Rockwell Automation T80016 Application Note Maintenance Override Programming - Manual

Trusted TM
AN-T80016
Issue 1 June 08 AN-T80016 1
Application Note
Maintenance Override Programming
TÜV Rheinland publishes guidelines on the maintenance facilities of safety related programmable
systems, currently available on the Internet at
http://www.tuv-fs.com/modr_3_e.htm . Whilst this draft
paper is not a compulsory part of safety system operation, it has been revised over many years and
serves as a guide to sensible system design. The paper has been extended to cover maintenance
facilities applied over communications networks.
Maintenance overrides are used to disable inputs or outputs from a safety related system for
maintenance activities, to avoid trip actions caused by spurious signals or logic tests. The preferred
method for applying overrides is by hardwired switches on a panel, either via inputs to the safety
system application or directly affecting the circuit. Whilst this is the most secure method, it is also
inflexible; there may not be room for all necessary switches and modifications to the switch panel are
complex and expensive.
This document describes methods for implementing maintenance override facilities using
programming and communications, within the TÜV guidelines.
Issue Record
Issue
Number Date Revised by Technical
Check Authorised by Modification
1 June 08 Nick Owens Andy Holgate Pete Stock Initial Issue