How can i use my power supply ( 24VDC 5A ) in redundant mode ? I need to design it
It's not the right way:
The problem, with the design above is that any supply capacitors in the device being powered must discharge before the relays will activate the secondary power supply.
It may be sufficient to arrange for the relay to de-activate at 23V using a comparitor circuit or by monitoring the feedback channel of the primary power supply.
Also it can't start without the secondary power supply, when the relays switch on the primary supply suddenly has to start the load, typically this is a higher than normal current resulting in a voltage drop and the relays turn off again.
To do this you'll probably need to detect the current out of the main powersupply in addtion to its voltage, as during power on it will likely be under-voltage but supplying a high current.