Redundant supply auto switch over

Hello,


We are to build a panel where there is two externat power supplies for redundancy. 400V 20 Ampere. One primare and one seccondary.

If the primary suply is out our system shall automatically switch to the secondary.

Is this as easy as two contactors and one voltage monitor relay or is there something of the shelf? A contactor with 3NO and 3NC would be nice.

could be. contactor needs to be blocked (electrically and preferably mechanically) so not to be engaged both in the same time.
beware that voltage monitor must be before contactors.
because of nature of power supply (always on) i would maybe opt for some 20A motorized switch so that primary power contactor, which is supposed to be 99% on does not have to be actuated all the time.

IMHO it depends on your definition of "redundancy"
1. Is the switching time critical? Would a motor operated circuit breaker or a contactor do?
2. As redundancy means reliability also wouldn't you need a phase monitoring relay?
3. Should the contactor switch to the main supply if it cycles of and on more than a couple of times within a second?
Lot of manufacturers have automatic source changeover blocks that besides the two switching devices have also a control unit.
It usually consists of phase monitoring relay (phase sequence and voltage), couple of timers so as the switching back to the main supply is done when it is stable for a given period (of seconds) and usually button for test and manual switching to the backup supply.

But if you have some electronics where the switching time is critical you probably have to install an online UPS to buffer the switching time. And when having redundancy probably these devices should be a pair, even the UPSes