Interposing Relay

http://in.answers.yahoo.com/question/index?qid=20071012231849AAXfSJm

Check out this thread:

http://cr4.globalspec.com/thread/55292

If the contacts of a particular switching and/or protection device is incapable of making and/or breaking a much larger current as that of another contactor coil or a circuit breaker's closing and/or trip coil, one has to use another relay, whose contacts are strong enough to handle such large making or breaking currents - but whose coil will have a making or breaking current within the capacity of the smaller contact. This second larger relay is called the inter-posing relay.

Let me give an example:

Today we use more of Nuemrical or Electronic Protection Relays. These relays are very compact overall. As such the out put (glass-package) relays within these protection relays are still smaller. And, the contacts of these glass package relays will be very thin, having very less contact areas and lesser contact gaps. As such, their making and breaking capacities are very limited.

Supposing one of the contacts of such a relay is wired to trip a circuit breaker, whose trip coil VA would be in the range of about 200 VA to 300 VA; assuming the worst case of 24V Trip Circuit Voltage, the steady state current of the trip coil would be around 12.5A; the switching in-rush current of such a trip coil would be around 125A to 250A; definitely, the very thin contact of the protective relay will not be able to handle this high "making arc" and would melt and weld.

In such a case, you would use an inter-posing relay, whose contacts would be strong enough to handle the making inrush current of the breaker trip coil. This inter-posing relay's burden would only be around 35VA whose inrush current will be within the capacity of the thin protective relay output contact.