The question was posed to me about emergency power and when a bypass device would be required in lieu of a transfer device. From this question, the second natural questions to ask was how the devices work. The first question, I’m going to refer you to code (NEC, UL 924, Emergency). As far as how the devices work, the following discusses how the following devices work: Shunt Relays, Emergency Bypass, and Emergency Transfer.
Often time, designs have dimming, switching, or control levels in rooms or zones with emergency requirement (i.e. conference rooms). The intent of the shunt relay is to provide a level of lighting control on an emergency circuit, while still allowing the circuit to switch to 100% ouput during power failure. Shunt relays are a bypass device.
Emergency Shunt Relay (Detail provided by LC&D, click here for information on the shunt)
Emergency power is fed into the shunt relay, connected to one end of a normally open relay, and then continued out to the switch/dimmer/ relay before being connected to the load. A wire is also run from the second side of the normally open relay, and is connected to the first set of wires downstream from the control device and upstream from the load. This second wire forms the bypass route for the emergency power to take during failure, while the normally open relay forces current through the control device up until the moment of failure.
The shunt relay is also connected to normal power; however, this connection is designed only for power sensing purposes. Once the shunt relay is connected to normal power, it will hold the normal open relay open until power lose is detected on the normal circuit. At the moment of power lose, the relay will close, and electricity will natural flow down the path with least resistance, the path without the lighting control. At this point, lights on the emergency circuit will turn to 100% and will remain at 100% until power is restored to the normal circuit, which will cause the relay to open, forcing power through the lighting control.
This solution is typically seen in conference rooms requiring emergency lighting. The zone designated to turn on in the event of an emergency will be powered by an emergency circuit. The shunt relay will be connected prior to the device controlling the lighting zone, and the zone will operate according to the control device until power loss is detected.
Emergency Bypass takes the Shunt Relay one step further. The device is still positioned to direct power around a switch or control device. However, with the transfer device, the normal power feed is providing power to the light, and the device will transfer the source to emergency power during failure.
Emergency Bypass – (Detail provided by Nine-24, click here for more information on the transfer)
When looking at the diagram, you will see it looks a little different. The Black, Red and White are the Normal power connections. White is neutral, and the black and red are normal power. The black connection is uninterrupted, while the red connection has the switch, photocell, and time clock tied to it. The red wire is providing the power to the lights during normal operation, and the power will be limited by the devices downstream (it will dim with the photocell, off/on with the switch/timeclock, etc). The black wire is there for the purpose of sensing, similar to the normal power connection on the Shunt Relay. During normal power operation, power flows from the source, through the control device, down the red wire, and out the yellow wire to the load. The circuit is completed from the load, back to the white/blue wire, and then out again from the white wire back to the panel.
The emergency power is connected by the blue, yellow, and white/blue wires. Blue provides the emergency power into the Bypass, and yellow takes that power out to the lighting load. The white/blue wire is the neutral path from the lighting load back to the normal power during normal lighting operation, and it is the neutral path back to the emergency power during emergency power.
When the Bypass devices does not detect power on the black normal wire, it means there was some form of power failure. At that point, inside the bypass device, the device will transfer the connections, and power is provided from the emergency source by the blue wire, and out to the fixture from the yellow wire. The circuit is completed from the load back to the emergency panel (it will not follow the white/blue wire back to the device).
In this situation, the control device is avoided and the lights will turn to 100% output. The lights will remain at 100% until the black wire detects normal power restored, and the transfer device will reroute power back through the control devices in the room.
Finally, Emergency Transfer
Emergency transfer, like the shunt relay and bypass device, is designed to allow lights to turn to full output upon power failure. The transfer works the same way the bypass device works, in that it takes a normal and an emergency input to the device. However, it transfers the hot wire and the neutral wire for full emergency control.
Emergency Transfer (Detail provide by Nine-24, click here for more information on transfer device)
The detail provided above looks a little different from the last two, but don’t let the number of connections confuse you. The Nine-24 device provide additional contacts to allow input from different systems. In the detail above, a Fire Alarm Panel is shown. In this case, a signal from the fire alarm system could cause the lights to switch to 100% output rather than waiting for the Transfer device to detect power failure.
The lighting control is shown on the top left portion of the detail. Power is routed from the normal power panel , down through the dimming cabinet, and then into the BLTC device (note in this case it is labeled as a dimming cabinet, but this can be any type of lighting control). The normal circuit is tapped prior to the dimming cabinet and run into the top portion of the BLTC. This connection serves as the power sensing portion of the device, and will initiate the transfer to emergency power during power failure.
Emergency power is fed into the device, and the hot and neutral feeds are connected to the same ports the normal hot and normal neutral feeds were connected to. With the dual feeds connected to the transfer device, when power failure is sensed, the bypass will transfer the normal hot and the normal neutral to emergency hot and emergency neutral. Lights fed downstream from the BLTC device will turn to 100% output.
In summary, the Shunt Relay, the Emergency Transfer, and the Emergency Bypass device will all allow a controlled lighting zone to turn to 100% output during a power failure. A shunt relay allows a level of control to be applied to an emergency circuit, and the Shunt and Bypass transfer power from a normal source to an emergency source with the transfer only transferring the hot wire, and the bypass transferring the hot and neutral wires. The different devices are required for different occupancy levels per Code Requirements, and you should verify the type of emergency control that is required on your project prior to specifying a device.
Bonus – How do you turn a light on by turning the switching off?
Each of the override devices have a sensing element associate with the device. When connecting the device to normal power, if the sensing leg is connected to a switched leg, the device will think power was lost any time the switch is flipped to the off position, and the emergency light will be turned to 100% output. Turning lights on, by switching them off.