initiative for installation of infrastructure to SCE commercial customers. Through this program, SCE’s goal is to advance the vision for a clean energy future while providing medium- and heavy-duty fleet owners the opportunity to save money. The program is offering opportunities such as, no-cost installation of electric infrastructure or giving businesses the option to install, own, operate and maintain the infrastructure on site for a rebate at 80% of cost. SCE is also offering special incentives such as commercial rate options that make EV charging and TRU shore power more affordable during certain times of the day.

In addition to SCE’s infrastructure programs, CARBs Clean Off-Road Equipment Voucher Incentive Project (CORE) offers a streamlined voucher process for buyers to receive funding to offset the adoption costs of clean, commercial ready zero-emission equipment including transport refrigeration units, cargo-handling equipment, and more. CARB also offers eligibility opportunities to offset electricity costs through the Low Carbon Fuel Standard (LCFS) Program which is designed to encourage the use of cleaner low-carbon fuels in California, encourage the production of those fuels, and therefore, reduce greenhouse gas emissions.

Charge Ready Transport, LCFS and CORE are some of the many incentives available in California that provide opportunities to reduce your organization’s carbon footprint and improve the air quality of your community all while cutting operation and maintenance costs to your fleet.

Get ahead of the competition; learn more about SCE’s Charge Ready Transport and reducing greenhouse gas emissions here: https://www.sce.com/business/electric-cars/charge-ready-transport.

If you’re interested in learning how ESL can help provide safe utility power to electric or hybrid TRUs check out our eTRUconnect:  https://eslpwr.com/etruconnect/