Introduction
Bonding is one of the most misunderstood parts of electrical work, especially when deciding if something actually needs to be connected to earth.
A lot of people assume every bit of metal should be bonded, but that is not the case. In fact, bonding something unnecessarily can introduce risk into the system.
At EcoGrid Renewables, we test and verify bonding properly using resistance, continuity, and polarity (RCP) testing in line with BS 7671.
Here’s how we determine what needs to be bonded, the reasoning behind the 6.67 kΩ rule, and how to confirm compliance through testing.
1. What is an Extraneous-Conductive-Part?
An extraneous-conductive-part (ECP) is any metalwork that can introduce a potential from outside the electrical installation, such as copper pipework, structural steel, or metal ducting entering a building.
The purpose of bonding is to keep everything at the same potential if a fault occurs, reducing the risk of electric shock.
However, if a metal part cannot introduce an external potential, it does not need to be bonded. Testing allows you to confirm this accurately.
2. Why Over-Bonding Can Be a Problem
Bonding parts that do not need it can introduce a 0 V path between circuits that were never meant to be connected.
This can lead to circulating currents, parallel earth paths, or cause unwanted operation of RCDs and RCBOs.
Modern systems such as solar, battery storage, and EV chargers can all share earthing references, so unnecessary bonding can create faults that are difficult to identify.
That is why testing is essential rather than relying on assumption.
3. The Maths Behind the 6.67 kΩ Rule
Whether something needs bonding depends on the resistance between the metal part in question and the main earthing terminal (MET).
If the resistance is low, it means the part could introduce an external potential and must be bonded. If it is high, it cannot introduce a potential and does not need bonding.
This is worked out using Ohm’s Law and the limits set out in BS 7671 and IET Guidance Note 8.
The calculation is:
The formula is: R = (UL / IB) - RB
Where:
R = resistance between the metal part and main earthing terminal (Ω)
UL = 50 V (maximum safe touch voltage)
IB = 0.0075 A (7.5 mA conventional body current)
RB = 1 000 Ω (typical human body resistance)
R = (50 / 0.0075) - 1000 = 6666 Ω
Where:
UL = 50 V (maximum safe touch voltage)
IB= 0.0075 A (7.5 mA current that can cause shock)
RB= 1 000 Ω (average human body resistance)
Rounded, that gives 6.67 kΩ.
If the resistance between a metal part and the MET is greater than 6.67 kΩ, the potential current flow through a person would be too low to be harmful, meaning it does not need bonding.
If it is below 6.67 kΩ, it is classed as an extraneous-conductive-part and must be bonded.
We use the IET Guidance Note 8 method to verify whether metalwork is truly extraneous.
If the measured resistance to the MET exceeds roughly 6.67 kΩ, it is not treated as an ECP and protective bonding is not required.
If it is below that value, bonding is required in accordance with the intent of Regulation 411.3.1.2 of BS 7671.
This method removes guesswork and ensures compliance is based on measurable results, not assumptions.
4. How to Test to Verify Bonding
Step 1 – Identify the Part
Find any metal parts that might be connected to earth through buried pipework, structural steel, or other conductive routes.
Step 2 – Measure Resistance to the MET
Use a low-resistance continuity tester to measure between the part and the main earthing terminal. Make sure to null your test leads before taking readings.
Step 3 – Interpret the Reading
Greater than 6.67 kΩ: The part is not extraneous and does not require bonding.
Less than 6.67 kΩ: The part is extraneous and must be bonded.
Step 4 – Record the Results
Log the readings in your EICR or verification paperwork, including the test method, instrument type, and configuration.
5. Common Misunderstandings
“Everything metal must be bonded.”
Only parts that can introduce a potential from outside need bonding.
“Bonding everything is safer.”
Over-bonding can cause circulating currents, magnetic interference, and other unwanted issues.
“It’s safer to over-bond than under-bond.”
Bonding should only be applied when the test result proves it is required. The 6.67 kΩ rule gives a clear decision point based on data, not opinion.
6. Why Verification Matters
RCP testing provides the proof needed to verify correct bonding.
It ensures protective devices operate correctly, that only necessary parts are bonded, and that the installation meets the intent of BS 7671.
EcoGrid Renewables includes full bonding verification in every inspection we carry out, confirming resistance, continuity, and polarity across all key systems.
Conclusion
Bonding is about maintaining safe potential differences, not connecting every piece of metal in sight.
By applying the 6.67 kΩ rule and verifying with RCP testing, you can prove whether a part needs bonding or not.
This approach prevents unnecessary connections, reduces risk, and ensures compliance with BS 7671.
If you are unsure whether your property’s bonding is compliant, we can carry out a full inspection to verify and document every connection to current standards.