Practical Check: Universal Safety Relay for Diagnostics and Information

Even if no one is standing on the safety mat or the light curtain is supposedly free, the machine sometimes does not start. Cable breakage, forgotten emergency shut-off unlocking, or faults in the wiring are typical causes. But no matter how simple the reason, troubleshooting and diagnosis are often a time-consuming and costly process. For many service technicians this is a recurring and nerve-wracking challenge – especially when the cause of the fault is not clearly indicated.

Safety Relays: Today and Tomorrow

Conventional safety relays from the group of safety switching devices are used wherever only a few safety functions need to be monitored, no bus technology is installed or safety controllers would simply be too expensive – for example in simple processing machines. But despite all the economic advantages – ordinary safety relays also have disadvantages. And this in two respects: On the one hand, a classic relay can only process one type of signal. This means that a separate, suitable relay is needed for each safety door, each light curtain and each magnetic switch. Not exactly ideal in terms of storage and procurement of spare parts. On the other hand, classic relays only have rudimentary possibilities for diagnosis and status evaluation. Without any bus connection, the relay is directly wired to the control system and only communicates the basic status information: machine stopped. For the work of the service technician, this means: If the machine does not start as usual after shut-off, the actual troubleshooting begins. Now it is time to dive into to machine, armed with torch and measuring device. This is a tedious, sometimes dangerous undertaking, which usually costs a lot of time and money.
This is why universal safety relays represent such an enormous progress. With their multi-functionality and diagnostic capability, these devices do not only have a positive impact on material and operating costs, but also have a very tangible effect on the daily work of technical staff.

Preventa XPSU

The universal Preventa XPSU safety relay from Tech Group Schneider Electric provides a practical description of how modern relay solutions work. For example, on a safety door, which is standard equipment in many machines, a safety relay is usually installed between the door switch, the contactor for the motor control and the control unit. Since a general control system cannot be used so easily for safety applications, the safe evaluation takes place in a safety switching device that not only monitors the switch, but also only releases a machine again when the necessary start conditions are fulfilled.

Diagnostic and Status Information

The universal safety relay is connected to the control via a simple, direct line. But instead of the one, binary status signal, the diagnostic output of the Schneider relay generates modulated signals that are transmitted to a standard digital input of the PLC via the hardwired connection, similar to a Morse telegram. The control decodes the signals of the telegram and communicates them to the operator in the form of a diagnostic or status message. If the machine does not restart properly after an interruption in operation, the service technician or plant electrician simply looks at the display on the machine and immediately recognises, for example: the power contactors for the safety function must not be switched on due to a fault. In total, the universal relay can communicate up to 40 different diagnostic messages to the control unit – and thus help to drastically reduce safety-related downtimes.
Every mechanical device is subject to wear and tear – even safety technology products. Therefore, it is important to keep a close eye on the service life of all components. In practice, however, this is often not easy. Users often only notice the end of a device’s service life when a failure has already occurred. And failures are always problematic. They cost time, money and often nerves. Universal safety relays therefore represent a real quantum leap for everyday operation. On the basis of their diagnostic data, the PLC can carry out an evaluation through stored switching frequencies and inform the user at an early stage according to the real switching frequency. In this way, maintenance work can be planned at an early stage and, in the event of failures, there is no need to react hectically in order to reduce downtimes.

Communication with the Control

For successful communication of diagnostic data between the universal safety relay and a control system, it is important that the data can be made available and used easily. In order to obtain a fast and reliable interface here, ready-made function blocks are already available for common programming systems. In this way, you simply integrate the corresponding function module into your control system program and do not have to worry about decoding the clock signals yourself. The status message is then shown on the display in such a way that the user receives direct instructions for action – such as “Machine running” or “Waiting for start button to be pressed”.

Flexible Setting of Function Modes and Start Conditions

Not only door switches, emergency stops and contactors wear out, a relay also has to be replaced at some point. Here, too, universal relays mean a considerable reduction in workload. Preventa XPSU, for example, has two small but decisive rotary knobs on the front that transform the supposedly simple component into a true all-rounder. Behind the six digits arranged around the knob titled “Function” is the possibility to set the relay to up to six different safety functions. By setting the rotary knob to “1”, for example, the relay is ready to monitor an emergency stop. If you set it to “6”, a light curtain is monitored – the right function depending on requirements. The advantages of this flexibility are obvious: if, as is practically always the case, you have installed different safety features on your machine, from now on you will still get by with only one type of safety relay. This results in enormous simplification regarding stock keeping and re-ordering of spare parts.
The second rotary knob on the front of the relay refers to the starting conditions. Whether automatic start, monitored start or optional start-up test – the required start function is available in the device and only needs to be selected. This means that the same safety relay can be used to implement compact machines right up to modular machine concepts without any problems. One advantage is that the connections on the relay do not differ from the required start function. This can therefore be easily adapted to the respective requirements. In terms of flexibility and user-friendliness, this is a real quantum leap.
A single question remains: how does one prevent unauthorised changes in the settings? There is a simple solution in place: the rotary switches are located behind a transparent cover that can be protected with a numbered locking strap. This is included in the scope of delivery of Preventa XPSU and – similar to a cable tie – can only be used once. Thanks to the unique number on the tape, a change can be easily verified.