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

How to Select the Correct Safe Guard

When should you install a fixed guard as opposed to an interlocked guard?

Is a bolted guard a permanent fixed guard?

When is it acceptable to replace physical guards with light curtains?

These are common questions people have when selecting the appropriate guarding for their machinery. Guidance on selecting the appropriate guard is available in the Work Health and Safety legislation, section 4 of the Code of Practice, "Managing the Risks of Plant in the Workplace" explains this process. This is based on clause 208 of the Work Health and Safety Regulations (Current legislation for all states and territories except for WA and Victoria).

Here's what the code of practice says about selecting your safe guard:

If access to the area of the machine is not needed during operation, maintenance or cleaning then a permanent fixed safe guard is required. What is a permanent fixed safe guard? The code of practice states this guard is welded or incorporated into the body of the machine, thus a bolted guard is not a permanent fixed safe guard.

If access to the area of the machine is require during operation, maintenance or cleaning then an interlocked guard can be used. This guard will have a safety control system that will cease any relevant hazardous energy to the machine when the safe guard is not in a closed position.

If it's not reasonable to use a permanent fixed safe guard or interlocked safe guard then a fixed safe guard can be used. A fixed safe guard can be removed and replaced with the aid of a special tool, such as a coded spanner or Allen key. Thus a bolted guard would be classed as a fixed safe guard.

If none of the above physical guarding options are practicable, then a presence sensing system, such as light curtains or laser scanners can be used. A common example of this would be a conveyor that transports goods into a robotic cell, if a physical guard was used then the goods would be blocked in entering the cell whereas a light curtain will allow the goods to enter the cell in a safe manner.

The most common query people have about the above guidance is: When should I use an Interlocked Guard or a Fixed Guard?

The interlocked guard is the first option when access is required because a safety control system is protecting the operator from the hazards on the machine. When using fixed guarding we are relying on human behavior to ensure three things:

  1. The hazardous energy has been isolated before the safe guard is removed
  2. The hazardous energy will remain isolated while the safe guard is removed
  3. The guard is replaced before the hazardous energy is resupplied to the machine

Thus two considerations should be made when deciding if a fixed guard is appropriate:

  1. Frequency of Access - The more frequently we rely on human behavior, the more likely the process will fail. If access is required multiple times a week or during normal operation it would be recommended to use an interlocked guard
  2. People performing the task - For access through a fixed guard the operator must be trained on the isolation procedure of the machine, this training must be refreshed and documented. If this knowledge can't be relied on then an interlocked guard should be used.

Published: 6 October 2016

How do you future-proof your safety systems?

Looking through machine safety standards there is plenty of guidance for the early phases of machine safety system life cycles, by this I mean you can find good guidance to explain the following activities:
  • Select the required integrity level; CAT/PL/SIL
  • Design the safety system
  • Verify the system design
  • Validate the safety system
But what guidance is available for the operation phase of the safety system? Safety systems can be operational for 10 to 20 years, sometimes even longer! Is it reasonable to expect application parameters won't change the requirements of the safety system over that extended period of time?

Requirements can change dramatically over the life of a safety system for example here are some parameters that could affect the suitability of the current safety system:
  • The uses of the machine 
  • Speed of throughput
  • Frequency/duration of safety demands on the system
  • Stopping times of the equipment
The need to design systems to take consideration of the above changes is becoming more prevalent. Functional safety standards such as AS 62061 mention these factors as prompters for safety system modification, but how can you reliably identify these parameter changes?

Relying on manual monitoring of the safety system parameters causes extra work and is susceptible to human complacency/error.

With the ability to have high levels of data sharing from modern safety systems to standard control systems, it is possible to create this parameter checking as an automated function of the control system. Thus if the use of the machine is changed in a way that effects the safety system's suitability, this will be flagged by the control system and initiate the appropriate modification process.

The most common example of the above concept is Stopping Performance Monitoring (SPM), which is a requirement out of IEC/TS 62046. SPM should be performed when presence sensing systems such as light curtains, safety mats or laser scanners are used as a trip device and the stopping performance of the machine can be subject to deterioration, due to wear of brakes, valves, etc. SPM could be achieved by the machine control system monitoring the stopping performance of the machine and comparing this result to the calculated stopping time used for the safety distance calculation of the presence sensing system. Once the calculated stopping time is exceeded the control system could initiate a safety stop, provide information to the operator of this condition and not allow operation until the system is restored to its acceptable state.

Preventative warnings could be provided by the control system as the stopping performance approaches the calculated stopping time, thus the braking system can be repaired in upcoming scheduled maintenance. Downtime is then avoided and the level of safety is maintained.

Require more information about how modern safety systems with increased integration can assist? 

Craig may be able to assist you with the above mentioned issues, so please reach out via email - cimrie@nhp.com.au.

Craig has been a Safety Specialist with NHP Electrical Engineering Products since 2007. He is also a committee member at Standards Australia and is a TUV Rheinland certified Functional Safety engineer.
Craig Imrie

Published: 6 July 2016