Protection Against Electric Shock: ADS, Disconnection Times and RCDs

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 a 230 V final circuit, the relevant Table 41.1 rating limit is 63 A if it contains socket-outlets, or 32 A if it supplies only fixed equipment:

• TN systems: 0.4 s
• TT systems using RCD fault protection: 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

Additional protection normally uses an RCD rated at no more than 30 mA. Common applications include:

• Socket-outlets rated up to 32 A for general use.
• Low-voltage circuits serving a room containing a bath or shower, subject to the applicable Section 701 provisions.
• AC final circuits supplying luminaires in domestic premises.
• Shallow concealed cables using the prescribed-zone-and-RCD method, rather than another permitted protective wiring arrangement.

The device senses a difference between the currents in its monitored live conductors. A fault to an earthed case may operate it before anyone touches the equipment; a person can also become part of the residual-current path. A balanced line-to-neutral contact may produce no detectable imbalance. Prompt disconnection reduces risk, but neither the 30 mA marking nor a working test button makes contact safe.

For a general non-delay device, the normal AC test at its rated residual current has a 300 ms maximum. This instrument criterion is distinct from the circuit’s ADS time: an actual earth fault can produce a much larger residual current and a faster response. Check both requirements in their proper context.

SELV as a Protective Measure

SELV combines a suitable safety source, separation from higher-voltage circuits and earth, and an extra-low voltage limit. The complete protective measure can provide basic and fault protection. Voltage alone does not establish SELV or guarantee safety in every location: bathrooms and other special locations impose lower limits and requirements for insulation or barriers. For example, equipment in bathroom zone 0 uses SELV no higher than 12 V AC or 30 V ripple-free DC under the applicable conditions.

More on the protective measures

• Every protective measure provides basic protection and an independent means of fault protection, or an enhanced provision, such as reinforced insulation, that delivers both. Fault protection guards against shock from metalwork made live by a fault, such as a casing whose insulation has failed.
• Barriers and enclosures giving basic protection need at least IP2X or IPXXB in the general case, preventing access by a standard test finger. Cracks, missing blanks or loose covers can compromise that protection and require assessment; not every outer crack exposes the same internal parts.
• On a TT system, RCDs are central to fault protection because the electrode's resistance is often too high for an overcurrent device to disconnect in time.
• At 230 V, learn the Table 41.1 circuit limits as 63 A with socket-outlets and 32 A for fixed equipment only. The TN value is 0.4 s; for the usual TT arrangement with RCD fault protection it is 0.2 s. The 63 A limit is the circuit rating, not the rating marked on an individual socket.
• Socket-outlets up to 32 A normally require 30 mA additional protection. Any permitted exception must meet its own conditions; it is not enough merely to label a socket for a particular appliance. Use the applicable RCD type and test settings when verifying the protection.
• Obstacles and placing out of reach are allowed only where access is restricted to skilled or instructed persons.
• For electrical separation supplying one item, keep the separated circuit free from intentional earth connections and separate from other circuits. The source must provide at least simple separation; it need not always be a safety isolating transformer. Insulation or barriers still keep live parts inaccessible, and touching both separated live conductors can still cause a shock.
• Extra-low voltage, such as power over a data cable, lowers the shock risk, but heat and fire risks remain.
• A small generating set may not deliver enough fault current to operate protective devices in time, so fault protection is rechecked for that source.