Safety Relays – Keeping an Eye on Machine Safety


Safety Relays, as the name suggests, are, generally, devices that have been designed into a production process of some kind to implement safety functionality should a trigger event or error in the manufacturing processing activity occurs.

When the safety relay is triggered by an activation signal or when it detects a failure within the safety network, it will initiate a safe and reliable response which should help reduce the outcome of any potential risk to persons or the process devices themselves to a negligible level.

With an array of safety relays designed into a system and each one monitoring a specific function, you can achieve a very high level of plant and machine monitoring. Adding these devices into your processes is an efficient way of ensuring systems and personnel are protected adequately and any potential risks are reduced to a minimum.


Basic functions of Safety Relays in Industry

  • Status and monitoring of movable guards that open or close for access to machinery
  • Controlling a powered guard system, preventing access until machinery has halted
  • Emergency stop functionality – two-handed operation functions
  • Stopping a conveyor, motor, press or other devices in a controlled safe manner

Assessing the Risks

It’s no small stretch of the imagination to envisage just how dangerous unguarded machines and processes can be, press tools, conveyor belts, bailers, CNC machines etc. are all powerful machines that could cause serious injuries. Of course, it’s not only the power of a machine that has to be considered, but noise and vibrations, heavy material, fumes and sharp implements are all cause for concern, basically, everything that could cause harm or damage to a person or process machine is a risk.

What this all means is that when you are designing a new installation, processing machine or some form of automated system you will need to find and assess all the possible risks that could occur during its use and then aim at reducing or eliminating them.

Trying to establish all of the issues of concern within your intended installation can hard to fathom, some will be blatantly obvious but others could arise from operators circumnavigating safety protocols or from issues relating to mechanic failure.  Either way, once you have done your very best to evaluate all of the potential areas of concern you then have to design safety mechanisms into the installation, items such as interlocks, light curtains and emergency stop buttons etc.

However hard you may try there is always the possibility of your safety system failing, for example, an operator accesses a guard door and the safety interlock doesn’t work correctly leaving the machinery still functioning. The safety system you so diligently built into the process has failed to eliminate the danger, so you have to analyse all of the potential risks for all the aspects of your safety control systems. Basically, you want the safety system to be safe even when aspects of it are failing or have failed and that’s where a safety relay comes in, monitoring the safety functions of your systems and checking for issues within the network.

Monitoring the Safety Network

You would be hard pressed to find any industrial manufacturing process or application such as a theme park ride that doesn’t feature safety relays within its safety monitoring network. Heavily regulated, there is a stringent set of standards to adhere too, OHSAS 18001 – replaced by ISO 45001 in 2018, that has been designed to help protect those who encounter these installations and machines, wherever they may be located.


Common Failures

  • A break in a wire
  • Faulty and failing contactor
  • Safety interlock failure


The above list makes up the majority of common issues you may encounter in your application, there are certainly others that you may come across of course. One thing, however, that all of these issues have in common is the method by which they are detected. A safety relay issues a series of small timed electrical pulses through the wiring network and determines from the flow of current and pulse timing if the safety devices are failing or have indeed failed, perhaps via a break in a wire or maybe a contactors contacts have welded together.

Safety Relays in Action

Light Curtains – utilised to protect personnel from injuring themselves when in close proximity to potentially dangerous machinery. When the infrared beams of these devices are broken by a passing object or person a stop signal is generated.

Safety Mats – these detect the presence of weight on their active surface, which can be utilised for a number of purposes, such as triggering a stop command when activated or as a supplementary switching device that initiates a light curtains mute function to allow access.

Two Hand Control – these devices insist on the use of both of the operator’s hands to activate a switching device, ensuring they can’t reach into any dangerous areas of machinery during its operation.

Emergency Stop – perhaps the most common control device available in a variety of guises, from the traditional red button within a yellow housing to variants that include a grab-wire allowing you to activate an emergency stop at any point on a process line.

Interlock switches – available in multiple forms, from spring-locking to electrical and air operated, these detect when safety components such as machine guards are in place and can lock to prevent them from opening.

Magnetic switches – durable and resistant to fluid and dirt ingress, with no mechanical interface between mating parts these switches are suitable for a wide variety of applications, relying on being aligned, or not, to activate the required safety response.

The Safety Relay Takes Control

When any of the mechanism above is activated a safety relay will then deal with ensuring that the machinery and processes being monitored react in the manner required by the safety procedures in place. Be that allowing the machinery to operate safely in the knowledge that the operator is where they should be or by bringing the processes to a controlled and failsafe stop before allowing anyone to access to the protection zones.

There are risks in most industries and businesses, however slight, and in today’s modern world it’s a priority for any venture to ensure employees, customers and their environments are kept safe, as well as protecting valuable business assets from avoidable damage.

The costs of machinery downtime due to failures that could have been prevented are self-evident, as are the cost of litigation should an employee or member of the public become injured. It’s a simple mantra – if a risk can be reduced or eliminated, then it should be controlled or removed entirely…common sense really.

Kempston Controls, as one of the leading distributors of industrial control products, stocks an impressive range of all the safety relays and associated interlocks, switches, light curtains and ancillary components from leading suppliers such as Allen Bradley Guardmaster, SICK, Siemens, Schmersal, Omron, Pizzato and many more. Everything you will need to create your own safety relay monitored machine guarding installation, all in one place.

Call us on +44 (0) 1933 411411 or email

Kempston Controls is committed to finding you the ideal solution for your application. Call us today on +44 (0) 1933 411 411, email us at, or alternatively contact us here, we will be happy to help.

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