Interposing Relay

J

James Ingraham

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.
 
B

Bill Schwarz

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
 
K

Khursheed Iqbal

Very good explain regarding the interposing relay's.

here is mention some application of Interposing relay's

from low voltage drive circuit to high voltage high current drive applications uses interposing relay's, like SCADA based control application drive circuit. Valve & solonoid drive applications, communication Control devices applications use rack base interposing relay's 20, 25, 30, or as per the requirements & designed.

Thanx
Khursheed Iqbal
Hi-Tech Alternate Energy Systems.
www.hitech.com.pk
 
Hi everyone,

You people told about the way one can interface high current (and voltage) applications to plc at <i>output terminal</i>. But how can we connect high current (and voltage) applications to <i>input terminal</i>?
 
The same way: just power the relay coil with the high voltage external circuit and use the contacts to switch the low voltage to the PLC input.
 
D
Interposing relays are commonly used in the Petroleum Industry to seperate Electrical and Instrumentation systems. An interposing relay cabinet is usually installed in the Motor Control Centre for the purposes of starting/stopping/drive status and ESD of electrical drives. This therefore provides a standard interface to separate the above disciplines.
 
V
I would add: Use them strictly where needed, because they reduce loop reliability, use "expensive" instrumentation electrical power (with batteries back-up, etc.), warm up the environment (when you have an air conditioning failure, this can be critical), need spare parts. but definitively provide excellent galvanic insulation,are EMI interference immune, etc.
 
Is there a difference between an interposing relay and say an "ice cube" relay? I'm trying to figure out how to feed the 60kV panels, BUT put a second 24 dc source voltage for 12kV annunciators. Don't ask why, I just got here.
 
S
The phrase "interposing relay" is describing the relay by it's purpose in the system architecture. A relay is needed between the source and consumer of the information represented for some reason. Different voltages, desire to keep them electrically isolated, etc.

An "ice cube relay" is describing a physical type. The small, 1-4 pole relays, typically plug in to a DIN rail socket, and handle low signal to maybe 20A. Typically, with coils no higher than 240V. The term probably arose when the typical control relay had a steel frame, phenolic body, replaceable (and often reversible) contacts, and weighed a pound or two. There's certainly no reason an "ice cube" relay couldn't be used if you need an interposing relay, so long as it's ratings are adequate.

Like I said in another post, it's like asking for a comparison between "dog" and "brown" -- the terms are describing different characteristics, and are not mutually exclusive.
 
B

B.Ratan,Dy GM

>can anyone give the detail of interposing relay

..Interposing relays are used to connect digital signals (eg relay contacts at different voltage levels )of different systems eg.MCC system and CONTROL system. ....
 
Thanks Steve. I understand what you're saying, I assume this is just a question of semantics. The function of the interposing is accomplished by an ice cube relay. Right?
 
S
You have the idea, assuming you're not thinking that the interposing relay MUST be an ice cube relay. It might need to be a contactor or one of those little mini-contactors depending on contact and coil ratings. But yes, within their ratings, there's nothing wrong with using an ice-cube relay as an interposing relay.
 
Interposing Relay is a normal relay function and it will be used in order to acquire required power ratings. and Isolation barrier is used to separate data acquisition system to hazardous area transducers. it is having signal conditioner.
 
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.
Sir can you please explain the "Driving Relay"?
 
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