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620.91 Emergency and Standby Power Systems (C)Disconnecting Means (Elevators)
If there is a secondary source of power used for emergency purposes, how is the elevator disconnecting means affected?
The disconnecting means referenced in 620.51 must be capable of disconnecting all sources of power for maintenance purposes. Hydraulic elevators have the capability of using a battery pack to lower the elevator in a loss of power situation. The battery attachment is utilized as an extra level of safety to keep from stranding people in the elevator for long periods of time. Under normal operation, the main line power from the disconnecting means controls the raising of the elevator through a pump motor and the lowering of the elevator through a solenoid and a drain valve. To send the cab upward, the pump motor pumps hydraulic fluid into the piston that forces the elevator upward. To return the cab back down, a drain valve at the bottom of the piston is opened by a solenoid valve and as the fluid drains back into the reservoir, the elevator lowers. If the main line power is lost, this battery pack attachment can supply enough power to actuate the solenoid. See
Figures 4 and 5.Please note
Figure 5. By not having the auxiliary contact, the manual opening of the Elevator Disconnect will be viewed by the Elevator Controller as a loss of power. A control relay is used to sense the main line power. When de-energized, this control relay’s contacts will become closed and allow the battery pack attachment to activate the solenoid. This could be a serious problem if a maintenance person is the one that opened the disconnecting means to work on the elevator. If he or she is going to work in the pit, he or she will bring the elevator up a floor or two to enable access. Depending upon the timing involved, this could potentially result in the elevator lowering with the maintenance worker in the pit. This is why 620.91(C) requires an auxiliary contact. See Figure 6.As you can see, during normal operation having the auxiliary contact compromises nothing. See
Figure 7.During a normal loss of power (from the utility or other devices turned off upstream) the battery pack attachment would be able to operate the solenoid and lower the elevator. See
Figure 8.
As you can see in Figure 8, by having the auxiliary contact in the switch, the main line power and the battery pack attachment would both be disconnected when the switch is turned off so that the elevator will not be able to move. This is why 620.91(C) is important. Inspection plays a key part in this section in that the electrical contractor usually installs the disconnecting means and the elevator contractor usually installs the battery attachment. It is critical that the wiring for this be verified at the time of installation by both the electrical inspector and the elevator inspector. Both should have jurisdiction over this circuit to insure the highest level of safety.
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