What is Auxiliary Contact in a Contactor? Explained Technically!

Contactors have auxiliary contacts and main contacts. When I was an inexperienced engineer I did not know their definitions. Later I understand the difference. Let me explain it to you too.

What is Auxiliary Contact?

Auxiliary contact is the contact in the auxiliary circuit that is operated mechanically. It is physically linked to the main contacts and activated at the same time. It does not carry so much current. Auxiliary contact is also referred to as supplementary contact or control contact.

It is used for auxiliary functions on a contactor, relay, or circuit breaker. Auxiliary contact is available in various versions as normally open (NO), normally closed (NC), or changeover (C/O). It can be available internally on a device or can be mountable. Multiple auxiliary contacts can be mounted to electrical devices.

Below you can see the positions of the main contacts and auxiliary contacts of a 3-pole contactor. In this example, the contactor has 3 NO main contacts, 1 NO auxiliary contact, and 1 NC auxiliary contact.

Auxiliary contacts are mainly used to signal switching or malfunction states, for interlocking circuits or logic and sequence controls with low to high loading.

Many protection or control devices have additional auxiliary contacts. This enables remote signaling of the main contact switching states and control of further functions that allow for remote interrogation and error messages.

Main Contacts in a Contactor

The main contacts are the contacts in the main circuit that conducts the current when closed. They are designed to switch different types of loads (motors, heating systems, lighting, capacitors, etc.) with different rating data (power, current, voltage) over a suitable lifespan. Main contacts carry large currents.

The demands of the main contacts result from the corresponding product standards.

Differences Between Auxiliary and Main Contacts

Auxiliary contacts vs main contacts:

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