What is a generator ATS? What does it do?
An automatic transfer switch or ATS for a diesel generator is a device that switches the load from the mains to the generator and back again. The mains is monitored either in the generator control panel, or in the ATS. When the mains either fails, a phase fails, or the mains goes in/out of preset parameters, the generator will start and the ATS will changeover, from the mains to the generator, to ensure that the generator does not backfeed the grid.
Back feeding the grid when it has failed is a sever risk to the life of any grid employees working on the lines, poles or transformers. Imagine the grid cut the power to your street, because a transformer needs replacing. It’s going to take them around an hour. You start your generator, which you have connected directly into your fuse board and you haven’t stripped the main breaker in the house. The power will travel back into the grid, directly to the man working on the transformer, potentially killing him.
It is essential, in all circumstances, that a foolproof method of isolating the grid from your generator when the mains has failed. Never be tempted to save a few pounds and risk the lives of others. The consequences for them, for yourself and your family could be disastrous.
This is the internals of a small ATS panel that uses a motorised switch to change over - other panels may also use contactors, MCCB's and ACB's, depending on their size and cusotmer requirements.
How does an ATS panel work?
There are various designs of ATS panel. Typically, a mains failure relay is fitted in an electrical enclosure, with two contactors. The contactors are mechanically and electrically interlocked. This means there is a mechanical device, called an interlock, that ensures the two contactors cannot be closed at the same time. The contactors need electrical power to close (you can get both DC and AC types).
The electrical interlock is designed in the circuitry, to prevent the contactors from energizing at the same time. While the mechanical interlock should stop both of them closing even if this were to happen, this secondary circuit is an added protection. Usually, a normally closed contact is fitted to each contactor. These contacts, normally (ie when the contactor is not energized) are closed. When the contactor is energized, they open. If you have two contactors, A and B, the circuit to close contactor A is wired through the auxiliary of B, thereby when B is closed, its not possible to energize A. The same is true for B, this is wired through the auxiliary of A. This provides the electrical interlock.
When the mains fails, the ATS sends a remote start (normally volts free) to the generator. The generator runs up and when at the required power and voltage, the ATS will open the mains contactor and close the generator contactor. When the mains returns, the generator contactor is opened and the mains contactor is closed.
Because both contactors are never closed together, even when the mains returns, there will be a short break in electrical supply as the contactors change over.