Fault Protection Explained

Fault protection (historically "protection against indirect contact") ensures that if a fault causes normally safe metalwork to become live, the supply disconnects fast enough to prevent a dangerous shock.

How it is achieved

The most common method is automatic disconnection of supply (ADS), which relies on:

• effective earthing and bonding, so a fault creates a large fault current;
• a low enough earth fault loop impedance; and
• a protective device (fuse, MCB, RCBO or RCD) that operates within the required disconnection time.

Why disconnection times matter

The faster the disconnection, the shorter the time a dangerous voltage can exist on touchable metalwork. BS 7671 specifies maximum disconnection times depending on the circuit and system.

On an EICR

Inspectors verify disconnection can be achieved by measuring earth fault loop impedance and testing RCD operation. Failures here — because impedance is too high or an RCD does not trip in time — are safety-critical and coded accordingly. Fault protection depends directly on earthing arrangements and RCD protection.