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ERP8321 Relay Module


ERP8321 is a redundant AC power switch module, which can be used to make an industrial controller resilient to the failure of a single power circuit.

ERP8321 is not an uninterrupted power supply by itself, but switches to the back-up power when the primary power fails.


ERP8321 needs two independent power sources for their operation. Although it is intended to operate as a redundant AC power switch module for 24VAC, ERP8321 has on-board terminals to connect two independent 230VAC sources, namely “230VAC IN1” & “230VAC IN2”. It has on-board terminals for the connections of two transformers. On-board fuses protect transformer’s primary and secondary connections. The transformers are of 24VAC and the secondary connections are represented by "TX SEC1" and "TX SEC2".

"230VAC IN1" connects to the main power input port and the other power circuit connects to the backup power-input port. If the main power to the ERP8321 fails for any reason, the redundant AC-power switch automatically uses the backup power source. When power is restored, the redundant AC-power switch automatically changes back to the main power source.

ERP8321 has two status LEDs: 'LED CH1' for primary source and 'LED CH2' for secondary source. LED will glow blue if both 230VAC in and 24VAC are healthy. But, if only the 24VAC side fails for any reason, the status LED will glow orange. The switchover is by the means of relays. If all relays are healthy, the LED will glow steadily, otherwise LED will blink with duration of 1 second.

It provides status signals for healthy/faulty conditions, which a DDC controller can use for its status monitoring. Generating an alarm in such events on the operator workstation is such an example. Statuses are available for 230VAC, 24VAC & relay fault. The relay fault will be available as a common fault, which means, if any relay is faulty, "Relay health" status will be "0", otherwise the status will be "1".

Produt Datasheet

  • Redundant power switch for 24VAC.
  • On-board terminals for 230VAC inputs for both main and back-up supply.
  • On-Board fuses for both AC230VAC in and 24VAC with 5mmx20mm dimension.
  • Immediate switchover to the backup supply in case of the mains supply failure.
  • Supports current rating up to 15A.
  • Switch-over as fast as <10ms.
  • On-board LED indications for main and backup supplies.
  • Status signals to the DDC controller for graphics display on the workstation.
  • On-PCB texts for all terminals such as “230VAC IN 1”, “TX SEC1”, “24 VAC OUT”, “L”, “N”, “STATUS”, etc.,
  • Direct panel mountable.
  • Reduced assembly time and easier on-site installation.
  • Smaller panel size: This module is very compact with on-board fuses, relays and terminals, which helps to use a smaller enclosure for the assembly.
  • On-board provision for panel indicator for main and backup power circuits.

Power requirement

  • Power Supply Input: '230VAC source-1' and '230VAC source-2'
  • Switching time: <8ms (Primary-to-secondary on primary fail), <10ms (Secondary-to-primary on primary restore)
  • Status signal type: Multiplexed for industrial controllers (0~5VDC or 0~10VDC).
  • Max Power Consumption(Only for the operation of ERP8321): 6VA


  • Type : Power, 24VAC
  • Rating: 15A @ 24VAC.

Status LED

  • Blue: "Normal" (Both 230VAC & 24VAC are present).
  • Orange: "230VAC present, but 24VAC failure".
  • Steady: Relays are healthy.
  • Blinking: Relay is faulty.

The ERP8321 should be installed by a suitably qualified technician in conjunction with any guidelines for the equipment, which it is to be connected to. Field wiring should be installed to satisfy the requirements set out by the manufacturer of the equipment that the module is being connected to. Use screened cable wherever necessary.

The image below shows the terminal designations and typical wiring diagram of the 'AC redundant power switch module', ERP8321.

Wiring Diagram of ERP8321