relay wiring question

a little more information would be appreciated too :)

is the back up power battery? generator?

incandescent or fluorescent?

have any make / model numbers you would care to divulge?

any reason you cannot ask the lead electrician who is working on the project?

the simplest method (but not the only one) is to use a single relay. the relay is normally energized. when main power is lost the relay "drops". the contacts switch to the backup AC source and start the source as needed. (i.e. a battery operated inverter) when main power is restored the relay "picks" switching the load (lamp) back to main power and shutting down the backup source (and recharges batteries as needed) usually a test switch is located in the fixture which when depressed simulates a power failure.

For a school this function should be inplemented using a transfer switch rated for the application. The transfer switch is basically a large double throw relay with a control circuit with test switches and control relay contacts for starting/stopping a generator.

Bill Kaszeta Photovoltaic Resources Int'l Tempe Arizona USA snipped-for-privacy@pvri-removethis.biz

Emergency lights generally require 2 switch functions: one to switch the lights on when power fails, and one to turn the lights off when they might damage the batteries by being left on too long. (For example, if it is a 12 volt battery, it should *never* be discharged below 10.5 volts.) Although it does not necessarily require two relays, it can be done with two relays.

See the diagram below. AC power, when it is available, energizes relay 1. Relay 1, by being energized, does two things:

Relay 2, by being energized, completes a path from the battery to its own coil, via the RY2-1 normally open contact. It also provides a path to the lights through the de-energized normally closed RY1-2 contact. N/O +--[RY2-1]---+ | | N/C AC---+-------+ +--[RY1-1]---+--[RY1-2]-+ | | | N/O | | [RY1] [CHARGER]==[BATT] [RY2] [LITES] | | | | | AC---+-------+ +------------+----------+

When AC power fails, RY1 de-energizes. The RY1-1 contact opens. Current still has a path through the RY2-1 normally open contact, so RY2 remains energized. The RY1-2 normally closed contact closes when RY1 de-energizes, and the lites turn on. The battery slowly discharges through the lites. When battery voltage drops low enough, RY2 doesn't get enough current to remain energized, so it drops out, opening the path to the battery. Thus the battery cannot discharge any farther.

In practice, RY2 is usually energized with a solid state circuit, (transistor/zener/voltage comparator) because setting the voltage that the relay will de-energize is most easily accomplished that way.

Ed