What is Interposing Relay? & How it is used in PLC?

What is Interposing Relay (IPR)?

  • Interposing Relay is the auxiliary relay for isolating the two different systems or two different devices because these devices may have different Zero Voltage references.
  • These two devices may be a Programmable Logic Controller and Contactor.
  • The main purpose of interposing relay is to minimize the load on the controller relay.
  • This relay allows lower control signals to switch on or switch off the load which is higher than operating capability.

Interposing Relay operation is discussed in two different cases as shown below.


Here in this case we are operating a PLC with a contactor with a single-phase AC supply.  The PLC relay is DC type with an output voltage of 24 V. But we need a DC type interposing relay with a coil voltage of 24V DC but the contact AC type with 230 V.

The PLC operates interposing relay in the first stage & Contactor through its Auxiliary contacts.


The PLC relay can only avail 1A at 110 VAC, but the relay connected to the controller needs 3A at 110 VAC.

Here, an interposing relay with contacts rated for operation at 5 A at 110V AC can also be used as an interposing relay between the controller and PLC.

  • Interposing relay coil requires a minimum voltage and lower current than the driving relay.
  • The interposing relay contacts must handle load controller requirements
  • Along with directly performing logic functions, an electromechanical relay can also be used as an interposing system between dissimilar probes, sensors, controllers, and/or control devices.

Let us consider an example of a relay used to interpose between dissimilar systems.

A fine toggle switch is used for controlling high-power lamps for an off-road automobile.

What is Interposing Relay? & How it is used in PLC? 1

In the above-shown circuit diagram, the relay doesn’t execute any logic function but amplifies the dashboard toggle switch signal and sends that signal to the bank of high-power lamps.

A toggle switch of heavy duty is installed inside the dashboard of an automobile to make and break a lamp circuit for safety and reliability purposes.

Working of Interposing Relay in PLC?

  • Interposing relays in PLC systems are used between dissimilar sensors, controllers, and/or control devices for controlling high-power circuits and the status of high-power lines.
  • But dragging high power lines to the control panel is not possible because it is quite dangerous and expensive.
  • Interposing relay system safeguards the PLC panel
  • The Programmable Controller requires an AC supply of 120 Volts to activate but the proximity switch requires a 24V DC supply to operate which is shown below figure A.
What is Interposing Relay? & How it is used in PLC? 2
  • Interposing relay is used between switch voltage and PLC input voltage to compromise between them.
  • When the proximity switch detects the object lying close to it and activates output to energize the relay coil.
  • The circuit is completed for 120 VAC to reach input channel 0 on PLC when the relay contact is magnetically closed.
  • A commutating diode and relay coil are used in parallel to vanish or to destroy energy stored in the coil during De-energization when the proximity switch is turned off.
  • De-energization is the elimination of harmful energy from machinery equipment before switching off. A similar effect also occurs in the output circuit of PLC.
  • A transistor output of PLC has the capability to handle only 24 VDC low current is insufficient for the operation of many devices.
  • The circuit considered as an example shown in below figure B consists of a three-phase contactor. The coil requires an AC source of 120 V at a moderate current level for proper function.
  • The relay connected interposes between the controller’s low-rated output channel and the high-rated demands of the contactor coil.
  • The commutating diode is again used to destroy energy stored when the PLC is De-energized.
What is Interposing Relay? & How it is used in PLC? 3

What are the functions of IPR?

The function of interposing relay is to separate

  • Two dissimilar or non-identical systems.
  • One voltage level from another.
  • Energize the starter to switch on the electric motor.

What is an IPR cabinet?

Interposing Relay (IPR) cabinet is used in industrial automation systems along with automation devices for proper controlling of load circuits such as Electrical feeder contactors for motors & compressors, Air Conditioning systems, Lamps, Fans, etc. It can be used to control light circuits and AC circuits.

Why is the IPR panel used?

Interposing relay panels in industrial control are used to communicate the signals and status between the Automation system such as DCS & PLC and electrical modules in Machine control centre (MCC) such as motor control units, motors, pumps, lamps, and so on.

What are the Applications of Interposing Relay?

Interposing relays in Industrial Automation systems are used to

  1. To switch on or switch off any electrical drive such as such as motor driver, light driver, and contactor driver through PLC or DCS
  2. To turn on or turn off the heater through DCS.
  3.  To separate low rating controllers and high rating loads in small circuits.
  4. Interpose between digital and analog circuits
  5. Interpose between low-rated control equipment and high-ratedcircuit Breaker
  6. To control motors and high-power lamps.
  7. To protect the switchgear system,
  8. To operate high-rated isolators, and actuators using low rated PLC system, Microcontroller, etc.

What are the Advantages of Interposing Relay?

  1. It provides isolation between different circuits to avoid propagation of noise from circuits and maintains safety and protection.
  2. It can also function as a static device for higher operational speed.
  3. Interposing relay governs high rated machine to run from the command issued by rated PLCs
  4. Interposing relay enables communication link between PLC having dissimilar voltage ratings.
  5. Interposing relay system safeguards the Programming Controller panel from high-intensity field current that affects the PLC system internally.

Rabert T

As an electrical engineer with 5 years of experience, I focus on transformer and circuit breaker reliability in 110/33-11kV and 33/11kV substations. I am a professional electrical engineer with experience in transformer service and maintenance. I understand electrical principles and have expertise troubleshooting, repairing, and maintaining transformers, circuit breakers, and testing them. Tweet me @Rabert_infohe

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