What are Automatic Mains Transfer Panels?

The most common use of the Automatic Mains Transfer Panel is to switch the source of mains supply for UPS systems between the utility mains supply and a Standby Diesel Generator supply.
Under normal circumstances the UPS system will derive its supply from the utility mains supply. Dependent upon the sophistication of the Automatic Mains Transfer Panel there will be a number of different sensors for determining a loss, partial loss, or variation of the utility mains supply.

The most basic sensors are utility mains failure, covering anything from a single phase or a three-phase system failure to all three phases failing simultaneously. The detection of these failures is often achieved by voltage sensing relays which will operate when the utility mains has reached a predetermined point of failure. This settings should be adjusted in conjunction with the operating characteristics of any uninterruptible power supplies. If the warning point of the utility mains failure is outside of the UPS system’s operating parameter’s one could find that the load is being supported by the UPS batteries while the standby diesel generator is lying idle.

engineer2In its simpler form the Mains Transfer panel consists of two contactors (or motorised switches) with their associated control sensors. One contactor will supply the utility mains source to the site distribution board; the other contactor will supply the standby diesel generator supply to the distribution board. When there is a utility mains failure, initially the mains contactor will de-energise and the standby diesel generator will receive a start command.

When the standby diesel generator control sensors have determined that the generator supply is at the correct frequency and voltage, a signal will be sent to the standby diesel generator contactor to energise it and feed the generator supply to the site distribution board.

When the utility mains supply is re-established, a timer will start to ensure that the supply is stable, and after a preset time the utility mains contactor will energise and the standby generator contactor de-energise. The standby diesel generator will continue to run until it has completed its cool down cycle.

During the period of supply loss from both the utility mains supply and the standby diesel generator, the UPS system will continue to provide the power supply to your the essential load. Just remember never to let the diesel generator run out of fuel. Obvious? Don’t you believe it !

For timing purposes, the duration of supply loss between the failure of the utility mains supply and the provision of a supply from the standby diesel generator can be as short as 30 seconds, providing that the generator starts first time. The changeover time for returning to the utility mains supply can be measured in milliseconds.
And then life and power goes on as before.

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