What is Auxiliary Contact and how it works with MCB?

A miniature circuit breaker can provide much more than current protection, it can also provide remote tripping, status indicator, under-voltage/overvoltage protection, and other features.

What is Auxiliary contact?

Auxiliary contacts (or switches) are supplementary contacts that are added to a circuit to protect the main contact. This accessory allows you to check the status of a Miniature Circuit Breaker or Supplementary Protector from a remote. Simply explained, it aids in remotely determining whether the breaker is open or closed. This device can be used for a variety of purposes other than remote status indication. 

The Miniature Circuit Breaker will turn off the supply to the motor and protect it from the fault if the power circuit has a fault (short-circuit or overload). However, a closer examination of the control circuit reveals that the connections remain closed, supplying electricity to the contactor coil unnecessarily.

Auxiliary contact
Auxiliary Contact of MCB

What is the function of the auxiliary contact?

When an overload triggers a MCB, the wire to the MCB may burn. If this happens frequently, system may begin to smoke. Auxiliary contact are devices that allow one switch to control another (typically larger) switch.

The auxiliary contact has two sets of low current contacts on either end and a coil with high-power contacts inside. The group of contacts designated as “low voltage” is frequently identified.  

Auxiliary contact, similar to main power contactor coils, which are rated for continuous duty throughout a plant, contain time delay elements that prevent arcing and possible damage if the auxiliary contact opens while the main contactor is still energized.


Auxiliary contact uses:

The main uses of Auxiliary contact are:

  • Auxiliary contact is used to get the feedback of main contact whenever a trip occurs
  • Auxiliary contact keeps your circuit breakers and other equipment protected.
  • Auxiliary contact provides better protection against electrical damages.
  • Auxiliary contact reduces the possibility of electrical failure.
  • Auxiliary contact contributes to circuit breaker durability.

Difference between the main contact and the auxiliary contact:



In an MCB, it is the principal contact mechanism that connects the load to the supply.Control, indicator, alarm, and feedback circuits use auxiliary contacts, also known as helpful contacts
The main contacts are NO (normally open) contacts, which signifies they will only establish contact when the MCB’s magnetic coil is powered.Both NO (Normally Open) and NC (Normally Closed) contacts are accessible in auxiliary contact
Main contact carries high voltage and high currentAuxiliary contact carries low voltage and low current
Sparking occurs due to high current
No sparking occurs in auxiliary contact
Main contacts are main terminal connection and motor connectionsAuxiliary contacts are utilised primarily in control circuits, indication circuits, and feedback circuits.


S 200 Auxiliary Contact in MCB:

  • S 200 auxiliary contact- the toggle position of the MCB (On/Off) determines the switching position of the auxiliary contact.
  • The auxiliary contact is utilized to indicate the condition of the MCB because it is connected to the switching mechanism.


S 800 Auxiliary Contact in MCB:

  • Auxiliary and signal contacts are built within the MCB. The signal contact will indicate that the MCB has tripped in the case of an overload or short circuit.
  • Because the signal contact is tied to the MCB’s trip mechanism, the signal contact will not provide an indication when the MCB is operated manually. The signal contact can also be reset without having to reset the MCB.

Some useful questions:

1.What are different types of contacts available in auxiliary contact?

Different types of contacts available in auxiliary contact are:

  • Normally Open (NO) contact,
  • Normally Closed (NC) contact,
  • Change-Over (CO) contact.

2.What is Change-Over Contact?

MCB the changeover switch is a three-position switch with the middle position being the OFF position, the top position being connected to supply 1, and the bottom position being connected to supply 2. It serves as a switch between two power sources.

Rabert T

As an electrical engineer with 5 years of experience, I focus on transformer and circuit breaker reliability in 110/33-11kV and 33/11kV substations. I am a professional electrical engineer with experience in transformer service and maintenance. I understand electrical principles and have expertise troubleshooting, repairing, and maintaining transformers, circuit breakers, and testing them. Tweet me @Rabert_infohe

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