Mitsubishi FX Programmable Logic Controllers, 2nd Edition

//INTEGRAS/ELS/PAGINATION/ELSEVIER UK/PLC/3B2/FINALS-7-6-04/0750656794-CH01 0.3D – 112 – [111–115/5]

19.6.2004 9:10PM

10.2 Safety relay specification

To meet the requirements of a Category 2 fault situation, the safety relay must have the

following specification:

1. A safety check prior to the system being started up will only allow the system

to continue starting up, should no fault be found. This is done as follows:

(a) Ensure the emergenc y stop circuit is made;

(b) A cross-check circuit, i.e. R3, which checks whether R1 or R2 is held stuck in the

closed position;

(c) By using positive guidance contacts, check whether any of the output contacts

have become welded together. The positive guidance contacts ensure that none of

the normally closed contacts ...