Dear Raghunathan,

protecting 24V system from short circuits to 415V, mmh...

Let's have an estimation:
Assume you use 5KP24A, which is a 5kW transzorb. Datasheet can be found here:

http://www.vishay.com/docs/88459/88459.pdf

Further assume that you use a super-fast acting 700VAC fuse (200kA max blow current) from 'SIBA'. Datasheets can be found here:

http://www.siba.de/pdfdokume...rungen.pdf

Then you will find, that transzorb needs to shunt short circuit current for about 4msec. 5KP24A is able to withstand about 3kW, when duration of shunting is 4msec. As clamping voltage of this 24V transzorb is about 40V, then 3kW / 40V = 75A is allowed to flow.

Now following problem arises: How to limit short circuit current to 75A? Unfortunately, resistance of fuse is well below 1Ohm. But what would be needed is a current limiting resistance of 415V / 75A = 5.5Ohm, as an order of magnitude. So, you have to insert a very high power resistor of about 5.6Ohm.
Unfortunately 24V power supply current will cause a voltage drop accross this resistor, so that only 24V systems can be protected, which tolerate this voltage drop.

What would be needed finally?
This super fuse, this 5.6Ohm super high power resistor and this 5KP24A transzorb.
But remember that clamping voltage of this transzorb is about 40V, which still might be too high for the 24V system. Then another 24V transzorb is needed and an additional current limiting series component between the two transzorbs.

Kai