Chapter 5 of NFPA 110 covers the equipment that generates the electrical power in emergency and standby power systems. The Emergency Power Supply (EPS) is the source of the electrical power and includes everything necessary to generate the power (i.e. generator set, fuel supply, and accessories), whereas the Emergency Power Supply System (EPSS) are the components (i.e. transfer switches, circuit breakers, paralleling switchgear) that distribute the power from the EPS. The EPS fuel supply (energy source) and the equipment used to convert the fuel to electrical energy (energy converter) are covered in this chapter. The EPS accessories (starting system, control panel, remote annunciator, cooling system, exhaust system, etc.) are also covered.
NFPA 110 allows the use of only liquefied petroleum, liquefied petroleum gas, or natural/ synthetic gas for EPS energy sources. (5.1.1) Diesel fuel is the preferred choice in Level 1 installations where the probability of the interruption of the off-site fuel supplies (i.e. natural gas line) is high. Additionally, diesel gensets are capable of accepting loads more quickly than similar sized gas gensets and may be a better option where 10-second start times (e.g. Type 10 systems) are required.
While NFPA 110 does allow the use of natural gas for both Level 1 and Level 2 systems, it is key for designers to realize the limitations of gas engines as they relate to the classification of the Emergency Power Supply System (EPSS). For example, employing a large natural gas generator for a Level 1, Type 10 system may not result in power availability and transfer within the required 10-second time frame. Part of this is due to start-time delays for natural gas required as a safety measure to reduce the risk of gas explosions on start-up. Large natural gas generators also experience difficulties meeting the requirement in NFPA 110 to handle the full-rated load in a single step. (220.127.116.11.2)
However, diesel fuel presents its own challenges, especially in regards to on-site storage of diesel fuel in larger systems or where extended system runtimes are required (e.g. Class 48 or Class X systems). Some authorities having jurisdiction (AHJs) have interpreted that 96 hours of fuel be available whenever you have a Level 1 system in a high seismic zone. (A.4.2)
NFPA 110 requires that the main fuel tank carry 133% of the fuel required by the EPSS at the full rated load to meet the class requirements of the EPSS. (5.5.3) For example, a 3,000 kW generator may consume 200 gallons of diesel fuel per hour at 100% of the power rating and would therefore require 25,536 gallons of fuel to be stored on-site to meet the 96-hour and 133% requirements (200 x 96 x 1.33 = 25,536). In a large facility with multiple large units, this can result in hundreds of thousands of gallons of diesel fuel storage. In addition to the obvious cost issues, fuel preservation and stabilization quickly become an issue.
The capacity requirement applies to the main fuel tank and does not intend that the required fuel supply capacity be shared between the main fuel tank and any provided day tank. Fuel supplies for Level 1 systems shall not be used for any other purpose. (5.5.1) However, there are certain exceptions which permit the fuel supply to serve other systems, provided that other systems cannot draw down the fuel level to a point that would compromise the continued operation of the Level 1 EPS. (18.104.22.168)
For NFPA 110, energy converters can only be rotating equipment consisting of a generator driven by a diesel-cycle, otto-cycle (spark-ignited), or gas turbine cycle prime mover. This combination is generally known as a generator set (i.e. genset) and consists of the prime mover (engine) and rotating machinery (generator). (5.2)
NFPA 110 requires that generators be built using components that have proven compatibility and must be separately prototype tested as a complete assembly to prove that they operate reliably as a unit. (22.214.171.124) The supplier must show proof of performance under normal and adverse conditions before installation, which helps avoid problems that may not be discovered until after the installation, start-up, or much later. This makes it very difficult to ensure that units not built by an established generator set manufacturer are compliant with requirements. All mtu generator sets undergo factory acceptance testing including rigorous prototype testing to ensure reliable operation.
