Quick, effective closed transition switching is crucial for any site or application that relies on a backup power supply from a generator to ensure power is not lost.

A closed transition transfer switch is designed to seamlessly switch between two power supplies, often using make-before-break switches, and a controller that senses if the supplies are synchronised, before allowing switching.

Closed transition switching is very common in hospitals, data centres and other critical facilities with loads sensitive to momentary power loss between two supplies, such as during periodic testing.

For any site reliant on a backup generator supply, it is mandated that frequent tests are performed to ensure the operation of that supply in an emergency event. Generators must be tested several times a year, and in the case of many hospitals, a test must be performed monthly.

Without closed transition transfer switching, during each of these test periods there is typically a short interval where power is lost. Closed transition switching allows for testing of the emergency power system without interruption of power to critical loads, as well as maintaining continuity of power when transferring back to utility supply. Since the power to load is maintained, testing can be completed without most users even knowing it’s occurring.


In most applications, the backup supply comes from a generator. When switching back to the mains supply, there is a period where both supplies are connected as part of the seamless changeover, and the generator effectively becomes a power generation source feeding into the utility mains supply.

One mode of failure of a closed transition transfer switch can be where both sources become connected permanently, leading to equipment damage on the grid, which is why strict requirements are necessary.

While specific requirements vary from state to state, any fault when both supplies are connected must be cleared quickly before any protection devices on the utility network have time to respond and cut power to a wider area of users.

On average, utility circuit protection will begin to react to a disturbance within 50-60ms, so it is critical for downstream protection to react to a fault at a much faster rate.

Rapid fault clearance

Thanks to Terasaki’s patented double break contact technology, TemPower 2 Air Circuit Breakers (ACBs) are proven to provide superior fault clearance times of less than 30ms (a figure applicable for all models except a 5000/6300A frame size).

This makes it the preferred option for many utilities, as it will disconnect from their network long before their protection systems can engage. This rapid fault clearance also minimises thermal and mechanical stress on busbars and reduces arc flash energy during a fault, meaning that it’s not only much safer but has a much longer service life.

TemPower 2 ACBs also have a breakthrough self-monitoring temperature system for checking the condition of the main contacts and conductive path, known as ‘3C Technology’.

The status and wear of the contacts is determined by direct temperature measurement using integrated thermistors, and alarms can be set to notify of required maintenance before it becomes critical. This real time temperature data can be exported to any SCADA or BMS.

Servicing your ACB

Servicing the ACB is simple with NHP’s certified service technicians. All serviceable parts in TemPower 2 ACBs are available for inspection once the body is racked out, so the average time of repair is dramatically reduced to 15 minutes or less in some applications.

For critical infrastructure, these figures are invaluable. Terasaki TemPower 2 ACBs from NHP are a trusted essential for any critical facility sensitive to momentary power loss during scheduled testing, providing protection, performance and peace of mind for both electrical utilities and facility managers alike.

This sponsored editorial was brought to you by NHP. For more information, please call 1300 NHP NHP or email

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