Do PLCs use relays?

Relays are used to isolate one voltage level from another. A PLC may be used to control the operation of a medium-voltage motor, perhaps 2,300V or 4,160V. A relay is used to energize the starter, which, in turn, switches the motor voltage while the PLC controls the relay.

What is the difference between a PLC and a safety PLC?

The main differences between a regular PLC and a safety PLC are internal redundancy, self-monitoring and certification. A safety PLC can be used for both standard control and safety-related functions, but a standard PLC cannot be used for safety-related functions.

What makes a safety PLC?

For a PLC to be considered a safety PLC, it must meet a set of rigorous international standards, specifically IEC 61508 – Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems. This standard covers design, design methods, and the testing of hardware and software.

Why use a relay in conjunction with PLC outputs?

Relays can unburden safety PLCs of alarm and response tasks, which is helpful where programming even modest design changes is a hassle. In fact, smart relays can be more suitable than certain controls on otherwise simple designs needing safety functions — for example, where safety PLCs are prohibitively expensive.

What is a safety controller?

Safety controllers are capable of starting and stopping equipment based on signals from safety input devices. They also support additional safety inputs/outputs and can be used for safety control of equipment. Safety controllers are designed based on functional safety standards.

How do industrial relays work?

Relays control one electrical circuit by opening and closing contacts in another circuit. As relay diagrams show, when a relay contact is normally open (NO), there is an open contact when the relay is not energized. When a relay contact is Normally Closed (NC), there is a closed contact when the relay is not energized.

What is the difference between a PLC and a multi-function safety relay?

A multi-function safety relay with 24 inputs, 24 outputs, and multiple configuration options can often be used instead of a safety-rated PLC, providing a comparable solution in terms of performance, along with many other advantages. Multi-function relays can be simpler to use than a safety-rated PLC.

Can safety relays be used as the primary logic device?

Simple hard-wired systems, with very few safety instrumented functions, can be implemented with safety relays as the primary logic device. Using safety relays will limit the input signals to digital (on/off) input signals, meaning SIS sensors can only be discrete switches unless some form of analog to digital converter is used.

How does a safety PLC work with a ladder circuit?

If it is a safety PLC and you are doing the logic of the safety circuit within the ladder instead of with safety relays, then you are sending dual OSSD signals to a safety-rated input, and handling the function blocks and ladder within the code.

What is a safety relay and how does it work?

A safety relay is in other words a well-tried components, and that’s why you should use to build your safety system. On the other hand, a PLC for example is not a well-tried component. So when you are building an automation system or a machine you cannot use a PLC for the safety related parts of the machine.