Elevator Control Circuits
Q: What are the circuit connections that you use for elevator recall and elevator shut-down (shunt trip)?
A: Continuing from last month's discussion, it is hard to avoid the same lengthy/wordy explanations common in this industry, when describing elevator recall and shut down. Hopefully, the following illustration will be useful and you can refer to the letters associated with each feature. The NFPA 72-2002 code reference numbers are included, and you can find the previous 1999 edition rules in section 3-9.3. Both NFPA 72 editions complied with the rules set forth in the latest Safety Code for Elevators and Escalators (ASME A17.1, 126.96.36.199).
--Circuits for Elevator Recall for Fire Fighter's Service (NFPA72 6.15.3). Elevator recall rules require three circuits (pairs of wires) for attachment from the fire alarm system to the elevator safety control equipment. Mark these 1 through 3 and leave them in the sprinkler machine room for the elevator contractor to connect. Sometimes, they'll want you to provide them with a list of the detectors they are connected to.
Some people become confused because Circuit #2 usually recalls elevators to the first floor, and Circuit #1 usually recalls them to the second floor.
- Circuit #1 will recall the elevator(s) to the Alternate Level (floor) chosen by the fire department officials.
- Circuit #2 will recall the elevator(s) to the Primary/Designated Level chosen by the fire department officials.
- Circuit #3 is used to flash a firefighters helmet symbol inside the elevator car to warn firefighters using that car, that the elevator(s) are no longer safe to use. Since there may be more than one hoistway or machine room, there may be more than one 'Circuit #3'. If there are, just label them Circuit #4, #5 and so on, but be sure to also provide the installed location of these detectors .
--Circuits for Elevator Shutdown (NFPA72 6.15.4). The purpose of the shunt-trip is to remove elevator-operating power prior to the activation of waterflow from any sprinkler head located in the hoistway (A and/or F) or elevator machine room (B and/or D). To prevent needless interruption of elevator service, only the elevator cars using equipment in that particular machine room are to be shut down.
All elevator cars in a hoistway must be shut down prior to the activation of the hoistway's sprinkler(s). While sprinkler heads are not always required in these areas, if they are, water spraying upon the elevator car or elevator equipment machinery can have undesirable consequences. For instance, wet brakes can cause the car to slide on its rails or move without warning with its doors open. If the spray occurs in a machine room, or on machinery located in the hoistway, the water may short circuit the electrical equipment causing the elevator to stop between floors.
Hoistway Detectors. NFPA 72 only allows smoke detectors to be installed in hoistways (A) if sprinkler heads are also installed in that hoistway. While this unfriendly environment is considered unsuitable for smoke detectors, the benefit they provide for initiating recall before the shunt-trip can occur outweighs their potential for nuisance recalls. However, the preferred detector to use for the shunt-trip circuit is a rate-of-rise/fixed temperature heat detector installed within two feet of each sprinkler head. It must have a lower temperature rating and a more sensitive (i.e. 50 foot) Listed spacing if it is to operate the shunt trip circuit before the sprinkler head operates.
For additional safety for any elevator occupants, a time delay is permitted to be employed to allow the car(s) to descend to the primary level before a shunt-trip is activated. A waterflow switch installed at each sprinkler head is also permitted to be used, if a retard feature is not employed. This shunt trip function is less likely to be needed since NFPA13 doesn't require sprinkler heads in non-combustible passenger elevator hoistways anymore.