Programmable safety systems enforce test schedules

A Pilz Automation Technology product story

Edited by the Engineeringtalk editorial team Jan 2, 2004

Machinery safety control circuits that use programmable safety systems can be viewed as having higher integrity than conventional safety circuits because a PSS can continually monitor the circuit.

In some respects a machinery safety control circuit that uses a programmable safety system (PSS) can be viewed as being of higher integrity than conventional safety circuits because the PSS can continually monitor the circuit; but if the circuit contains gate switches or emergency stop switches that are seldom operated, these could harbour faults that will remain undetected until the switch is operated.

Although the circuit should still fail to a safe condition, it would be preferable to know in advance that there is a fault.

In addition, should an accident occur and the HSE decides to investigate, one of the questions that may be asked is "When was a functional test last carried out on the safety components?".

Assuming that a machine already has a PSS specified, together with a suitable man-machine interface for displaying error messages, it is extremely straightforward to program the system to enforce a regular functional testing regime.

In a typical application, the display will show how many days are remaining until the next functional test is due (so there are no surprises) and then, at the appropriate day and time, the operator will be prompted to test and reset the safety components one by one.

Note that the PSS will not halt the machinery simply because the tests are due; rather the PSS will prevent the machine from restarting after it next stops Tests can be carried out on gate switches, emergency stop switches, light curtains, laser area scanners, safety mats, or any other safety device.

If the tests are not completed or if any component fails to function as expected, the machine will be inhibited from operating.

As an alternative, depending on the application, it may be possible to force the machine to run only at reduced speed or in some other limited mode until the tests have been completed satisfactorily.

All the necessary program instructions are already available to enable such enforced functional testing to be programmed.

Furthermore, Pilz Automation Technology has assembled some specific modules (function blocks) that make implementing a functional test regime even easier.

Each application is different and one aspect that needs careful consideration is the length of the interval between scheduled tests.

In one recent project use was made of historical data for component failures and accidental physical damage (such as often occurs with emergency stop switches); the result was to initiate testing every 21 days to ensure that components are tested at least once per month.

Increasingly companies are expressing an interest in this topic, whether the application is Category 2, 3 or 4 as defined in BS EN954-1.

If a PSS is already specified the implementation is extremely easy, but a safety system with traditional safety relays will need to be upgraded to a PSS because of the requirement for a clock function and the ability to retain in memory the details of the last time the test was conducted - even if the power supply is disconnected.

In arduous environments, such as heavy industries, quarrying and wet areas, accidental physical damage can be a significant problem.

Regular scheduled functional testing - with a means of enforcing the testing regime - can be a simple, cost-effective solution that minimises downtime and provides additional peace of mind.