Relay

What is a relay?

A relay is an electrically operated switch in which a small control signal, such as 24 V DC from a PLC, opens or closes a separate circuit carrying a higher current or a different voltage, while keeping the control circuit and the load circuit electrically isolated from each other. In industrial automation the relay is one of the oldest and still most widely used components: as an interface relay between control system and field, as a time-delay relay, as a monitoring relay and, in certified form, as a safety relay. The concept also lives on in power engineering, where digital protection relays are networked devices and part of the OT landscape.

🧠 How does a relay work?

An electromechanical relay consists of four parts: a coil, a movable armature, a return spring and a set of contacts.

1. Energising the coil - current through the coil creates an electromagnet that pulls in the armature.
2. Changing over the contacts - the armature pushes the moving contacts over: normally open contacts close, normally closed contacts open.
3. Dropping out - when the control voltage disappears, the spring pulls the armature back and the contacts return to their rest position.

Because coil and contacts are linked only mechanically, there is galvanic isolation between the control circuit and the load circuit. A 24 V DC output can therefore safely drive a 230 V AC valve, a solenoid or a contactor, and disturbances from the field stay away from the control system.

A relay with two changeover contacts is called DPDT. Industrial interface relays usually have one or two changeover contacts.

🔍 What types of relay are there?

A solid-state relay has no moving parts. An LED inside an optocoupler drives a semiconductor, giving galvanic isolation comparable to that of a coil. For AC loads, so-called zero-cross SSRs only switch on at the zero crossing of the sine wave. That limits interference and inrush currents, but it also means switching on can take up to half a cycle: 10 ms at 50 Hz.

A contactor works on the same principle as a relay but is built for heavy loads such as motors, with arc suppression and usually only make contacts. As a rough rule, devices switching more than about 15 A or several kilowatts are called contactors rather than relays.

🏭 Where are relays used in industry?

• Interface relays between PLC and field - a transistor output of a PLC or Remote IO module often supplies only 0.5 A at 24 V DC. A slim 6.2 mm interface relay lets it switch a load of a few amperes at 230 V AC and acts as a cheap, replaceable weak point: if a contact burns out, you replace the relay, not the I/O card.
• Driving actuators - valves, indicator lamps, heating elements and the coils of contactors in a motor starter or star-delta starter.
• Hard-wired logic - small machines, or backup functions, are sometimes still built with relay logic. Ladder logic in a PLC is literally derived from these relay diagrams.
• Safety circuits - emergency stop, guard monitoring and light curtains are wired through a safety relay or a safety PLC.
• Power distribution and motor protection - protection relays in substations trip a circuit breaker on overcurrent or short circuit. In an MCC, thermal or electronic overload relays protect the motors by dropping out the contactor (see thermal protection).

As a result, relays are found in almost every control cabinet, from a simple pump control to a process plant with hundreds of interface relays on DIN rail.

🔧 How do you choose the right relay?

In practice a relay rarely fails at the coil; it almost always fails at the contacts. So selection is mainly about the load. Typical values for an industrial interface relay:

A practical approach:

1. Define the load - resistive, inductive (coil, valve), capacitive or lamp. The utilisation categories in IEC 60947-5-1 help: AC-15 for AC electromagnetic loads, DC-13 for DC electromagnets.
2. Check the DC breaking capacity - direct current has no zero crossing to extinguish the arc. A contact rated at 6 A at 230 V AC often handles only a fraction of that at 24 V DC with an inductive load.
3. Choose the coil suppression - a flyback diode across a DC coil absorbs the inductive voltage spike that could damage a PLC output, but it slows down drop-out. Where response time matters, use a diode plus Zener diode or a varistor; for AC coils, an RC snubber.
4. Watch small signals - if the contact switches a control system input of a few milliamperes, choose a gold-plated version.
5. Protect the circuit - place a fuse or circuit breaker in the load circuit, matched to the contact rating. Overload makes contacts weld.
6. Choose an SSR for high switching rates - a heater control that switches every second makes more than 80,000 operations a day and therefore wears out a mechanical contact within a few days. An SSR handles this, but has a voltage drop of roughly 1 to 1.5 V and therefore needs cooling.

