Saving money by using the right safety relay

Even with something as apparently basic as a safety relay, money can frequently be saved by selecting the right product.

One of life's certainties is that engineers like to pay as little as possible for automation components.

Sometimes this is done by cutting corners - which is to be discouraged - but often money can be saved simply by choosing an alternative product.

Machinery safety systems are essential, and engineers accept that safety-related components are a little more expensive than non-safety components that perform a similar function - look at light curtains, laser area scanners, limit switches or relays, for example.

However, even with something as apparently basic as a safety relay, money can frequently be saved by selecting the right product.

Essentially there are three areas to consider when looking to save money with safety relays: by taking care not to overspecify a safety relay, purchasing costs can be minimised; manufacturers may have several similar relays that perform the same function but are priced differently; and costly unplanned downtime and lost production can be avoided by selecting a relay with a long life expectancy.

Before any safety relay is specified, the machine designer first has to carry out an assessment to establish the level of integrity of the safety related parts of the control system in accordance with BS EN954-1 (Category 1, 2, 3 or 4), or the SIL (safety integrity level) if working to IEC61508.

If one part of the machine is found to require Category 3 or 4 integrity, machine builders will often design the safety system as if the whole machine requires this high level.

However, the regulations permit the safety-related control system associated with each part of a machine to be considered in isolation, so it is acceptable to use an appropriate specification (and, in some cases, cost-saving) safety relay for those sections where the circuit integrity can be lower.

In reality, it is not always easy to take advantage of this situation because an emergency stop circuit, say, will be required to stop the entire machine, regardless of where on the machine an emergency stop button has been pressed.

But for complex machines, there is more likely to be scope for closely-matching the safety relay to the machine's needs.

For example, a robot cell may be Category 3 or 4 and therefore require a suitable high-specification safety relay.

However, if there is an outfeed conveyor that operates only at low speed and low power, an access gate could be monitored by a safety relay that meets the requirements of Category 2.

Although this is only one example, it shows how it is feasible to save costs by specifying each relay on a case-by-case basis, rather than selecting a relay to meet the highest integrity requirements and then using this wherever a safety relay is required.

Pilz Automation Technology led the market when it introduced the PNOZ fail-safe relays, which are now recognised as the "Classic" range.

Many engineers "grew up" with this family of relays and continue to specify these units as they are not aware of the more recently introduced X-range.

Thanks to the use of surface-mount electronics, the X-range relays cost less to manufacture and are therefore marketed at a lower price.

For example, customers who have previously used the Pilz Automation Classic range safety relays of type PNOZ 5, PNOZ 9 or PNOZ-Z can realise cost savings in excess of 30% by switching to the PNOZ X7.

Furthermore, the potential for savings does not end with the purchase order; wiring is also easier and quicker, and the PNOZ X7 is just half the width of the PNOZ 5 and PNOZ 9 relays.

Panel builders are increasingly realising the value of a wireman's time and the true cost of cabinet space, so these savings are now very attractive.

All safety relays, whatever technology is employed inside their casings, are designed so that they will "fail safe".

This phrase is perhaps misleading because, by failing to a safe condition, the relay is correctly performing its final function (unlike other components that fail unpredictably).

A typical fail-safe situation would be where a relay is connected to a guard switch that is being repeatedly operated because of a requirement to gain regular access to part of a machine.

Eventually, after many thousands of cycles, the silver contact tips of the relay will wear away, leading to copper-to-copper contact and subsequent welding.

However, a safety relay will have two sets of contacts that are designed so that one set always opens and closes first, which ensures that this set wears faster.

This arrangement means that the second set will still be in good condition to provide the safety function, and the relay can be identified as having "failed".

In a production environment, such an event will lead to unplanned downtime - and lost production - while the relay is replaced.

It is therefore prudent to use high-reliability safety relays from a reputable supplier in order to minimise the number of such unplanned stoppages.

This is especially so for relays that are frequently activated, though relays for emergency stop circuits are switched only occasionally and therefore tend not to "fail" due to wear.

The main point to remember, however, is that the cost of downtime and lost production will most probably far exceed the original purchase price of the safety relay.

Safety is not generally thought of as an area of a machine's design that is ripe for making cost-savings.

Nevertheless, done properly, it is possible to reduce the purchase cost of the components, save cabinet space and wiring time, and lower the cost-of-ownership.

Everybody likes to save money, so taking a fresh look at even the most critical of components can be a very worthwhile exercise.

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