Buyer's guide to automatic transfer switches (ATS): PC vs CB class, current rating, poles, switching time and how to specify a dual-power backup supply.
An automatic transfer switch (ATS) is the device that moves a load from its normal power source to a backup - a generator or a second utility feed - the moment the main supply fails, and returns it when mains is restored. For panel builders, facilities managers and OEMs exporting backup-power equipment, specifying the right ATS is what keeps a hospital wing, a pumping station or a data cabinet running through an outage. This guide covers the parameters that change the part number and the field failures nobody wants.
IEC 60947-6-1 splits ATS units into two controller classes. A PC-class (contactor-based) switch uses contactors to make and break the load; it is compact, cheaper and fine for resistive and light-motor loads, but the contactors themselves need a separate short-circuit protector upstream. A CB-class (circuit-breaker-based) switch uses molded-case breakers as the switching elements, so it both switches and protects, with a higher short-circuit withstand. Choose PC class for cost-sensitive, lower-duty changeover and CB class where the switch must also clear a fault or feed a large motor. Get this wrong and you either overspend or leave the busbar unprotected.
Size the ATS for the load's continuous current with headroom for inrush, then pick the pole count to match the supply. Single-phase loads need 2-pole; three-phase four-wire systems feeding neutral need 4-pole so the neutral transfers too. Use the table as a starting point:
| Load type | Typical current | Recommended ATS class | Poles |
|---|---|---|---|
| Lighting / resistive | up to 63 A | PC class | 2P or 4P |
| Small motor (<7.5 kW) | 16-32 A | PC class | 3P or 4P |
| Large motor / compressor | 63-250 A | CB class | 3P or 4P |
| Whole-panel / generator | 100-400 A | CB class | 4P |
Switching time - the delay between mains loss and backup connection - ranges from a few cycles (open-transition) to seamless (closed-transition). An open-transition (break-before-make) ATS briefly de-energises the load; acceptable for lighting and most motors, but a server rack or a process controller wants a closed-transition unit or a UPS buffer. Also decide between utility-to-generator (the classic backup case) and utility-to-utility (two grid feeds). The switching logic, which senses voltage and frequency on both sources, must be built into the controller, not added in the field.
To get a part that drops into your panel, specify these on the RFQ:
Mount the ATS close to the incoming supply, with clear labelling of Source A and Source B, and never parallel the two sources - the whole point is break-before-make isolation. Size the builder's supply cables for the transferred load, not just the local branch. For generator-fed systems, coordinate the ATS controller with the generator start signal so the breaker only closes once the alternator is at rated voltage and frequency. Keep the controller's control-power feed independent of the load it switches.
ELEHPD supplies PC- and CB-class automatic transfer switches from 63 A to 400 A, in 2-pole and 4-pole versions, with generator and dual-utility controllers built to IEC 60947-6-1, ready to integrate with our MCCBs, contactors and busbar systems into a complete backup-power panel.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.