NFPA 110 requires that the EPS be installed in accordance with NFPA 70, National Electric Code (NEC). The NEC has two specific articles for standby power systems addressed by NFPA 110: emergency (Article 700) and legally required (Article 701). The scopes of Articles 700 and 701 correlate well with a Level 1 EPSS and a Level 2 EPSS, respectively. For mission critical facilities, Article 708 (Critical Operations Power Systems) may also apply. NEC Article 455, Generators applies to all installed generators.
Additionally, NFPA 110 requires that the EPS comply with NFPA 37, Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines, except as modified in the standard. (5.6.1) NFPA 37 provides details for stationary engines including mounting, housing, and locating engines and details for fuel systems and exhaust piping materials and installation.
NFPA 110 requires that energy converters (i.e. generator sets) have the required capacity and response to pick up and carry the load within the time specified by its Type after loss of primary power. (5.4) Generator set manufacturers generally reflect this capability in their Ratings. While all manufacturers comply with most industry basic standards, some rate their generator sets in ways that require careful consideration for the purpose that the generator set is intended to serve.
An EPS may have several different ratings indicating its overall operating capacity:
- Standby ratings are typically used for 60 to 70 percent of the rating for 24-hour periods up to 500 operating hours per year.
- Emergency standby ratings are used for 60 to 70 percent of the rating for 24 hours and for up to 200 hours per year
- Prime power ratings are used for 60 to 70 percent of the rating for extended periods with 10 percent overload for one hour and for unlimited hours per year
- Mission critical ratings are used for 85 percent of the rating for extended periods and for up to 500 hours per year
- Continuous ratings are used for up to 100 percent of the rating for extended periods for unlimited hours per year
For a more complete understanding of generator set ratings, see Rating Definitions for mtu Generator Sets, download the Technical Article: Understanding Generator Set Ratings for Maximum Performance and Reliability, or Contact Us to schedule a Lunch and Learn or speak with one of our application engineers.
In addition to the generator set, Chapter 5 of NFPA 110 also covers accessories that are required to be installed, maintained, tested, and operated to achieve maximum reliability and compliance with the NFPA 110 standard including:
- Temperature Maintenance equipment such as jacket water heaters (i.e. block heaters) and battery warmers. (5.3) Section 5.3.5 requires that the ambient air temperature in an EPS equipment room or outdoor housing for a Level 1 system not be less than 4.5o C (40o F), which may require that the entire room or enclosure be heated.
- Fuel Supply equipment such as fuel storage tanks (i.e. sub-base tanks, day tanks, main tanks), low fuel sensing switches (i.e. floats, senders, etc.), and other fuel supply accessories. Section 5.5 specifically covers fuel tank supply and sizing, additional fuel system requirements are covered in Section 7.9.
- Governors and engine controls required to maintain the rated frequency when load is applied to the EPS. Section 126.96.36.199.2 requires that the EPS (genset) accept the entire emergency load in one step.
- Engine Control Accessories such as solenoid valves, gauges, instruments, and wiring are also covered in Section 5.6.3.
- Starting Equipment such as electric starters, batteries, and battery chargers are covered in Section 5.6.4.
- Controls such as control panels, remote annunciators, remote controls, and alarms are covered in Section 5.6.5 and 5.6.6.
- The controls are required to provide: automatic remote start capability; a “Run-Off-Automatic” switch; shut down and lock out the EPS under certain conditions; audible and visual alarms to signal personnel of faults locally and remotely. (188.8.131.52)
- All installations shall have a remote manual stop (E-Stop) of a type to prohibit inadvertent actuation by an unauthorized person located on the exterior of the enclosure or outside of the generator room in an indoor location. (184.108.40.206)
- Level 1 systems require both local and remote annunciation (220.127.116.11)
- Cooling Systems such as unit mounted radiators, remote radiators, heat exchangers, and related accessories are covered in Section 5.6.7.
- Exhaust System piping, mufflers, and silencers are covered in Section 5.6.8.
- Generators and Voltage Regulators are covered in Section 5.6.9.
- Mounts and Vibration Isolators are covered in Section 5.6.10