Earthing, Bonding and Protection Against Electric Shock
This domain covers how a fault is cleared and how touch voltages are kept low: system earthing arrangements, disconnection times, earth fault loop impedance, main protective bonding and supplementary bonding. Expect a calculation and a question about a bathroom or a service entry.
The two layers of protection
Basic protection stops contact with live parts in normal use: insulation, barriers, enclosures, distance. Fault protection deals with a fault: automatic disconnection of supply, double insulation, SELV or PELV, or electrical separation. Both are required, and additional protection by a 30 mA RCD is required for socket outlets, for mobile equipment outdoors and for buried cables without other protection.
Disconnection and loop impedance
- TN systems: 0.4 s for final circuits not exceeding 32 A at 230 V, 5 s for distribution circuits and larger final circuits.
- TT systems: 0.2 s for final circuits not exceeding 32 A, 1 s for distribution circuits, achieved with an RCD because the loop impedance is too high for overcurrent devices.
- Typical maximum Ze values are 0.8 ohm for TN-S and 0.35 ohm for TN-C-S. For a TT system the electrode resistance must satisfy RA not exceeding 50 divided by the RCD residual operating current, so 1667 ohm for a 30 mA device, though 200 ohm or lower is commonly aimed for to keep the reading stable.
- Zs = Ze + (R1 + R2). Compare it with the tabulated maximum for the device, allowing for conductor operating temperature, commonly by requiring the measured value to be no more than 0.8 of the tabulated figure.
Bonding
Main protective bonding connects metallic service pipes, structural steel and other incoming metallic services to the main earthing terminal, typically with 10 sq mm copper, increased per the tables where the supply neutral is large. Connect on the consumer side, close to the meter and before any branch, with a labelled clamp. Supplementary bonding is used in locations such as bathrooms where disconnection times may not be met for every simultaneously accessible part, and must achieve a resistance between parts not exceeding 50 divided by the operating current of the protecting device.
Worked example
A 32 A type B ring final has a tabulated maximum Zs of 1.37 ohm. Measured Ze is 0.35 ohm and R1 + R2 is 0.90 ohm, giving Zs of 1.25 ohm, which passes, and 0.8 times 1.37 is 1.10 ohm, so the measured value is marginal and the connection resistances should be rechecked before the result is recorded.
Traps
Using a water pipe as an earth electrode; bonding across a plastic section; clamping after a branch; disconnecting bonding to measure Ze and failing to restore and label it; applying the 0.8 rule as a substitute for the tabulated value; using an ordinary loop tester at a point with an inadequate category rating.