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Training and Consulting Safety Technical Articles Service Bulletins Transfer Switch Sequence of Operations	When a power outage occurs, primary voltage supplied from the utility disappears. Input voltage to the utility voltage sensors goes to zero. These are typical voltage sensors. You may have from one to three sensors grouped together in the upper left corner of the control panel. They will be mounted in slots A1, A2 and A3. If your voltage sensors have a red LED at the top, the LED wil glow when power is available at the sensor. Some older sensors do not have the LED. When the voltage sensors sense a loss of power, they signal the start-stop time delay to begin timing for start. This is a typical start-stop timer. Older model switches may have mechanical timers. When timing for start, the top red LED will light. When timing is complete, the red LED will go out and the bottom LED will glow green. When the start-stop timer times out, it signals the generator to start the engine. On small Onan generators, such as a 7.5 JB or 15 JC, a 2 to 3 wire converter is used. On larger units, the converter is not needed. This is a 2 to 3 wire converter. Most generators over 20 kilowatts will not require this. When the generator starts, voltage is supplied to the emergency voltage sensor. This is a typical emergency voltage sensor. It is identical to the utility voltage sensors and mounts next to them in slot A4. When the emergency voltage sensor detects available generator power, it signals the transfer-retransfer time delay to begin timing for transfer. This is a typical transfer-retransfer timer. Older model switches may have mechanical timers. When timing for transfer, the top LED will glow red. When timing is complete, the red LED will go out and the bottom LED will glow green. When the time delay times out, it will signal the switch to transfer. The contactor assembly located on the back wall of the switch enclosure will then disconnect from the utility and connect to the generator. The generator is now powering the load. When utility power is restored, the utility voltage sensors will detect the utility voltage and signal the transfer-retransfer timer to begin timing for retransfer. Utility voltage sensors Transfer-retransfer timers. When timing for retransfer, the bottom LED will flash and the center LED will glow red. When timing is complete, the LEDs will go out. When the transfer-retransfer time delay times out, it will signal the switch to retransfer. The contactor assembly will then disconnect from the generator and connect to the utility. The load will now be running on commercial power. When retransfer is complete, the start-stop timer will begin timing for stop. Start-stop timer. When timing for stop, the center LED will glow red and the bottom LED will flash. When timing is complete, all LEDs will go out. When the start-stop timer times out, it will signal the generator to stop the engine. Small ONAN units use a 2 to 3 wire converter to do this. 2 to 3 wire converter. This is typically not found on larger generators. The system is now completely reset and ready for the next power failure. HomeBack to MainBack to Step 1

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Transfer Switch Sequence of Operations

When a power outage occurs, primary voltage supplied from the utility disappears.
Input voltage to the utility voltage sensors goes to zero.

These are typical voltage sensors. You may have from one to three sensors grouped together in the upper left corner of the control panel. They will be mounted in slots A1, A2 and A3.

If your voltage sensors have a red LED at the top, the LED wil glow when power is available at the sensor. Some older sensors do not have the LED.

When the voltage sensors sense a loss of power, they signal the start-stop time delay to begin timing for start.

This is a typical start-stop timer. Older model switches may have mechanical timers. When timing for start, the top red LED will light. When timing is complete, the red LED will go out and the bottom LED will glow green.

When the start-stop timer times out, it signals the generator to start the engine. On small Onan generators, such as a 7.5 JB or 15 JC, a 2 to 3 wire converter is used. On larger units, the converter is not needed.

This is a 2 to 3 wire converter. Most generators over 20 kilowatts will not require this.

When the generator starts, voltage is supplied to the emergency voltage sensor.

This is a typical emergency voltage sensor. It is identical to the utility voltage sensors and mounts next to them in slot A4.

When the emergency voltage sensor detects available generator power, it signals the transfer-retransfer time delay to begin timing for transfer.

This is a typical transfer-retransfer timer. Older model switches may have mechanical timers. When timing for transfer, the top LED will glow red. When timing is complete, the red LED will go out and the bottom LED will glow green.

When the time delay times out, it will signal the switch to transfer. The contactor assembly located on the back wall of the switch enclosure will then disconnect from the utility and connect to the generator. The generator is now powering the load.

When utility power is restored, the utility voltage sensors will detect the utility voltage and signal the transfer-retransfer timer to begin timing for retransfer.

Utility voltage sensors

Transfer-retransfer timers. When timing for retransfer, the bottom LED will flash and the center LED will glow red. When timing is complete, the LEDs will go out.

When the transfer-retransfer time delay times out, it will signal the switch to retransfer. The contactor assembly will then disconnect from the generator and connect to the utility. The load will now be running on commercial power.

When retransfer is complete, the start-stop timer will begin timing for stop.

Start-stop timer. When timing for stop, the center LED will glow red and the bottom LED will flash. When timing is complete, all LEDs will go out.

When the start-stop timer times out, it will signal the generator to stop the engine. Small ONAN units use a 2 to 3 wire converter to do this.

2 to 3 wire converter. This is typically not found on larger generators.

The system is now completely reset and ready for the next power failure.

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