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Better ways to connect safety switches in series

...and there could well be other safety devices such as safety mats, light curtains and laser area scanners.

Traditionally designers may have created safety circuits in which all of the emergency stop switches were connected in series and wired back to a single safety relay on the control panel. Similarly the guard switches may have been connected in series with a single safety relay - or even connected in series with the emergency stop switches.

While such arrangements are likely to meet the basic safety requirements, they have their drawbacks. For example, if one of the guards is opened or an emergency stop switch is pressed, there is no simple way to tell which device has been operated - unless auxiliary contacts have been individually wired back to a PLC or other controller to monitor the inputs. And without any diagnostic data, the machine is likely to take longer to restart.

Wiring auxiliary contacts on a complex machine can add significantly to the cost in terms of the wiring time and the requirement to purchase extra I/O for the PLC. However, Pilz has products available that can help to reduce the wiring and I/O requirement while still delivering diagnostic data and maintaining high levels of safety.

The Pilz PSEN ix1 multiple-sensor interface may be connected to four emergency stop switches or guard switches, and a maximum of 12 PSEN ix1 interfaces can be cascaded to enable up to 48 safety devices to be connected to a single safety relay in safety circuits conforming to the requirements of BS EN 954-1 Category 3. LEDs on the front of the interface unit give a clear indication of the safety device that has been actuated.

Two versions of the PSEN interface are available, the PSEN ix1 for use with Pilz PSEN 1 series switches or other devices having two normally-open contacts, and the PSEN i1 for use with Pilz PSEN 2 series switches or other devices having one normally-closed and one normally-open contact. This gives users a broad choice of input device and also helps end users that wish to upgrade existing safety circuits in order to benefit from improved diagnostics - and reduced downtime.

While the PSEN ix1 interface may be connected to a conventional Pilz X-range safety relay, the similar PSEN i1 interface can be used with a Pilz PNOZelog safety relay, a Pilz PNOZmulti modular safety controller or a Pilz PSS programmable safety system with or without a SafetyBUS p safety fieldbus. If either the PNOZmulti or the PSS is used, the transistor indication outputs from the interface unit can be used to convey diagnostic data to the controller, which results in even faster recovery from a stoppage.

The PSEN interface units certainly have their advantages for BS EN 954-1 Category 2 and Category 3 applications, and Pilz has an alternative for Category 4 applications. The Pilz PSENcode range of coded non-contact safety switches includes a uniquely coded version featuring matched switches and actuators that only work in combination with each other. Virtually impossible to defeat, the switches provide an exceptional level of integrity.

Whereas a Category 4 application would normally require two mechanical switches per guard (to ensure mechanical redundancy), with each connected to a separate channel on a safety relay (for electrical redundancy), a single PSENcode switch can be used on each guard – and because the switches are intelligent and can remember their operating state, ten or more of these switches can be connected in series to a single safety relay. This leads to considerable savings on hardware costs, as well as substantially simplified wiring. Indeed, the switches can be connected to a Pilz PNOZ X-range or PNOZelog safety relay, a PNOZmulti modular safety controller, a PSS system or a SafetyBUS p network.

In addition, the PSENcode switches have a signal output that can be connected to a PLC, process controller or display so that an operator can immediately identify which guard has been opened. Each switch also has high-brightness LED indication for local fault diagnosis.

Whether customers chose to use PSENcode switches on Category 4 applications or multiple-sensor interface units for Category 2 and 3 applications, they will enjoy high plant availability at an economical price.

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Pilz Automation Ltd
Pilz House, Little Colliers Field
Corby, Northants, NN18 8TJ
United Kingdom

Telephone: +44 1536 460766
E-Mail: sales@pilz.co.uk

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