84.Difference between Relay and Contactor

Relay – “A device by means of which contacts in one circuit are operated by a change in conditions in the same circuit or in one or more associated circuits”

Contactor – “A device for repeatedly establishing and interrupting an electric circuit under normal conditions”

What Makes Contactors Different from Relays

The textbook definitions are similar enough it doesn’t really help us. Both perform the same task of switching a circuit! So What really differentiates the two devices?

1. Load Capacity

Relays are generally classified as carrying loads of 10A or less, while a contactor would be used for loads greater than 10A, but this definition, while simple, gives an incomplete picture. It leaves out any physical differences, or standards.

2. Open/Closed Contact Standards

Contactors are almost exclusively designed to operate with normally open (Form A) contacts. Relays on the other hand can and often are both Normally Open and/or Normally Closed depending on the desired function. This means that with a contactor, when it is de-energized there is (typically) no connection. With a relay there very well could be.

3. Auxiliary Contacts

To confuse things a bit, contactors are often fitted with auxiliary contacts which can be NO or NC however these are used to perform additional functions related the control of the contactor.  For example, the contactor may transmit power to the motor, while the auxiliary contact is in the control circuit of the motor starter and commonly used to turn on a pilot light indicating the motor is operating.

4. Safety Features (Spring-Loaded Contacts)

Because contactors are typically carrying high loads, they often contain additional safety features like spring-loaded contacts to help ensure the circuit is broken when de-energized. This is important because in high load situations contacts can weld themselves together. This can create the dangerous situation of a circuit being energized when it is supposed to be off. Spring-loaded contacts help to reduce this chance, as well as ensure all circuits are broken at the same time. Since relays are typically for lower power, spring-loaded contacts are much less common.

5. Safety Features (Arc Suppression)

Another safety feature commonly included in contactors, due to the high loads they typically carry, is arc suppression. Magnetic arc suppression works by extending the path an arc would have to travel. If this distance is extended further than the energy can overcome, the arc is suppressed. Since relays aren’t designed for high loads, arcing is less of a concern and arc suppression is much less common on relays.

6. Safety Features (Overloads)

Lastly, contactors are commonly connected to overloads that will interrupt the circuit if the current exceeds a set threshold for a selected time period, usually 10-30seconds. This is to help protect the equipment downstream of the contactor from damage due to current. Overloads are much less common on relays.