Shenler Interface Relays: Protecting PLC Outputs in Control Panels

Wire a PLC transistor output straight to a contactor coil or a solenoid valve, and two things go wrong at once: the output has no galvanic isolation from the field wiring, and the coil's inrush and inductive turn-off spike exceed the ~0.5 A that output can switch. The transistor fails - often taking the rest of the I/O card with it - and the line stops. An interposing relay between the PLC and the load prevents all of it.

Shenler's RNC and RFT interface relays are the slim DIN-rail modules that sit in that gap: they isolate the PLC output, amplify its low-current signal to switch a real load, and show status with an LED [1]. This guide covers how a Pakistani panel builder specifies them - coil, contact, isolation, and coil suppression.

Why a 0.5 A PLC Output Needs an Interposing Relay

A PLC digital output - a transistor point or an onboard relay - is rated for a modest current, typically 0.5 A per point, and its common rail is referenced to the PLC's own supply. IEC 61131-2 [3] defines those output ratings and the isolation classes for programmable-controller I/O. Drive a contactor coil, a solenoid, or a motor starter directly and you ask the output to do three things it should not: carry the coil inrush, absorb the inductive turn-off spike, and bridge the field wiring to the control electronics.

An interposing relay breaks that chain. The PLC output drives only the relay's low-power coil; the relay contact - galvanically isolated per IEC 61810-1 - switches the real load. One cheap, replaceable relay stands between a failed field device and a failed I/O card.

What an Interface Relay Is (IEC 61810-1)

An interface relay is a slim electromechanical relay plus a socket, built as one pluggable DIN-rail module. Shenler's RFT and the ultra-slim RNC carry a coil matched to the PLC I/O voltage (24 VDC is standard), an LED for coil state, and a screw or spring contact to the load. When the relay wears out you unplug it and keep the wired socket - no rewiring [1].

The relay is an electromechanical elementary relay to IEC 61810-1 [2], so its contact rating, endurance, and dielectric strength are declared against that standard, while its role as a control-circuit device falls under IEC 60947-5-1 [4].

Coil and Contact Selection

Two ratings define an IEC 61810-1 interface relay:

• Coil - match it to the PLC output voltage, almost always 24 VDC; a non-polarity coil with integrated LED (as on the RFT and RNC) simplifies wiring and diagnostics.
• Contact - size it for the load, not the PLC: the RNC handles up to 6 A, the RFT 8–12 A. Derate for inductive AC and for DC, where arc-breaking is harder.

Aspect	RNC module	RFT interface relay
Width	ultra-slim 6.3 mm	slim
Contact rating	up to 6 A	12 A (1-pole) / 8 A (2-pole)
Coil	matched to PLC I/O (24 VDC typical)	matched to PLC I/O
Indication	LED	LED + lockable test button
Best for	high-density PLC I/O	higher-current interposing

Slim 6.3 mm Modules and Panel Density

A control panel can carry dozens of I/O points, and every interposing relay competes for DIN-rail space. The RNC's 6.3 mm width nearly doubles the relays per rail versus a standard module, and a bus jumper links the common coil terminals so you land one supply wire instead of one per relay [1]. A push handle and a recognizable LED make replacement and fault-finding fast on a live panel - the detail that matters when a line is down and the crew is counting minutes.

Coil Suppression for Inductive Loads

The load side is not the only place inductive kickback bites. When the PLC de-energises a 24 V relay coil, the coil generates a reverse spike well above 24 V that stresses the PLC output. Shenler's plug-in AMD/BMD coil modules suppress it at the source:

Coil / load type	Suppression module	Effect
DC coil (24 VDC)	Free-wheel diode	Clamps the reverse spike; slight release delay
AC coil	RC snubber	Limits the spike without polarity concern
Surge-prone load	MOV / varistor	Clamps high-voltage transients

Common Specification Mistakes

The recurring errors push the PLC output past its 0.5 A limit:

• Driving a coil or solenoid straight from the PLC. The output has no isolation and cannot carry inrush or absorb kickback - interpose a relay (IEC 61131-2 [3] output limits).
• Sizing the contact to the PLC, not the load. The coil matches the PLC's 24 V; the contact must match the real load current and type.
• Quoting a contact rating without the load category. An amp figure is only valid for the stated AC/DC voltage and load - IEC 61810-1 [2] declares it per category.
• Omitting coil suppression on inductive loads. No diode, RC, or MOV means the coil's turn-off spike hits the PLC output every cycle.
• No isolation between field and control. The interface relay provides the galvanic barrier that a shared-common wiring scheme does not.

What to Specify When Enquiring

A Shenler interface-relay enquiry that quotes cleanly against IEC 61810-1 states:

1. Coil voltage - the PLC output voltage, typically 24 VDC
2. Contact rating and poles - sized to the load current, AC/DC, and load type
3. Module width / density - RNC 6.3 mm for high I/O counts, RFT for higher current
4. Coil suppression - diode, RC, or MOV per the coil and load
5. Accessories - bus jumpers, marker tags, and the socket type
6. Point count - how many I/O points, for jumper and rail planning

Sourcing and Support in Pakistan

Pacific Engineering & Automation is the authorized Shenler reseller in Pakistan. We supply the full interface-relay range - RNC and RFT modules (6 to 12 A), sockets, coil-suppression modules, and bus jumpers - and help match the coil, contact, and suppression to the PLC and the load before the panel is wired. Shenler interface relays sit directly downstream of Yaskawa VIPA PLC outputs (see the Yaskawa VIPA PLC guide) and mount in the same Tempa Pano control panels covered in the LV distribution board guide.

To specify an interposing-relay scheme for your control panel, request a catalogue or quotation or contact our engineering team.

Field-derived case studies will be added to this post as Pacific Engineering & Automation accumulates engagement records. The current version is grounded in published specifications, regulatory documentation, and standards body references.

Sources

1. Shenler / Shenle Co., Ltd. - official product portal
2. IEC 61810-1:2015 - Electromechanical elementary relays, Part 1: General and safety requirements
3. IEC 61131-2:2017 - Programmable controllers, Part 2: Equipment requirements and tests
4. IEC 60947-5-1:2016 - Low-voltage switchgear and controlgear, Part 5-1: Control circuit devices and switching elements