In the Know

Maintaining a Healthy Generator Engine at Your Facility | Part 1

All diesel powered generators when exercised should be loaded to at least 60 - 70% of their standby nameplate rating at least once a month in order to be a reliable sources of power. Exercising generators at less than 50% will ultimately result in “wet stacking” or “slobbering”. In order for a diesel engine to operate at peak efficiency it must be able to provide fuel and air in the proper ratio and at a high enough engine temperature for the engine to completely burn all of the fuel.

NFPA 110, Emergency and Standby Power Systems makes specific reference to this issue in the Appendix of the 2013 edition at A.8.4.2: Light loading creates a condition termed wet stacking, indicating the presence of unburned fuel or carbon, or both, in the exhaust system. Its presence is readily indicated by the presence of continuous black smoke during engine-run operation...

If the connected load will not provide sufficient engine loading to mitigate the problem, there are two solutions: (1) connecting a portable load bank or (2) increasing the connected loads served by the generator. 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 is the simplest solution in that the load bank can be rented from a generator service company and trailered to the site for connection to the generator or paralleling switchgear. Most load banks are manufactured with incremental switching that allows the generators to be “ramped-up” so as not to damage the engine by unintentional “block-loading”.

NFPA 110,, 2013 edition, 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 at not less than 50 percent of the EPS nameplate kW rating for 30 continuous minutes and at not less than 75 percent of the EPS nameplate kW rating for 1 continuous hour for a total test duration of not less than 1.5 continuous hours.

Using a resistive and reactive load bank should be considered when conducting tests in lieu of just a resistive load bank. Most generator sets are rated at 0.8 PF with the generator (alternator) having a kVA rating of 125% of the kW rating. The resistive and reactive load bank will allow you to test both the engine and the generator to its maximum rating.

Procedures for connecting a portable load bank should be well thought out to mitigate problems in case of a failure of the normal/utility source during a load bank exercise.

If possible, output cables from the generator or breaker(s) should never be disconnected during a test. If an outage should 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.

NFPA 110, 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 can be “paralleled” with the building load if possible so that a reconnection of generator cables is not necessary. The proper way to connect the load bank is to a dedicated bus or breaker downstream from an overcurrent protection device inside a switchboard or to a bus inside a NEMA 3R connection box mounted at a convenient place outside the generator building.

Another method is to have an additional switchboard installed so a load bank or portable generator can be connected to the bus. The switchboard would consist of three breakers with kirk-key interlocks that allow only two breakers to be closed at once. The first breaker would be connected to the permanent generator, the second breaker connected to the building load, and the third for connection to the load bank (or portable generator). In the event that utility power fails during the test, generator power could be reconnected to load quickly. If there was a failure of the permanent generator, the third breaker could be used to quickly connect a portable generator to the facility. 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 fails.

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). Additionally, testing may cause interruption to normal business activity.

Adding a permanent pad-mounted load bank mounted outside the generator building is another alternative. One major advantage is that the loadbank can be hard wired into a switchboard and, through proper control wiring, be automatically disconnected in case of a utility failure.

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