Earthing systems and protective bonding

Earthing and bonding have different jobs

Protective earthing connects exposed-conductive-parts to the intended fault-current path so the selected protective device can disconnect in time. Protective bonding connects relevant metalwork to limit differences in potential between parts a person could touch together. Neither guarantees that every metal part remains at zero volts during a fault.

A protective conductor is not the intended line or neutral load path. It can nevertheless carry current during ordinary operation, for example from equipment filters. Bonding can also carry diverted neutral current. Check the hazards before disturbing a connection, even after the installation’s main switch has been opened.

The earthing arrangements

• TN-S has separate neutral and protective conductors from the source. A traditional public supply may provide its protective path through a cable sheath or armour.
• TN-C-S combines neutral and protective functions in part of the supply, then separates them for the consumer installation. In the common PME arrangement, the distributor earths the combined PEN conductor at multiple points.
• TT uses the installation’s own electrode system, independent of the supply’s neutral earthing. RCD fault protection is usual because the earth-return impedance is often too high for timely overcurrent-device operation. Overcurrent fault protection is possible where the required low impedance is reliably achieved.
• TN-C retains a combined PEN. Consumer circuits use separate neutral and PE conductors; an RCD is not a fault-protection solution within a TN-C section.

The MET, or main earthing terminal, brings the protective system together. Connected earth bars can serve different boards; every CPC need not physically terminate on the same screw or bar. The earthing conductor connects to the appropriate distributor terminal or local electrode arrangement.

Decide what needs main bonding

An extraneous-conductive-part is outside the electrical installation and can introduce a potential, usually earth potential. Assess incoming metal services and structural steel for that property. Metal alone does not establish it, and a plastic service entry does not exclude other connections to earth. Follow any additional requirements of the pipework system.

A resistance assessment may use Rx = (U0 ÷ I) − Rb, with stated assumptions for voltage, body resistance and current. Interpret a reading with the actual paths and conditions that can change. It is not the same test as confirming continuity of an installed bond.

Place the main pipe connection near entry before branches. For an internal gas meter, suitable consumer-side pipework within 600 mm of the outlet, where practicable, is the preferred position. For an external meter, consider the appropriate point near entry into the building. Sound metal conducts both ways from a clamp: the location is not a one-way boundary of electrical protection. Verify continuity as well as location.

Select the conductor for its duty

On a non-PME supply, copper main bonding starts from half the required earthing conductor area, with a 6 mm² minimum and no requirement under this rule to exceed 25 mm². PME main bonding instead follows the supply PEN/neutral sizing rule and distributor requirements; 10 mm² copper is common domestically, not a universal design size.

For separate supplementary bonding, the mechanical minima are 2.5 mm² copper with suitable mechanical protection and 4 mm² without it. Check the other sizing requirements too; the finished design may need more.

A copper earthing conductor buried without protection against either corrosion or mechanical damage has a 25 mm² minimum. Under the simple protective-conductor selection method, copper line conductors up to 16 mm² have a same-size copper protective conductor, subject to the applicable conditions. Calculation is another method: S = √(I²t) ÷ k.

Worked example (illustrative figures). At 800 A for 0.2 s, with k = 115, I²t = 128,000. Its square root is 357.8; dividing by 115 gives about 3.1 mm². Select the next suitable standard size and check the other requirements.

TT protection and sound connections

For RCD fault protection, RA × IΔn ≤ 50 V is a design condition. RA includes the electrode and relevant protective conductor resistance. Actual fault current may exceed the RCD’s rated residual operating current, so touch voltage can exceed 50 V before disconnection. The required disconnection time must also be satisfied. Electrode resistance above 200 Ω may be unstable; a lower value is not automatically a pass for every design.

Do not use a water or gas service as the designed earth electrode. Provide suitable accessible connections for inspection and testing, and the required tool-operated test-disconnection facility at the MET. Select the test method safely; an installation can have several electrodes.

Use the correct clamp on a properly prepared contact surface, verify its connection, and fit the required Safety Electrical Connection – Do Not Remove label. Temporary continuity bonding during pipe work does not replace permanent protection or assessment of electrical current and voltage hazards.

Special arrangements

For a bathroom with ordinary low-voltage circuits, supplementary bonding may be omitted only when the required disconnection conditions, 30 mA RCD additional protection and effective main bonding of all extraneous-conductive-parts are satisfied together. SELV and other special arrangements have their own requirements.

An open PEN can raise bonded metalwork towards line voltage. Assess PME carefully for outdoor equipment and contact with true earth. PME must not earth the metalwork of a caravan or boat; use the permitted arrangement for that location, rather than assuming each needs its own rod.

Functional earthing supports operation or EMC. It may share a conductor with protective earthing if every protective requirement is met; the protective duty takes precedence. Identify the designed purpose before altering any link.

An island supply needs suitable independent earthing, a source reference and fault protection. A distributor earth may remain connected physically, but island operation must not rely on it alone. Any controlled neutral-to-earth link belongs in the designed source arrangement, not arbitrarily in a consumer circuit.