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Test That Transfer SwitchFeb 28, 2003 12:00 PM, By Larry Blaine, SF Griggs & Associates

If you've been putting off conducting maintenance checks, you could be doing moredamage than you think.

An automatic transfer switch (ATS) is a critical part of any emergency power supplysystem (EPSS). The ATS is the device that selects a power sourceeither normal utilitypower or standby generator powerand conducts the power to critical loads. TheNFPA, Joint Commission on Accreditation of Healthcare Organizations (JCAHO), andall major ATS manufacturers publish literature calling for the monthly test of ATSs, yetmany facility owners ignore these guidelines and refuse to perform the tests. There are

good reasons for both the monthly-test requirement and for the reluctance of manyoperators to comply.

NFPA 110, the standard for emergency and standby power supply systems, states themonthly test of a transfer switch shall consist of electrically operating the transfer switchfrom the standard position to the alternate position and then a return to the standardposition (Paragraph 8.4.5.1). The standard also requires a monthly generator run. Thebest way to perform both these tests is by operating the test toggle on the ATS whenthe generator isnt running, and confirming that the ATS properly signals the generatorto start and run before transferring building load.

Reasons for ATS testing.

There are three primary reasons for the monthly test requirement. First, a test is anactive demonstration that the EPSS is still able to function as intended. If a switch isgoing to fail to transfer, its better to have the switch fail while connected to a live normalsource during a test than to have it remain connected to a dead normal source duringan actual power outage.

Second, an automatic transfer switch is an electromechanical device with moving parts.Like the pistons and crankshaft in an automotive engine, the moving parts in an ATScan seize if theyre left in one position for months or years. Regular exercise will helpensure that moving parts will continue to operate smoothly.

Third, a transfer switch test transfers building load onto the generator for the duration ofthe generator test. Most standby power generators of more than 100kW are dieselengine driven, and all major diesel generator manufacturers recommend a load of atleast 50% of maximum capacity during monthly generator tests. NFPA requires the

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same thing, for the simplereason that running undera light load is very bad fora diesel engine.

Running a dieselgenerator in an unloadedconditionwith no ATStransfer, thus with nobuilding load connectedcauses carbon buildup inthe cylinders and exhaustsystem, a condition knownas wet stacking. Thisproblem reduces efficiencyand power, and in extremecases it can cause anexhaust system to catchfire. To reduce a wet-stackcondition once it hasdeveloped, some facilityowners will rent a resistiveload bank and connect it tothe standby generator fora full-load run of sufficientduration to burn out thebuildup. The cost for suchan exercise is substantial,and the relief provided is only temporary.

Why some facility operators wont test.

Some people mistakenly think that monthly tests will shorten the life of a transfer switch,and are therefore reluctant to conduct them. More often, the reason for refusing to testproperly is a fear of the two brief interruptions in power that an ATS test entails.

Theres no truth to the idea of a regular test shortening the life of an ATS. Any device tobe used as a transfer switchswitching a single electrical load between two sources ofpowermust comply with UL standard 1008, the only standard applicable to this duty.

Any switch that bears a UL 1008 label can transfer a minimum of 3,000 times, with atleast 1,000 of those operations under at least 100% of rated load. (For smaller ampacityswitches 6,000 transfers must be under 100% load or greater).

A switch that transfers back and forth each month will see 12 transfer operations peryear, generally under something less than 100% of rated load. It would take a transferswitch at least 83 years to reach the same level of full-load use that representativeswitches endure in standard UL testing. And, as mentioned above, a switch thattransfers regularly is less prone to seizure than a switch that has remained static for along period.

The most-cited reason for failing to test transfer switches is that power interruptions cancause problems with lighting, transformers, computers, and other sensitive loads. Itsthis fear, primarily, that causes many facility operators to conduct their monthlygenerator runs without switching building load onto the generator.

While problems of this nature can be frightening, they should be no more frighteningthan the prospect of a transfer switch that fails to operate at a critical time due to lack oftesting, or a generator that fails due to wet-stacking. There are several ways ofmitigating or eliminating the problems caused by these power interruptions, both in newconstruction and in existing facilities.

Solving the problem.

For new construction projects the issue of transfer-test-caused interruptions in power isan easy one to solve. The engineer responsible for specifying the electrical equipmentshould specify closed transition, or make-before-break, transfer switches. A transferswitch of this kind transfers when in its test mode, with no interruption in power.Switches of this kind were once a special product available from only one manufacturer,

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