Generation
Automatic transfer switches and ATS panels
An automatic transfer switch moves the load between mains and a generator, or between two sources, and is specified by current, poles, transition and the load it switches.
- Rated current
- A, at the switched voltage
- Poles
- 3, or 4 with switched neutralset by the earthing system
- Transition
- open, delayed or closedset by the load and the operator
- Utilisation category
- for example AC-33BIEC 60947-6-1
- Short circuit withstand
- kA, with the protecting device
A transfer switch decides how the load sees an outage. In open transition it breaks from one source before it makes onto the other; a delayed transition adds a pause so motors and transformers lose their residual voltage first. In closed transition the two sources are paralleled for a moment, which needs synchronising and often the network operator's consent.
Whether the neutral is switched follows from the earthing of the generator. Where the set's neutral is earthed at the set, a four pole switch keeps the two earthing points apart and stops neutral current from splitting between paths.
Changeover on a GB distribution network
No one may run generation in parallel with a public network without the distribution network operator's agreement, a rule Engineering Recommendation G99 takes from regulation 22 of the ESQCR. G99 Issue 2, of 10 March 2025, sorts standby sets into operating modes, and the mode decides what the transfer equipment must do.
In short term parallel operation, mode OM3, the set may run in parallel for at most five minutes in any month and no more than once a week. Beyond that it counts as long term parallel, unless the operator agrees otherwise. OM3 needs a parallel time interlock separate from the changeover control and under and over voltage and frequency protection, but no loss of mains relay.
In a changeover with no parallel at all, mode OM4, the interlocking must be fail safe, the changeover may not rely on removable fuses, and the set needs an independent earth electrode. The operator may inspect the installation and witness its commissioning.
Break classes for safety services, BS 7671
| Class | Changeover within |
|---|---|
| No break | continuous supply |
| Very short break | 0.15 s |
| Short break | 0.5 s |
| Medium break | 15 s |
| Long break | more than 15 s |
BS 7671 regulation 560.4.1, as set out in IET Guidance Note 7. A set started on mains failure reaches the medium break class and no shorter, so loads that need a shorter break sit behind a UPS.
What gets specified
| Parameter | Typical options | What decides it |
|---|---|---|
| Rated current | 125 A to 4000 A | The load and the busbar it feeds |
| Poles | 3 or 4 | The earthing system of both sources |
| Transition | Open, delayed, closed | Motor loads, UPS input and the operator |
| Sources | Mains and generator, generator and generator, two mains | The supply arrangement |
| Utilisation category | AC-31B, AC-32B, AC-33B | Resistive, mixed or motor loads |
| Short circuit withstand | kA with the upstream protection | The fault level at the switch |
| Controls | Mains sensing, test, exercise, remote signalling | The site's control system |
Applicable standard
| Standard | What it covers |
|---|---|
| IEC 60947-6-1 | Low voltage transfer switching equipment |
The quotation confirms the edition and the utilisation category the switch is tested to.
What we need to quote
- Rated current and voltage
- 3 or 4 poles, and the earthing system of both sources
- Open, delayed or closed transition
- The loads switched, or the utilisation category from the specification
- Fault level at the switch and the upstream protection
- The G99 operating mode agreed with the network operator: no parallel, or short term parallel
- Enclosure, and how the controls report to the site
Building or expanding a facility?
Send the whole equipment schedule as one project, each item with its own requirements.