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Generator School · 10

Automatic Transfer Switches — types and redundancy

The ATS is the device that decides whether your building is fed by the grid or by your generator. It's the single most important piece of switchgear in a standby system — and the type you choose has a big impact on reliability, switching speed, and recovery options when something goes wrong.

01

Contactor-type ATS

Two electromechanical contactors (mains and generator), interlocked so only one can be closed at a time. Fast (50–100 ms transfer), compact, cheap. Suits residential and light commercial up to ~250 A. Disadvantage: contactors are not rated for short-circuit interruption — they need upstream fuses or MCCBs. Not suitable for prime / continuous duty.
02

Motor-operated MCCB / ACB ATS

Two MCCBs (Moulded-Case Circuit Breakers) or ACBs (Air Circuit Breakers) with a motor operator that drives the handle from open to closed. Slower (0.5–2 second transfer) but the MCCBs also provide short-circuit protection and isolation — no upstream protection needed. Standard fitment above 400 A and the only acceptable option for data centres, hospitals, and prime-power sites.
03

MCCB ATS vs ACB ATS

MCCB ATS is rated up to ~1600 A and is the typical fit for commercial buildings (sub-stations, data centres, hotels). ACB ATS is rated above 1600 A and used in heavy industrial / utility applications. ACBs have higher fault-current withstand and are typically draw-out (you can physically wheel the breaker out for service while live).
04

Redundancy — why a lightning strike doesn't have to be a blackout

Modern ATS controllers can fail (lightning, firmware glitch, sensor failure). When they do, the contactors / MCCBs / ACBs themselves are still functional — they just don't have a brain to drive them. A well-designed ATS includes: • Manual operating handle on every contactor / MCCB / ACB — so you can physically transfer while the controller is dead • Independent secondary controller path (an AMF-only fallback, e.g. simple DSE module beside the main DSE) so a controller swap is hot-swappable • Mechanical interlock that prevents both sources being closed at once even if a controller goes crazy
Quick answers

FAQs

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Sometimes, depending on the contactor model. For critical sites we'll typically recommend upgrading the whole switch to a motor-operated MCCB / ACB type rather than half-fitting an old design.

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