What is the difference between earthing and bonding?
Earthing is not bonding, and bonding is not earthing, they are different concepts.
Earthing is provided to connect exposed-conductive-parts to the main earthing terminal, to provide a path to earth for fault current in the event of a fault.
Protective bonding is provided to connect extraneous-conductive-parts to the main earthing terminal, to prevent a potential difference occurring and creating a hazardous touch voltage.
It’s a common misconception that all water pipes require protective bonding to be installed. If the incoming water service pipe is made from plastic and there is not any fortuitous contact with Earth, it is not an extraneous conductive part and does not require bonding.
This IET Wiring Matters article provides guidance on protective bonding and extraneous-conductive-parts.
What is the maximum external earth fault loop impedance?
The external earth fault loop impedance is the path of resistance external to the installation. This value is declared by the Distribution Network Operator (DNO). Typical values are quoted as listed below:
- 0.8 Ω for a TN-S earthing arrangement
- 0.35 Ω for a TN-C-S earthing arrangement
- 21 Ω for a TT earthing arrangement.
It’s important to remember these are typical values only - it will be different when tested on site. The DNO will be able to provide more information.
This IET Wiring Matters article provides further guidance.
Can a gas engineer install protective bonding to gas pipework?
It’s unlikely that, unless specifically trained and experienced to do so, a gas engineer will have insufficient knowledge of electrical installations to carry out the work.
It is a requirement of the Health and Safety at Work Act 1974, and the Electricity at Work Regulations 1989 that all persons carrying out electrical work must be competent to do so.
The term ‘skilled person’ is used throughout BS 7671:2018, they are different terms, but they all mean the same thing, the person carrying out the electrical work is required to have adequate education, training and practical skills.
An assessment of the existing installation will be required in accordance with Regulation 132.16, to determine if the existing installation, including the distributor’s equipment and earthing and bonding arrangements, are adequate for the altered circumstances.
Does the metal furniture in commercial catering kitchens need bonding?
Whilst it is commonly seen, there is no requirement in BS 7671:2018 to install earthing or protective bonding to metallic kitchen furniture. It is not an exposed conductive part and is unlikely to be an extraneous-conductive-part, therefore, it does not require earthing or bonding.
This IET Wiring Matters article on protective bonding habits provides further guidance.
Does a metal oil tank need protective bonding?
The oil tank does not require protective bonding but it may be necessary to bond the service pipe if it’s considered to be an extraneous-conductive-part. For example, if it’s metallic and in contact with Earth, it’s likely to meet the definition of an extraneous-conductive-part. It’s defined in Part 2 of BS 7671:2018 as:
‘a conductive part liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation.’
If required, the conductor shall be sized in accordance with Regulation 544.1.1 and the connection is required to be within 600 mm of the point of entry to the installation as identified in Regulation 544.1.2.
Some fuel storage tanks may require lightning protection in accordance with BS EN 62305 - specialist advice should be sought to determine the requirements.
This IET Wiring Matters article on protective bonding habits provides further guidance.
Do I have to bond a water or gas pipe if the incoming service pipe is plastic?
It’s a common misconception that all water pipes require protective bonding to be installed. Protective bonding is installed to prevent Earth potential from outside the installation being brought inside.
A plastic pipe is not conductive, therefore it cannot introduce Earth potential into the installation. However, it must be verified that there are not any fortuitous connections with Earth throughout the installation.
This IET Wiring Matters article provides guidance on protective bonding and extraneous-conductive-parts.
How do I perform a calculation to check if the armouring of SWA is suitable as a circuit protective conductor (CPC)?
As the armouring of a steel wired armoured cable is manufactured from steel, the copper equivalent cross-sectional area must be calculated, taking the thermal constraints of the material into account.
Table 54.7 of BS 7671:2018 provides guidance on calculating the cross-sectional area of the armouring of a steel wired armoured cable, Table 54.4 provides values of k for the sheath or armour of a cable being used as a protective conductor.
IET Guidance Note 1, Appendix B, provides examples of cable sizes which are suitable to use the armouring as the CPC.