Kontaktorius ar relė: kuo skiriasi ir kada kurį naudoti valdymo spintoje

Contactor or relay: how they differ and when to use each in a control cabinet

Byinteq

Both a relay and a contactor work on the same principle: a control voltage is applied to a coil, the electromagnet pulls in the armature and the contacts switch. That is why they are often confused, and sometimes one is fitted in place of the other. The result is burnt contacts, a stuck relay or an unnecessarily large and expensive cabinet. This article covers the practical differences and simple rules for choosing which to use.

Short answer

  • Relay – for control circuits and signals: boosting PLC outputs, galvanic isolation, logic, small loads (usually up to ~16 A).
  • Contactor – for power circuits: motors, heating elements, lighting groups, three-phase loads and higher currents.

In other words, a relay often controls a contactor coil, and the contactor then switches the motor.

Key differences

Feature Relay Contactor
Purpose Control signals, low loads Power loads, motors
Contacts Usually changeover (CO), 1–4 groups Main poles NO (3 or 4) + auxiliary NO/NC
Contact current Typically up to ~16 A From a few to hundreds of amperes
Arc suppression Minimal Arc chambers, double contact break
Mounting Socket, DIN rail, PCB DIN rail or mounting plate
Standard EN/IEC 61810 EN/IEC 60947-4-1

Utilisation categories: AC-1, AC-3 and why they matter

A contactor's current rating is always stated together with its utilisation category under IEC 60947-4-1. The same contactor has several rated currents:

  • AC-1 – resistive or slightly inductive loads (heating elements, resistors). The current rating is highest.
  • AC-3 – squirrel-cage asynchronous motors: starting and stopping the motor while it runs. Use this rating for motors.
  • AC-7a / AC-7b – household and similar loads in modular contactors (AC-7b – household motors).

For example, Siemens 3RT2015-1BB42 has 7 A AC-3 (3 kW motor at 400 V) but 18 A AC-1. If you choose a contactor for a motor based on the AC-1 figure, the contacts will burn out quickly.

Relay datasheets often list only the nominal current for resistive loads. Switching a motor, transformer or lamps with a high inrush current requires a much larger margin – or, better, a contactor.

When a relay is enough

  • A PLC or controller output needs to switch a circuit at a different voltage (e.g. using a 24 V DC output to switch a 230 V AC signal lamp).
  • Several switched contacts are needed for logic or signal distribution.
  • The load is small: a small fan, a solenoid valve, a contactor coil.
  • Fast replacement is needed – a plug-in relay in a socket is changed in a minute.

Typical examples: Schneider Electric RXM2AB2BD (2 CO, 12 A, 24 V DC coil, with LED) or Finder 40.52.8.024.0000 (2 CO, 8 A).

When a contactor is needed

  • An electric motor (especially three-phase) is being switched.
  • The load is three-phase or the current exceeds what a relay can handle.
  • Auxiliary contacts are needed for status monitoring or interlocks.
  • Together with a thermal overload relay, it forms a motor starter.

For example, Schneider Electric LC1D09BD – TeSys D series 3-pole contactor, 9 A AC-3 (4 kW at 400 V), 24 V DC coil, auxiliary contacts 1 NO + 1 NC.

Modular contactors – the intermediate option

Distribution boards often use modular (installation) contactors – the same format as circuit breakers. They are suitable for lighting groups, heating and water heaters. For example, Finder 22.32.0.024.4340: 17,5 mm pločio, 2 NO, 25 A (AC-1 / AC-7a), 10 A (AC-3 / AC-7b), su rankiniu Auto-On-Off perjungikliu.

Coil voltage: AC or DC

In both cases it is important to match the coil voltage and current type exactly. In PLC cabinets, 24 V DC coils are most commonly used – they can be driven directly from a PLC output if the output current is sufficient (check the coil power in the datasheet). 230 V AC coils are convenient for simple circuits without a controller. Some contactors have universal AC/DC coils.

A DC coil driven by a transistor output must be protected against the voltage spike when switched off – with a diode, varistor or RC snubber (often sold as an accessory).

Common mistakes

  • The motor is switched through a simple relay – the contacts burn out or weld together.
  • The contactor is selected according to the AC-1 current for the motor.
  • Confusing AC and DC coils.
  • Forgetting coil protection against overvoltage.

How to find them on inteq.lt

Full range – in the section Relays and contactors, you can filter by coil voltage, contacts and current. Do you have the code of an old contactor or relay? Enter it in the search by manufacturer code – the system will show that product or equivalents. If the marking is unreadable, upload a 📷 photo or write to us via užklausos formą. More about replacements – in the article How to choose a relay replacement.

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