Electrical protective measures form the backbone of safe electrical installation design. BS 7671 requires that every installation incorporates appropriate measures to protect people against electric shock, thermal effects, overcurrent, and other hazards. Understanding these measures is essential for every electrician, whether you are designing a new installation or assessing an existing one.
Basic Protection
Basic protection prevents direct contact with live parts during normal operation of the installation. The primary methods of basic protection include:
- Insulation of live parts using materials that prevent accidental contact with conductors
- Barriers and enclosures that physically prevent access to live components, meeting minimum IP ratings as specified in BS 7671
- Obstacles which prevent unintentional bodily approach to live parts (limited to skilled or instructed persons)
- Placing out of reach where live parts are positioned beyond arm's reach (also limited to controlled environments)
In domestic and most commercial installations, insulation and enclosures are the standard methods of basic protection. The other methods are only acceptable in specific circumstances with restricted access.
Fault Protection
Fault protection operates when something goes wrong, such as insulation failure or a fault between a live conductor and exposed metalwork. The main methods include:
- Automatic disconnection of supply (ADS) which is the most commonly used method. It relies on protective devices such as MCBs, RCBOs, and RCDs disconnecting the circuit within specified time limits when a fault is detected
- Class II equipment (double insulation) which provides two layers of insulation, eliminating the need for an earth connection
- Electrical separation using an isolating transformer to supply a single item of equipment, preventing a fault current path to earth
- Extra-low voltage (SELV and PELV) using voltages low enough that the risk of dangerous shock is minimised
For the vast majority of installations in the UK, automatic disconnection of supply combined with earthing and bonding is the primary fault protection measure.
Earthing and Bonding
Earthing and bonding work alongside protective devices to ensure fault protection operates correctly:
- Earthing provides a path for fault current to flow back to the source, enabling protective devices to operate
- Main protective bonding connects extraneous conductive parts (such as gas pipes, water pipes, and structural steelwork) to the main earthing terminal
- Supplementary bonding provides additional connections between exposed and extraneous conductive parts in specific locations such as bathrooms
The effectiveness of the entire fault protection system depends on earthing and bonding being correctly installed and maintained. Poor earthing is one of the most common defects found during electrical inspections.
Residual Current Devices (RCDs)
RCDs provide an additional layer of protection by detecting imbalances between line and neutral current, which indicate that current is flowing through an unintended path, potentially through a person. BS 7671 requires RCD protection for:
- All socket outlets rated up to 32A in domestic installations
- Cables installed in walls at a depth of less than 50mm
- Cables installed in walls without earthed metallic covering
- Circuits in bathrooms and other special locations
RCDs are not a substitute for proper basic and fault protection. They are an additional safety measure that provides enhanced protection against electric shock in situations where other measures alone may be insufficient.
Additional Protection Measures
Beyond basic and fault protection, BS 7671 also addresses:
- Protection against overcurrent using fuses and circuit breakers sized to prevent cables from overheating
- Protection against thermal effects ensuring that equipment and wiring do not create fire risks
- Protection against voltage disturbances including overvoltage from lightning and switching events
- Protection against electromagnetic influences relevant to installations with sensitive electronic equipment
Frequently Asked Questions
What is the difference between basic protection and fault protection? Basic protection prevents contact with live parts during normal operation (such as insulation around wires). Fault protection operates when something goes wrong, such as insulation failure, to disconnect the supply before a dangerous level of shock can occur.
Why is supplementary bonding required in bathrooms? Bathrooms present a higher risk of electric shock because wet skin has lower resistance, and the presence of earthed metalwork (taps, pipes) creates potential shock paths. Supplementary bonding ensures that all metalwork is at the same potential, reducing the risk of shock from simultaneous contact with different metal parts.
Can RCDs replace earthing? No. RCDs and earthing serve different but complementary functions. Earthing provides the fault current path needed for protective devices to operate, while RCDs detect residual current that may indicate a person is receiving a shock. Both are needed for a properly protected installation.