Choosing the right contacts

30. June 2026 by Dr. Hendrik Köpf - Reading Time 2 min.

Different applications and loads present different technical requirements for overcurrent protection devices. Selecting suitable switching devices and signal and control contacts is difficult. One special option is the so-called A3 auxiliary contact. We will answer some frequently asked questions about switch types and schematic symbols to help the users decide on their design and documentation.

1. What is the difference between switching, auxiliary and control contacts, and how are they used?

1.1 What is the difference between signal contacts, mirror contacts and digital outputs?

According to the IEC 60934 standard, the main switch operates the auxiliary and control contacts. Figure 1 shows such a configuration. The labelling of the terminals on the individual components is specified according to IEC 60947-1, Annex L. If an input and an output must be labelled, the smaller number always indicates the input. In the case of the main contacts, this is a three-pole circuit breaker with a disconnecting function. The main contacts are labelled with individual numbers. 

Fig.1: Industrial relay contacts and their designations

1.1.1 What are auxiliary and control contacts?

The auxiliary and control contacts are marked with a single-digit number followed by a sequence number. The single-digit number indicates the function. The next digit is the tens digit. The digits .3 and .4 are assigned to normally open contact elements, while the digits .1 and .2 are assigned to normally closed contact elements. The changeover contact elements 41, 42 and 43 are determined based on their individual functions.

1.1.2 What is the purpose of mirror contacts or positively driven contacts?

A mirror contact is a special type of auxiliary or control contact as defined in the IEC 60947-4-1 standard. It is a normally-open auxiliary contact associated with a normally-closed main contact. The two contacts are coupled in such a way that they are never closed at the same time. A mirror contact enables reliable monitoring of the main contact.

Mechanically coupled contacts consist of a normally-open and a normally-closed auxiliary contact, which form a positively driven contact pair. 

1.2 Special feature: auxiliary contact

In the case of the A3 auxiliary contact, this is a special feature of E-T-A circuit breakers for equipment protection. The A3 auxiliary contact can be tapped directly at the load tap/LOAD of the control device. 

The auxiliary contact is usually routed past one or all the circuit breaker’s trip elements. This means, it is either not protected at all or only to a limited extent. Users employ an A3 auxiliary contact to operate high-current signal units or control systems. 

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