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Home / Technical Articles / What is the purpose of equipotential bonding?

Purpose and Practice

Equipotential bonding is essentially an electrical connection maintaining various exposed conductive parts and extraneous conductive parts at substantially the same potential.

Purpose of Equipotential bonding
Purpose of Equipotential bonding

An earthed equipotential zone is one within which exposed conductive parts and extraneous conductive parts are maintained at substantially the same potential by bonding, such as that, under fault conditions, the difference in potential between simultaneously accessible exposed and extraneous conductive parts will not cause electric shock.

Bonding is the practice of connecting all accessible metalwork – whether associated with the electrical installation (known as exposed metalwork) or not (extraneous metalwork) – to the system earth.

Lightning equipotential bonding
Lightning equipotential bonding

In a building, there are typically a number of services other than electrical supply that employ metallic connections in their design. These include water piping, gas piping, HVAC ducting, and so on. A building may also contain steel structures in its construction. We have seen earlier in this chapter that when an earth fault takes place in an installation, the external conducting surfaces of the installation and the earth mass in the vicinity may attain higher potential with reference to the source earth.

There is thus a possibility that a dangerous potential may develop between the conducting parts of non-electrical systems including building structures and the external conducting parts of electrical installations as well as the surrounding earth.

This may give rise to undesirable current flow through paths that are not normally designed to carry current (such as joints in building structures) and also cause hazardous situations of indirect shock.

It is therefore necessary that all such parts are bonded to the electrical service earth point of the building to ensure safety of occupants. This is called equipotential bonding.

There are two aspects to equipotential bonding: the main bonding where services enter the building and supplementary bonding within rooms, particularly kitchens and bathrooms.

Main bonding should interconnect the incoming gas, water and electricity service where these are metallic but can be omitted where the services are run in plastic, as is frequently the case nowadays. Internally, bonding should link any items, which are likely to be at earth potential or which may become live in the event of a fault and which are sufficiently large that they can contact a significant part of the body or can be gripped.

Small parts, other than those likely to be gripped, are ignored because the instinctive reaction to a shock is muscular contraction, which will break the circuit.

In each electrical installation, main equipotential bonding conductors (earthing wires) are required to connect to the main earthing terminal for the installation of the following:

  • Metal water service pipes
  • Metal gas installation pipes
  • Other metal service pipes and ducting
  • Metal central heating and air-conditioning systems
  • Exposed metal structural parts of the building
  • Lightning protection systems.

It is important to note that the reference above is always to metal pipes. If the pipes are made of plastic, they need not be main bonded.

If the incoming pipes are made of plastic but the pipes within the electrical installation are made of metal, the main bonding must be carried out, the bonding being applied on the customer side of any meter, main stopcock or insulating insert and of course to the metal pipes of the installation.

Such bonding is also necessary between the earth conductors of electrical systems and those of separately derived computer power supply systems, communication, signal and data systems and lightning protection earthing of a building.
Equipotential bonding terminal for bathroom (OBO)
Equipotential bonding terminal for bathroom (OBO)

Many equipment failures in sensitive computing and communication equipment are attributable to the insistence of the vendors to keep them separated from the electrical service earth. Besides equipment failures, such a practice also poses safety hazards particularly when lightning discharges take place in the vicinity.

In such cases, large potential difference can arise for very short periods between metal parts of different services unless they are properly bonded. Some of the case studies in a later chapter deal with this issue.

If the incoming services are made of plastic and the piping within the building is of plastic, then no main bonding is required. If some of the services are of metal and some are plastic, then those that are of metal must be main bonded.

Supplementary or additional equipotential bonding (earthing) is required in locations of increased shock risk. In domestic premises, the locations identified as having this increased shock risk are rooms containing a bath or shower (bathrooms) and in the areas surrounding swimming pools.

There is no specific requirement to carry out supplementary bonding in domestic kitchens, washrooms and lavatories that do not have a bath or shower. That is not to say that supplementary bonding in a kitchen or washroom is wrong, but it is not necessary.

For plastic pipe installation within a bathroom, the plastic pipes do not require supplementary bonding, and metal fittings attached to these plastic pipes also would not require supplementary bonding. However, electrical equipment still does require to be bonded and if an electric shower or radiant heater is fitted, they will require supplementary bonding as well.

