4.5 Double Cutting
The following should generally be double cut:
all safety-critical line circuits in their entirety.
all safety-critical on-track circuits in their entirety, except for signal lighting. This includes external loop inputs to electronic devices and their external outputs.
any other circuit fed from a power supply that feeds either of the above.
This is because signalling circuits are susceptible to earth faults, due, for example, to mechanical damage or insulation degradation permitting contact with a relay rack, lever frame, apparatus case body or running rail. As signalling supplies are not generally earthed, two earth faults would be necessary to create a hazardous failure, e.g. by bridging out contacts, although the first fault could go undetected.
By duplicating contacts in both legs of a circuit, four faults would be required to cause such a failure (and these faults would probably short circuit the supply and disable the circuit). This precludes the use of common returns for safety-critical line circuits. Earth returns should not be used for new installations, nor under any circumstances in electrified areas. Where practicable, contacts of the same relay should be used in each leg of the circuit in order to double cut. Where different relays are used in each leg, for consistency the first relay to operate and release should be placed in the feed leg. (For polarised circuits see Section 10.5.3.) The requirement to double cut does not apply to:
contacts used solely to impose non-safety-related controls on safetycritical circuits;
back contacts used solely for down proving;
back contacts used solely for cross proving;
contacts used solely for correspondence proving;
contacts used solely to economise power consumption;
contacts on the internal side of an isolating transformer, or transformer rectifier, feeding external circuits (see Section 4.2);
signal lighting circuits (see Section 4.4);
internal circuits on a dedicated power supply (see Section 4.2);
non-safety-related circuits on a dedicated power supply; nor to
systems that use alternative equivalent measures to mitigate the risk of earth faults, such as earth fault disconnection devices
