Why do I need an ATS?

Automatic switchover to the backup source without manual action; required for category 1 (PUE 1.2.17).

How fast does an ATS transfer?

Contactor — 0.5–2 s; with generator start — 10–60 s; with UPS — no break.

Can a generator work without an ATS?

Yes, via a manual transfer switch — with full break of both intakes.

ATS and backup power

Automatic Transfer Switch: schemes, transfer times, PUE requirements, generator auto start, earthing.

Cable cross-sections and ampacity Circuit breaker ratings Electrical schematic symbols (GOST/IEC)IP protection ratings RCD types and selection Earthing systems TN-C, TN-S, TN-C-S, TT, IT Wire colour code Breaker selectivity Generator selection: petrol or diesel ATS and backup power UPS: types and selection Solar power plants: types and calculation 🌍 World rules

What is ATS

ATS (Automatic Transfer Switch) automatically switches the load from the mains to a backup source (second line, generator, UPS) when voltage fails. In Russia — АВР.

Schemes

Scheme	Transfer time	Application
Manual transfer switch	1–3 min	Country houses
Contactor ATS	0.5–2 s	Homes
ATS + generator auto start	10–60 s	Homes with generators
ATS + online UPS	0 ms	Servers, boilers, medical

PUE requirements

Category 1 consumers (PUE 1.2.17) need two independent sources with ATS; the special group (operating theatres, fire pumps) — a third source (1.2.18). Fire-protection devices — category 1 with ATS (7.2.36).

Transfer times

Lighting and household: up to 15 s OK;
Gas boilers and pumps: 0.5–5 s;
Servers: 0 ms — UPS only;
Operating theatres (SP 158): 0.5–5 s.

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Data is for reference. Final design decisions must follow current regulations and a qualified engineer.