What is the difference between earthing and bonding? 

Getting earthing and bonding right saves lives. Both are core parts of UK electrical safety under BS 7671, but they do different jobs. This plain-English guide from Elec Training explains what each term means, how they w ork together, and what good practice looks like on real sites. If you want to deepen your competence, the topics below map directly into the NVQ Level 3 electrical installation units and are verified during electrical inspection and testing. 

Earthing, the fast path for fault current 

Purpose: Earthing gives fault current a low-resistance path to earth so protective devices disconnect quickly. If a live conductor touches an exposed conductive part, that fault current must be high enough to trip the MCB, blow the fuse, or operate the RCD within the time limits of BS 7671. That is how we keep touch voltages short lived and safe. 

Key parts of an earthing system: 

• Earthing arrangement: TN-S, TN-C-S (PME), or TT, each with different source and earth characteristics. 

• Main earthing conductor: Connects the installation’s main earthing terminal to the supply’s earthing point or an earth electrode. 

• Earth electrode, TT only: Rod, plate, or other electrode providing the connection to general mass of earth. 

• Circuit protective conductors, CPCs: The E in twin and earth, or a separate protective conductor in SWA or conduit systems. 

What it achieves: A fault from line to an exposed part drives current through the CPC and e arthing system, volt-drop appears across the loop impedance, and the protective device disconnects in time. Without effective earthing, disconnection can be slow or may not happen at all. 

Bonding, keeping metalwork at the same potential 

Purpose: Bonding reduces dangerous potential differences between accessible metalwork during a fault. It connects extraneous conductive parts, like incoming water and gas services, to the main earthing terminal so everything rises and falls together in potential, cutting shock risk. 

Types of bonding: 

• Main protective bonding: Typically 10 mm² or 16 mm² copper in domestic settings, linking the main earthing terminal to incoming gas, water, and other extraneous services at their point of entry. 

• Supplementary bonding: Local bonding within special locations, for example bathrooms, where required by the Regs when disconnection times or RCD protection alone are not enough. 

• Equipotential bonding: The result, not a separate conductor, a zone where accessible metal parts are at or near the same potential. 

What it achieves: If a fault raises an appliance casing above earth potential, bonded pipework nearby rises to a similar p otential, reducing touch voltage across a person who could touch both at once. 

Quick comparison 

• Function: 
  Earthing carries fault current away to enable automatic disconnection. Bonding equalises potentials between metal parts to limit shock voltage. 

• Current path: 
  Earthing uses CPCs, main earthing conductor, and sometimes electrodes. Bonding ties extraneous parts to the main earthing terminal, not to carry load current, but to keep potentials aligned. 

• When it acts: 
  Earthing acts every time a fault occurs. Bonding sits quietly, then limits voltage differences during those same faults. 

Typical site checks that prove competence 

Whether you are writing up an EIC or an EICR, you will confirm: 

• Presence and size of the main earthing conductor and its terminations. 

• Presence, size, and labels for main protective bonding to water and gas, secured near the point of entry and before any branches. 

• Continuity of CPCs and bonding conductors, measured with a low-ohms test. 

• Earth fault loop impedance meets disconnection times for the protective device used, or that RCDs trip within limits. 

• TT systems have acceptable RA values, often relying on RCDs for ADS. 

• Any requirement for supplementary bonding in special locations is either met or removed by achieving the conditions that permit its omission. 

These are precisely the skills assessed in a city & guilds 2391 style inspection and testing environment. 

Common mistakes to avoid 

• Mixing up CPC and bonding: A CPC belongs to a circuit, bonding does not. Never substitute one for the other. 

• Bonding the wrong place: Bond where services enter the building and are still extraneous, not far inside after plastic sections. 

• Loose or painted-over clamps: Poor terminations defeat the point of bonding. Clean, tight, labelled. 

• Ignoring PME restrictions: Be aware of special requirements for outdoor EV points and certain metallic structures on TN-C-S. 

• Assuming continuity through fittings: Always test. Decorative fittings do not guarantee earth continuity. 

How earthing and bonding show up in real jobs 

• Consumer unit upgrades: Verifying earthing and bonding is step one. If bonding is undersized or missing, upgrade it before or with the CU change. 

• Bathroom remodels: Decide if supplementary bonding is required given the protective measures and disconnection times. Document your rationale. 

• EV charge points and PV: Confirm the supply earthing arrangement and apply the additional conditions for PME or TT solutions as specified by manufacturer and BS 7671 sections. 

How to tell if a home likely has correct earthing and bonding 

A quick visual is never enough, but signs include: 

• Clearly labelled main earthing terminal in or near the meter or consumer unit. 

• Correctly sized green-and-yellow bonding conductors clamped to gas and water services at entry, clamps tight on clean metal, labels present. 

• CPCs present at points, earths sleeved, accessories earthed where required. 

• Modern consumer unit with RCDs or RCBOs, schedules and test labels up to date. 

Only proper inspection and testing confirms it. Do not guess, test. 

Build and prove these skills with Elec Training 

If you are working toward fully qualified status, this topic runs through your portfolio and assessments. You can: 

• Gather on-site evidence and complete performance units on a NVQ Level 3 electrical installation pathway. 

• Practise safe isolation, continuity, R1+R2, Zs and RCD tests on a dedicated electrical inspection and testing course. 

• Explore the right electrical training course sequence for your starting point, from first steps to advanced verification. 

• Prefer to study locally, small groups, clear outcomes, see dates for Electrician Courses Kidderminster and speak to our team. 

Earthing and bonding are simple ideas that carry huge safety impact, they really is. Master the difference, test correctly, and document clearly. If you want structured coaching, practice boards, and assessor feedback, Elec Training is ready to help you tighten every connection, from theory to on-site evidence.