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can anyone give the detail of interposing relay... where it is used....

I'm sure there will be a dozen responses by the time mine gets posted, but here goes.

There's absolutely nothing different between a "relay" and an "interposing relay." However, by describing the relay as "interposing," you have provided slightly more information than was available before. Imagine you see a relay and think, "That relay is completely unneccessary. Why did they put it there?" It might well be an interposing relay. The purpose is to electrically seperate the devices. This could be because they have different 0V references, different voltages, AC vs. DC, or you just plain don't want to blow one side up if the other side goes crazy.

-James Ingraham
Sage Automation, Inc.

Excellent response, James.

I would just add a little clarification. Relays used as interposing relays are generally necessary because the circuit(s) being switched have voltages and/or currents which can't be accommodated by the "driving relay".

For example, let's say the relay of a PLC can only accommodate 0.5A at 220 VAC, but the solenoid which is to be connected to the relay requires 1.2A at 220 VAC. In this case, an interposing relay with contacts rated for operation at 1.2A at 220 VAC would be used as an interposing relay "between" the PLC relay and the solenoid. The coil of the interposing relay should require less voltage and current than the driving relay is rated for, and the contacts of the interposing relay must be rated to handle the requirements of the load (solenoid, light, contactor, motor, etc.).

As James says, many people ask why is the "second" relay necessary and I've seen them eliminated by site personnel, sometimes with catastrophic results. At one site, a 250 VDC solenoid coil drawing about 12.0 A was connected directly to a relay that was only rated for 3.2 A at 125 VDC, and an interposing relay which had experienced a couple of failed coils was eliminated. The PLC relay and the printed circuit card it was mounted on as well as the printed circuit cards on either side of the PLC relay card were all destroyed. It turns out that interposing relay manufacturer had produced a bad batch of relay coils causing the higher-than-expected failure rate of the interposing relay.

But, there are also applications which don't require interposing relays, but because they were part of a "standard" or generic design they were included with the equipment and do represent another possible point of failure. So, if you have questions, you need to consider the capability of the driving relay and the load before making any changes to existing circuits.

I hope this wasn't redundant, but just made things a little clearer.

A typical use of an interposing relay is in the output circuit of a PLC that is controlling solenoid valves. The PLC drives the relays, which in turn switch the valves. The PLC output is isolated from the solenoid valve coil by the relay. This may be desirable because the voltage ratings are different, or because the PLC output current rating is insufficient to drive the valves, or to prevent shorts in the field wiring from damaging the PLC output card, if it is not fused.

I would like to add one more use for an interposing relay. If you need to multiply the available potential free contacts, say one for driving a coil of a solenoid and the other for the lamp indication or your application forces you to implement one NO and one NC for the same PLC output, then an Interposing Relay is useful.

This adjustment/compromise is necessary when you are fairly short on Free PLC outputs as it is not economical to insert a new output module in a free slot and do the necessary programming for just one extra output. If you have sufficient outputs then it is a good design practice to wire the load to the PLC output directly, while the potential and load ratings (current) are non-issues.

Best Regards,

RK Sastry