NR TI 033 ISSUE 1 PDF

Introduction

The principle of double cutting of safety critical circuits to avoid the risk of an earth fault bridging out contacts in one leg of a circuit has been a long standing principle. GI/GN7517 details the measures to be taken to prevent such a risk and also lists some situations where double cutting may not be appropriate.

In a recent issue of typical circuits for LED signals, the decision was made to double cut the feeds to signal heads since this provided a safety benefit, however this was applied to both front and back contacts. Subsequent review has concluded that whilst double cutting the feed to, for instance, the red aspect provides a safety benefit with regard to earth faults, it increases the risk of failure preventing the critical indication being displayed.