The relevant standards are IEC 61810-1 (general requirements for electromechanical relays), IEC 61810-2 (reliability, from which B10 values are derived), IEC 61810-3 (force-guided contacts, formerly EN 50205), IEC 62314 (solid-state relays) and the IEC 60947 series for switchgear. For installations in the Netherlands, NEN 1010 and the panel-building standard IEC 61439 also apply.

🛡️ What role do relays play in machine safety?

An ordinary relay is not suitable as the only element in a safety function. A contact can weld shut, and you cannot see that from the outside. Functional safety therefore uses relays with force-guided contacts: the contacts are rigidly linked, so that if a make contact welds, the break contacts stay open with a contact gap of at least 0.5 mm. The break contact thus reports whether the make contact has really opened.

• Redundancy and monitoring - a safety relay uses two such relays in series and checks them at every start.
• Quantification - for ISO 13849-1 you calculate with the B10D value of the relay or contactor: the number of operations after which 10% fail dangerously. If the manufacturer only states B10, B10D may often be taken as twice B10.
• No SSR for isolation - a solid-state relay has leakage current in the off state and often fails short-circuited. Safe isolation, as in Lock-out Tag-out, requires mechanical separation with a visible or guaranteed contact gap.
• Electrical safety - work on relay circuits in the cabinet falls under a work permit and the Dutch standard NEN 3140; control circuits can remain live from an external supply even when the main switch is off.

From 20 January 2027 the Machinery Regulation applies, which in addition to functional safety also sets requirements for protection against tampering with the control system.

🔐 Why do relays matter for OT security?

Relays play two opposite roles in OT security.

Hard-wired relays as the last line of defence. A relay-based safety circuit has no firmware, no IP address and no protocol. An attacker who takes over the PLC cannot remotely disable a hard-wired emergency stop or a relay-based overpressure trip. Following defence in depth, designers therefore prefer to keep the final safety layer hard-wired and independent of the network.

Digital protection relays as a target. In substations, protection relays have been digital for decades; the first commercial digital protection relay appeared in 1984. Modern units are an IED that communicates using IEC 61850 and GOOSE messages. In the Kyiv power outage of 17 December 2016, the Industroyer malware contained a module that could make Siemens SIPROTEC protection relays unresponsive through vulnerability CVE-2015-5374, attacking grid protection itself. Such relays therefore need the same measures as other OT devices: segmentation, patch management and control over who may change settings.

❓ Frequently asked questions

What is the difference between a relay and a contactor?

A relay and a contactor both use a coil that changes over contacts. A contactor is built for heavy loads such as motors, with arc suppression and usually only make contacts, whereas a relay switches smaller currents and control signals. A relay often drives the coil of a contactor.

Why do you use an interface relay after a PLC output?

An interface relay protects the PLC output and allows a load with a different voltage or higher current. It provides galvanic isolation between control system and field, and when it fails you replace a cheap plug-in relay instead of an I/O card.

When do you choose a solid-state relay instead of a mechanical relay?

A solid-state relay (SSR) is the better choice for very frequent switching, such as a heater control or a pulsed valve, because it does not wear. An SSR is less suitable where overload, heat or leakage current is a problem, and it is not intended for safe isolation.

What does a flyback diode across a relay coil do?

A flyback diode absorbs the high inductive voltage that appears when the current through a DC relay coil is interrupted. It protects the transistor output of the PLC, but the relay drops out more slowly, which lengthens the response time of safety functions.

What are force-guided contacts in a relay?

Force-guided contacts are relay contacts that are rigidly linked mechanically, so that make and break contacts are never closed at the same time. This makes a welded contact detectable, which is why they are required in safety relays. The requirements are set out in IEC 61810-3.

Can a relay be hacked?

An electromechanical relay itself cannot be hacked, because it has no software or network connection; the control system driving it can. Digital protection relays in the power grid, however, are networked devices and have already been attacked, including by Industroyer in Ukraine in 2016.

📌 In summary

A relay uses a small, isolated control signal to switch a larger circuit, making it the link between control system, field and safety.

You choose a relay based on the load, the DC breaking capacity, the coil suppression and the number of operations. For safety functions you use relays with force-guided contacts inside a certified safety relay. In the OT landscape the hard-wired relay is a reliable last protective layer, while the digital protection relay, as a networked device, needs to be secured just as carefully as a PLC.