Best practices for conducting a load bank test.
As covered in my previous blog post, all diesel-powered generators should be loaded to at least 30% of their standby nameplate rating when exercised during their monthly test. If the connected load will not provide sufficient engine loading to hit the 30% minimum load level, one option is to connect a portable resistive load bank annually to meet the requirements of NFPA 110. Note: If a generator set is paralleled with other generator sets — or if loads from other generator sets can be safely transferred to the under-loaded set — then simply switching the other sets to the OFF position might suffice.
A portable load bank can be rented from a generator service company and trailered to the site for connection to the generator. Most load banks are manufactured with incremental switching that allows the generators to be “ramped-up” so as not to damage the engine by “block-loading.”
NFPA 110, Emergency and Standby Power Systems, 2013 Edition, Section 220.127.116.11 states: “Diesel-powered EPS installations that do not meet the requirements of 8.4.2 shall be exercised monthly with the available EPSS load and shall be exercised annually with supplemental loads [e.g., a load bank] at not less than 50% of the EPS nameplate kW rating for 30 continuous minutes and at not less than 75% of the EPS nameplate kW rating for one continuous hour for a total test duration of not less than 1.5 continuous hours.”
Procedures for connecting a portable load bank should be well thought out to mitigate problems in case of a failure of the normal/electric utility source during a load bank exercise. Output load cables from the generator or breaker(s) should never be disconnected during a test. Should an outage occur, there is no way the cables could be reconnected in order for the generator to provide power to required loads within the 10 seconds required for life safety or other critical loads. Furthermore, NFPA 110, 18.104.22.168 states: “Equivalent loads used for testing shall be automatically replaced with the emergency loads in case of failure of the primary source.”
In order to perform the load bank test safely, the load bank should be “paralleled” with the building load, so a reconnection is not necessary. The proper way to connect the load bank is to a dedicated bus downstream from an overcurrent protection device inside a switchboard or to a bus located inside a NEMA 3R connection box mounted at a convenient place outside the generator building.
A more expensive alternative — but one that is also more enhanced — is to have the circuitry engineered so a load bank or portable generator could be connected to the bus. In the event that a permanent generator failed during the test or was disabled for repairs, power could be back-fed into the bus. In some critical cases a facility would be wise to have both the load bank and a portable generator on-site during the exercise in case the permanent generator failed.
Increasing connected building load can be problematic, and sometimes not feasible from an investment standpoint because of the location of switchboards, transfer switches, and generators. However, there is some economic benefit when switching non-essential load onto the generator in that the electric utility bill is decreased during the test (this benefit has to be offset, however, by the cost of the diesel fuel used during the exercise).
Adding a permanent pad-mounted load bank outside the generator building is another alternative. This is often less expensive than adding additional building load. One major advantage is that the load bank can be hard wired into a switchboard and through proper control wiring be automatically disconnected in case of an electric utility failure.
The recommended procedure to follow for the load bank test would be to:
By adding the building load first you will not be in danger of losing power to the building loads in case of a normal power failure…the building will already be on its emergency source.
Note: NFPA 110, 8.4.2 and 8.4.9 require that all diesel generators be exercised monthly for 30 minutes at operating temperature, and every three years to at least 30% of kW nameplate load for a period of 4 hours. The load bank exercise, if needed, can be completed simultaneously, if managed properly.
Caution: Follow lockout/tagout procedures and wear PPE as prescribed by NFPA 70E and OSHA regulations as found in 29 CFR 1910 Subpart S. No person should be exposed to a live bus without wearing the proper levels of PPE.
Chisholm, president of MGI Consulting, Orlando, Fla., has provided emergency power supply systems (EPSS) consulting services and education to more than 1,500 health care facilities. He serves as a member of the National Fire Protection Association's Technical Committee responsible for NFPA 110, Emergency and Standby Power Systems and the Electrical Section of NFPA 99, Health Care Facilities. He also serves as a primary emergency power consultant to the U.S. Army Medical Department (AMEDD) and the Department of Defense.