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.
01Contactor-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.
02Motor-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.
03MCCB 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).
04Redundancy — 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
Frequently asked questions
Can I retrofit a manual operator to an existing contactor ATS?
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.
What's the minimum redundancy for a hospital?
Dual independent ATSs, each fed from a separate generator and a separate utility feed, with static-transfer-switch (STS) cross-coupling below them. AS/NZS 3009 spells out the design requirements.