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What does a generator control unit do and what’s the difference between a manual and an automatic transfer switch?

Well, the difference can be profound, especially if you run a business storing perishable goods, or rely on your mains supply to power internet servers or other critical electrical appliances.

Let’s begin with a brief explanation of what a generator control is, and how a transfer switch works.

Generator control panels comprise of an electronic control unit, specifically designed to control generator functions (start/stop and shut down the generator should a fault occur; common faults may include low oil pressure, high water temperature, low coolant level, over/under voltage, over/under frequency etc), and a breaker to protect the alternator with overload protection.

A generator supply should never be connected directly to a distribution board (in parallel to the mains supply). The generator supply should be fed through a circuit breaker mounted on the generator itself, through a transfer switch and on to the distribution board/consumer unit circuit breaker. A transfer switch is a device which allows safe transfer from a primary source of power (normally mains supply) to a secondary source (normally a standby generator), by physically ‘making’ and ‘breaking’ a connection between the primary and secondary power supply, whilst maintaining isolation of each source from the other.

A ‘classic’ manual transfer switch (for use with a manual generator control panel), features a three position selector switch . It may also be fitted with warning lights, or some other method of indicating whether the mains/generator supply is available.

The first position will make the connection between the mains supply and the distribution board. This is the default position for the transfer switch when the mains supply is healthy. The second position will break the connection between the mains and the distribution board. In this position the mains supply and the generator supply are completely isolated from the distribution board and each other, preventing the possibility of the mains and generator supplies being connected simultaneously. The third position makes the connection between the generator and the distribution board and should be selected once the generator is running.

A ‘manual’ generator control unit requires an operator to physically push a button, or turn a key to start and stop the set (if the control unit has a remote start function, a switch could be wired to a remote location, or a radio frequency remote control device could be fitted to turn the set on and off). Once the set is started, the operator must then alter the position of the transfer switch, passing the load from the mains supply to the generator supply. When a healthy mains supply is restored the reverse operation must be performed. The transfer switch should be returned to the mains supply position and the generator set should be shut down manually by the operator.

By contrast, an ‘automatic’ control unit (known as an automatic transfer switch) constantly monitors the state of the mains supply. If the mains supply is interrupted, the voltage of the mains supply drops below a pre-determined level (brown-out condition) or surges above pre-determined parameters. The electronic control unit will immediately disconnect the mains supply and start the generator set. Once the generator is operating and producing a stable supply, it will transfer the load to the generator supply by energizing the motorized transfer switch. When the mains supply returns to a healthy state, it will carry out the reverse operation. The electronic control unit will then shut down the generator set and resume its monitoring function, ready for the next event. This entire procedure is carried out automatically, without the need for operator input.

Automatic mains failure transfer switch systems are very convenient, but for business owners who rely on a stable mains supply they are more than a mere convenience, they are an important safety net. The true value of standby power generation combined with an automatic switching system (automatic transfer switch) is seldom realized until an interruption in the power supply has occurred and the resulting financial consequences are felt.

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