Case 3 – Fail safe interlock (previously with
feedback) to EN 945-1 Cat 3
Again the failure mode where a relay closes (or stays closed)
when not energised no longer exists, so the feedback contacts
are not required.This also means that in order to achieve
EN 954-1 category 3 it is no longer necessary to provide two
channels for interrupting the power, because no single faults
cause the motor to be driven. As in the previous case, the
enable input wiring has to be protected in order to avoid the
possibility of a short circuit to a positive d.c. supply or digital
logic signal which could cause inadvertent enable.
The benefits of using Secure Disable are:
- Two power contactors with connected movement
eliminated (cost and space saving)
- Feedback checking arrangement eliminated
- No need for drive early-disable arrangements
Again the additional cost of arranging protected wiring for the
drive enable input is small compared with these benefits. |
