What makes a contactor a safety contactor?

A common question is; Do I need to use safety contactors in safety-related control systems?

So, what makes a contactor a safety contactor? These devices are purpose built for safety applications with many design principles built into the product. Like most safety devices, third-party certification provides a good reassurance that the product is appropriate for safety applications. NHP safety contactors are independently certified by Suva Accredited Certification Body.

As required in AS/NZS 4024.1501/1502/1503 the use of basic and well-tried safety principles must be considered for any safety control system for Category 1-4. The design and construction of safety contactors incorporate many of these safety principles. Some of these principles include:

Pictured: 37KW 3P 110V AC COIL 4NC
AUXILIARY Safety Contactor

True auxiliary indication

The auxiliary contacts that provide feedback to the safety system should use proven techniques such as positive guided/mechanically linked or mirror contacts to ensure a true indication of the contactor's state. In AS/NZS 4024.1502 the use of these techniques is defined as a well-tried safety principle and is required for Category 1-4.

No manual operation

Unlike standard contactors that can be easily operated from the front of the device, safety contactors do not allow for manual operation from the front of the contactor. This design feature avoids the possibility of personnel creating an unsafe state due to unexpected start-up. In AS/NZS 4024.1502 the prevention of unexpected start-up is defined as a basic safety principle required for Category B-4.

Securely fixed auxiliary contact block

The auxiliary contacts on safety contactors are permanently or securely fixed to the device, this avoids the possibility of the auxiliary contacts becoming separated from the contactor due to environmental causes (eg. Vibration) and makes intentional tampering more difficult. In AS/NZS 4024.1502 the secure fixing of these contacts is defined as a basic safety principle, required for Category B-4.

Reliability data

When designing safety systems to the standards AS/NZS 4024.1503 or AS 62061, reliability data needs to be obtained for the safety devices. Safety contactors have reliability data in the form of a B10d value.

Easily identifiable

To reduce the chances of unintended misuse of the safety system, safety contactors may be easily identifiable compared to standard contactors, i.e.: The safety contactor may be a different colour. This feature reduces the chances of accidental tampering with the safety system.

Other design considerations when selecting contactors in a safety-related control system include:

  • Consider environmental influences of the application such as temperature, vibration, existence of dust or other contaminants, this is a basic safety principle from AS/NZS 4024.1502
  • Consider over-dimensioning the contactor to reduce dangerous failure modes, this is a well-tried safety principle from AS/NZS 4024.1502
  • Where available use contactor coils with built in surge suppression, this is a basic safety principle out of AS/NZS 4024.1502
  • Ensure all circuits have relevant protection devices

Published: 17 January 2017