How automatic disconnection of supply protects against electric shock, the maximum disconnection times, and the roles of the 30 mA RCD and SELV.
Automatic Disconnection of Supply
ADS is the most common protective measure in BS 7671. It works only when three parts are all in place:
- Protective earthing: exposed-conductive-parts are connected to the main earthing terminal by circuit protective conductors (CPCs).
- Protective equipotential bonding: extraneous-conductive-parts such as metal water and gas pipes are bonded so that parts you can touch stay at a similar potential during a fault.
- Automatic disconnection: a protective device such as an MCB, RCBO, fuse or RCD cuts off the supply quickly when an earth fault occurs.
If a line conductor faults to an earthed metal case, the fault current must be large enough to operate the device within the required time.
Maximum Disconnection Times
For final circuits up to 63 A with socket-outlets, or up to 32 A supplying fixed equipment, at 230 V:
- TN systems: 0.4 s
- TT systems: 0.2 s
For distribution circuits and final circuits above those ratings:
- TN systems: 5 s
- TT systems: 1 s
Basic, Fault and Additional Protection
- Basic protection guards against contact with parts that are live in normal use, using insulation, barriers or enclosures.
- Fault protection guards against shock when a fault makes an exposed part live, mainly through ADS.
- Additional protection is a backup for when basic or fault protection fails or is defeated by carelessness.
The 30 mA RCD
A residual current device rated at 30 mA is the standard means of additional protection. It is required for:
- Socket-outlets rated up to 32 A for general use.
- Circuits serving rooms that contain a bath or shower.
- Cables buried in walls at a shallow depth without earthed mechanical protection.
It detects the small leakage current that would flow through a person and disconnects within tens of milliseconds.
SELV as a Protective Measure
SELV is a protective measure that can provide both basic and fault protection on its own. Because the voltage is held at or below 50 V AC and the circuit is separated from earth and from higher-voltage circuits, a dangerous shock is prevented. It suits bathrooms, illuminated signage and similar uses.