SHOWCASE
he long discussed phenomenon of fault masking (unintended resetting) when connecting multiple volt-free contacts in series into a safety monitoring device is now covered in EN ISO 14119 and an associated technical report.
In simple terms, if gate switches are wired in
series, then it is still possible to reset the safety circuit even after a short circuit has occurred on one part of the circuit, as one of the other gate switches can still open both circuits allowing the fault to be cleared from the
Safety Relay / PLC. The effect is so
pronounced that for a system with more than one frequently opened guard the level of Diagnostic Coverage (DC) falls to zero, resulting in a performance level (PL) reduction to a maximum of PLc.
To prevent this reduction in PL there are three solutions:
1. Do not use series wiring (daisy chain wiring), instead use a safety relay for every interlock 2. Use interlocks which incorporate built-in diagnostics or network diagnostics 3. Use a trapped key interlock system that can open multiple doors from just one switch
Options 1 and 2 come with quite a cost
impact, so how can trapped key overcome the issue?
The Trapped Key Option The principle of using trapped keys to open multiple doors is shown in the diagram below.
Fortress Interlocks Ltd Tel: +44 (0)1902 349000 Web:
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Meeting Fault Masking Standards Using Trapped Keys T
linked to a single force break switch with dual, normally
In this example four locks are mechanically closed,
contacts. These contacts
cannot be closed until all keys are returned to the unit, and in reverse it is impossible to remove a single key without breaking the contacts. Once the keys are released they go to the 4 door locks. Only with the key in the door lock can you open the door and, with the door open, the key is trapped in the lock (and so unable to return to the switch unit). Because there is only one set of safety
contacts, there is no risk of fault masking and only one set of safety I/O is required to cover the whole cell. Another benefit of this system is that the switch can be mounted wherever it is most convenient (not necessarily at
the
door), allowing the unit to be mounted at the main control panel, significantly reducing wiring costs.
The Fortress mGard range is the
only Trapped Key product with TUV approval to PLe.
In Conclusion In my opinion, a trapped key interlock system is ideal in meeting fault masking standards set out in EN ISO 14119. It has advantages over using safety relays, or incorporating built-in diagnostics, in that it saves on the number of safety I/O required and results in large savings in wiring costs.
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