Case 4 – Secure Disable used in EN 954-1
Cat 4 applications
Secure Disable alone meets the requirements of category 3. It can form part of a category 4 application.The additional
requirements are:
- A further channel for inhibiting motor operation in the
event of an accumulation of faults, for example a contactor with contacts having connected movement and a test arrangement
- A method for testing that Secure Disable is intact.This can be done by testing that no voltage is present at the SD
input, since the only credible failure modes result in voltage appearing at that point.The test can be done by a conventional relay, but that relay must itself be tested
For an electromechanical system, the arrangement is the same
as for case 3.The difference from category 3 application is the
degree of checking in the control circuit which provides the
two inhibit channels.
Case 4 shows a possible arrangement. “Feedback 1” is used in
the same way as for the electromechanical systems, allowing a
test of the safety function whenever the control relay (or other
control circuit) is reset. “Feedback 2” is used to test the
monitoring relay, for example it may be included to latch in a
start circuit so that if the relay does not operate the circuit fails
to latch. In an electronic control system, this feedback could be
provided by a logic input on the controller.
The benefits of Secure Disable are:
- Power contactor with connected movement replaced by relay or controller logic input (cost and space saving)
- No need for drive early-disable arrangement
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