Standby Generators & Automatic Transfer Switch
A practical explanation of how a standby generator and an automatic transfer switch work together to restore supply after a mains failure, the key parts of a genset, and why life-safety loads in UAE buildings depend on them.
A standby generator is an on-site source of electricity that takes over when the normal utility supply fails. In most buildings it is a diesel engine driving an alternator, sized to carry essential loads until the mains returns. The device that connects and disconnects it automatically is the automatic transfer switch (ATS).
In the UAE, standby power is not only a convenience but, for many buildings, a code requirement. Life-safety systems such as fire pumps, smoke-control fans, fire alarms and emergency lighting must keep operating during a power failure, and a generator with an ATS is the standard way to guarantee that supply.
How it works
The heart of the system is the generating set, or genset: an engine (usually diesel) mechanically coupled to an alternator. The engine provides rotational power, and the alternator converts it into electricity at the building voltage and frequency. A fuel system, cooling system, exhaust, and a control panel that monitors voltage, frequency and engine health complete the package.
The automatic transfer switch continuously monitors the utility supply. When it detects a failure or an out-of-tolerance voltage, it sends a start signal to the generator and waits for the engine to reach stable speed and voltage. Only then does it transfer the load from the failed mains to the generator, so equipment is never connected to an unstable source.
A critical safety feature of the ATS is mechanical and electrical interlocking that makes it a break-before-make device: it disconnects from one source before connecting to the other. This prevents the generator and the utility from ever being paralleled by accident, which would be dangerous for the network and for utility workers, and avoids back-feeding the grid.
When the utility supply returns and stays stable for a set time, the ATS transfers the load back to the mains, then signals the generator to run on for a short cool-down period before shutting down. This sequence — start, transfer, run, retransfer, cool-down — is automatic, so essential loads see only a brief interruption while the engine starts.
Generators are sized for the connected essential load, allowing for the high starting current of motors such as fire pumps and lifts. They may serve only life-safety and essential circuits, or the whole building, depending on the design. Regular testing, fuel storage and maintenance are essential, because a standby set must start reliably on the rare occasion it is needed.
Main types
In the UAE
- In the UAE, the Fire and Life Safety Code of Practice (enforced through Civil Defence) requires emergency power for life-safety systems such as fire pumps, smoke-control fans, fire alarm and emergency lighting, which a standby generator with an ATS provides.
- Generator installations must be coordinated with the Department of Energy / distribution company connection arrangements and must not back-feed the utility, which is why break-before-make transfer is mandatory.
- Fuel storage, exhaust emissions, acoustic limits and ventilation of generator rooms are subject to UAE civil-defence and environmental requirements, and equipment must comply with applicable ESMA / recognised standards.
How GPR applies this
GPR installs and integrates standby generators and automatic transfer switches for commercial, industrial and residential buildings across Abu Dhabi, sizing the set for the essential and life-safety load including motor starting. We coordinate the ATS sequence, earthing and break-before-make interlocking with the distribution company, arrange generator-room ventilation, fuel and exhaust to civil-defence requirements, and commission and test the system so emergency power is reliable when it is needed.
Frequently asked questions
What does an automatic transfer switch do?
It monitors the mains, starts the generator on a failure, transfers the load to it once stable, and switches back when the mains returns — all automatically and with break-before-make interlocking.
Why must transfer be break-before-make?
To ensure the generator and the utility are never connected together by accident, which would be dangerous and could back-feed the grid. One source is disconnected before the other is connected.
How is a standby generator sized?
For the connected essential load plus the high starting (inrush) current of motors such as fire pumps and lifts, so voltage stays stable when large loads start.
What loads does a building generator usually supply?
At minimum the life-safety and essential loads — fire pumps, smoke fans, fire alarm, emergency lighting and essential lifts — and sometimes the whole building, depending on design.
Why does a standby generator need regular testing?
Because it may sit idle for long periods but must start reliably in an emergency. Routine test runs, fuel maintenance and servicing confirm it will perform when called upon.