Protective Devices & Emergency Stops – when should we test them?

Feb 4, 2015

The Testing of Safety Related Systems (including emergency stops and protective devices) for a machine application is a subject that attracts a lot of navel gazing on a regular basis and we hear wonderful answers with even more wonderful justifications for them. For this reason we have written this article giving a proper method for determining the suitable manual test intervals based on current best practice borrowed from applicable EN standards.

Do I need to test interlocks, emergency stops and other protective devices?

In short, yes. This is a requirement under Regulation 5 of PUWER 98. This states that equipment should be checked frequently to ensure that safety-related features are functioning correctly. A fault which affects production is normally apparent within a short time; however, a fault in a safety-critical system could remain undetected unless appropriate safety checks are included in maintenance activities.

Where safety-critical parts could fail and cause the equipment, guards or other protection devices to fail and lead to immediate or hidden potential risks, a formal system of planned preventative or condition-based maintenance is needed.

OK, assuming I believe you…. How often should we be testing our protective devices and emergency stops?

There is no statutory interval to complete checks. Regulation 5 requires that the frequency of maintenance activities should take into account the:

(a) intensity of use – frequency and maximum working limits;

(b) operating environment, for example marine, outdoors;

(c) variety of operations – is the equipment performing the same task all the time or does this change?

(d) risk to health and safety from malfunction or failure.

Where protective devices use automatic monitoring to achieve the necessary diagnostic coverage for the required safety performance (PL / SIL) , a functional test is carried out every time the device changes its state. This is all well if the device is operated frequently; but there may be protective devices where the demand rate will be infrequent.

Frequency in terms of safety related system defines high demand or continuous mode as where the frequency of demands for operation made on a safety-related system is greater than one per year. Continuous is regarded as very high demand. (3.5.16 of IEC 61508-4)

So with this in mind; if the demand made on the protective device (interlock) is infrequent < 1 per year; then the device shall be used with additional measures. This is as a result that between consecutive functional tests the probability of occurrence of an undetected fault is increased.

When a manual functional test is necessary to detect a possible accumulation of faults, it shall be made within the following test intervals

PLr (EN 13849-1) Category (EN 13849-1) SIL (EN 62061) HFT (EN 62061) Minimum Manual test rate Reference e 3 & 4 3 1 monthly EN 14119 (8.2) d 3 2 1 annually EN 14119 (8.2) ≤ c 1 1 0 monthly SES suggested value (1)

(1) Suggested manual test rate basis – a single fault on such safety related systems will lead to the loss of the safety function being performed.

Whilst this standard only deals with interlocking devices; Spiers Engineering Safety use the principles outlined to suggest minimum testing requirements for emergency stop devices (as these devices should be operated infrequently).

Extent of the manual checks on the safety-related system

When completing the manual checks on the safety related system(s) this must include verifying the control response; i.e. when the input device (e stop / interlock) is actuated the appropriate control response is realised – final removal of power is achieved for the dangerous part(s) of the machine.

With this in mind, the person(s) carrying out the manual functional test must

• know what the expected control response(s) are when a safety related input device is actuated
• have sufficient competence to verify that this control response has been achieved

To learn more about PUWER read our Guide: The ultimate guide to PUWER