What is a Test Pulse from a Safety PLC?

It's not complicated, but it took me a while to understand - so no worries.

A test pulse is a signal that is generated at a Programmable Safety Relay (PSR) or Programmable Logic Controller (PLC) with a known clock rate (i.e. 3 pulses/sec) that repeats.

This pulse is unique from other test pulses created by the controller, which are generated at different clock rates. Since many safety applications require dual-channel inputs from safety devices, test pulses are typically used in pairs.

The pulses (T1 and T2 indicated below) may be wired to a mechanical safety device with contacts such as an interlock or E-stop. With the output of the safety device contacts are wired back to the safety PLC inputs.

The clocked pulse output is expected to return to the PLC input at the same rate. This allows for fault detection of the safety device outputs for cross-wired, broken wire, or other issues if the pulse rate changes or goes away altogether.

Detection of faults for safety devices is very important in the safety world, more is better. Good fault detection increases the Diagnostic Coverage (DC) rating of the device which results in a safer product.

Detecting faults early results in failing safe fast - rather than later when hazardous motion could be more dangerous.

Some devices, such as light curtains, have built-in test pulses (aka output signal switching devices or OSSDs) and are self-monitoring. These devices do not require a PLC with test pulse capability and typically have a higher diagnostic capability than contact-based devices.

Why use test pulses? As most machines require a certain safety level requirement Structure Category (aka Cat.) and Performance Level (PL), the Diagnostic Coverage (DC) of a product plays a big role in what level may be reached. A standard interlock without test pulses will not reach a higher safety level, whereas the same interlock utilizing test pulses from a safety controller may be able to reach a higher level.

If you do not know what products to use, please contact an expert to discuss your machine/application (such as the Tech Support Team at Lezue USA 470.508.3600).