Fast Switching Device

Easy. Ditch the relays and use two rectifiers.

Quickest I can come up with

Hi:

If you don't want to make any modifications to your circuitry, just replace K1 for a Make Before Break relay, here is a model number from OMRON, just complete it adding the coil voltage at the end.

G2A-4L32A

Regards

...Or you could simply put a decent sized capacitor across the relay output lines to help keep the 24VDC buss alive for the couple microseconds it takes for the relay to engage the other buss.

1. You can use a 'make-before-break' relay. When A1 fails, it will switch on A2 first, then switch off A1. Before you say 'but...', there will be no short-circuit between the two sources, since A1 is dead. Yes?

When A1 comes back, the relay resets.... now the relay NC contact should close back after the NO makes....doable, you need to look for a suitable relay

2. You can get delayed-drop-off control equipment to be used downstream of the relay. You need to look around.

i am perplexed though. The small relay will have a much smaller operating time than the downstream equipment. Are they really dropping off in the trasition time? Or is it just an academic worry?

I would like to pick a nit with your first premise, neither you nor I know how A1 will fail, it could short to ground, it could open, it could short to the supply, it could take a lightning hit, anything is possible. Therefore Keeping A1 and A2 separate should be mandatory to prevent whatever took out A1 from damaging A2.