Every automatic transfer switch does the same job — move the load from mains to generator and back — but the switching device inside decides how big, how robust and how expensive the panel is. For the basics of how an ATS works, see ATS panels explained. This guide compares the three main types.
1. Contactor-based ATS
Two electrically and mechanically interlocked contactors, one for mains and one for the generator.
- Best for: small and medium loads — villas, shops, telecom sites, small buildings
- Pros: fast switching, simple, economical, easy to maintain
- Watch out for: contactor rating must cover the full load current with margin; not the choice for very high fault levels
2. Motorised changeover switch (load-break switch)
A single motor-operated changeover switch with three positions: mains – off – generator. The mechanism itself makes it impossible to close both sides together.
- Best for: medium to large buildings, commercial and industrial sites
- Pros: inherent mechanical interlock, compact, wide range of ratings, can often be operated manually in an emergency
- Watch out for: the switch must be rated for the prospective fault current at the board
3. ACB-based ATS
Two air circuit breakers with motor operators and mechanical interlock — the heavy-duty option.
- Best for: large loads and main distribution boards, typically from around 800 A up to several thousand amps
- Pros: switching and protection in one device, high fault ratings, withdrawable for maintenance, adjustable protection settings
- Watch out for: larger panel and higher cost; justified at high currents
3-pole or 4-pole?
- 4-pole switches the neutral as well. It is usually needed where the generator neutral is earthed separately, where there are multiple sources, or to avoid neutral currents flowing through the wrong path
- 3-pole keeps a common, solid neutral — common in simple installations
The correct choice follows the building's earthing system; your consultant will specify it, and we build to that specification.
Open or closed transition?
Standard ATS panels use open transition: the load is briefly disconnected while switching (break-before-make). Closed transition briefly parallels the two sources so the return to mains has no interruption; it needs synchronising control and utility approval, and is used for sensitive loads.
Sizing the ATS
Rate the ATS to the generator's full-load current: at 400 V, amps ≈ kVA × 1.44. A 500 kVA generator needs about 720 A, so an 800 A ATS. Each generator size page shows the matching ATS rating.
What we build
We supply ATS panels from 63 A to 4000 A — contactor, motorised and ACB types, 3- or 4-pole — built and tested in Sharjah with AMF controllers and full documentation. Send us your generator size and single-line diagram on WhatsApp for a quote.