Supplementary bonding is carried out to the earth terminal of equipment within the bathroom with exposed conductive part. A supplementary bond is not run back to the main earth. Metal window frames are not to be supplementary bonded unless they are electrically connected to the metallic structure of the building.

Metal baths supplied by metal pipes do not require supplementary bonding if all the pipes are bonded and there is no other connection of the bath to earth.

All bonding connections must be accessible and labeled:


Resource: Grounding, bonding, shielding and surge protection – G. Vijayaraghavan
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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Nick Lethbridge
    Mar 20, 2022

    I’ve been looking into the earth bonding of a gas meter and it was interesting to read the article.

    Can anyone explain why a built-in gas meter housing box, located 600mm above a block paved driveway, with a plastic (Polyethylene – MDPE) supply pipe and no extraneous conductive part, could result in a dangerous potential difference between a fault and the metal gas pipework within the property?

    There seems to be a lot of confusion regarding it and often the ‘external meter box at ground level with a metal pipe going into the house that could be covered by soil’ is raised (which I can understand).

    However, the UK NHBC technical guidance below states that the gas supply pipe (inside the building, within 600mm of the meter) does not need to be bonded:

    The internal gas metal pipework could possibly touch a metal water pipe, but the water supply pipe (if metal) should presumably already have a PEB.

    If metal objects (e.g. a gas meter) insulated from the ground need to have a PEB, then why not metal tables, chairs, coat stands, etc. that are on carpet, or metal window frames set within brick walls?

    Can anyone think of an example or is it simply an urban myth, similar to how external meters without a PEB in the UK are usually flagged as a defect (when everyone knows that the PEB is likely to be done inside the building within 600mm of the meter)?

    I’m guessing that it’s simply ‘accountability/responsibility avoidance’ rather than any actual risk (compounded by memories of when the gas supply pipes were metal and would need to be earth bonded), but I’m open to being convinced otherwise.

    How could internal copper gas pipework introduce an Earth potential when the gas meter is insulated from the ground?

  2. Selvaraj R
    Jul 14, 2021

    A wonderful article

  3. Anantha Kumar Rajamani
    May 05, 2021

    Nice article, Edvard Csanyi. Would highly appreciate, if you could please put up a techno-practical article on this subject (i.e., Equipotential bonding) w.r.t. HV Instrument Transformers (particulary Ring Type Current Transformers)

    We have a Brand X make of 11kV ring type CT (Ratio: 1200/1/1/1A) with the below GTPs

    Core-1 [1S1-1S2] (Application 87T and 64R):Cl. X; Vk >=450V, Imag <=30mA at Vk/2; Rct<5 ohms
    Core-2 [2S1-2S2] (Application 67, 67N): Cl. 5P20, 15VA, Rct <5 ohms
    Core-3 [3S1-3S2] (Application Metering): Cl. 0.5, 15VA

    It would be nice, if you could please illustrate the technical aspects of the equipotential bonding between a ring CT core and the rectangular copper busbar (passing through this window type CT), so that more technical knowhow can be shared

    Would appreciate if we could connect over LinkedIn or through any other mode of communication, so that some interactions can happen.

    Many thanks and kind regards.

    It would be nice if you could help, because we have a situation in one of our substations.

  4. Koudjil Mohamed
    Apr 08, 2021

    Thank for the information.
    But, even with equipotential bonding in shower rooms, a dangerous potential can appear between the hand touching the conductor metal and the barefoot touching the room floor.
    What can be done to tackle this.

  5. Rich Helm
    Jan 31, 2017

    Do you need to bond all rebar and fittings in and around an ice rink indoors?

  6. Nhorman Walls
    May 18, 2015

    Thanks for this video, I really appreciate it.

  7. Chris Waters
    Apr 14, 2015

    Nice piece of information here, Edvard. Equal potential is key to human safety.

  8. Ramdas Bagul
    Dec 03, 2014

    Article is very useful ! Thanks all of you!

  9. Ty Brown
    May 30, 2014

    Why would you run a #8 COPPER WIRE AROUND a fiberglass pool with no steel in concert and no conductive items in deck or pool

    • ray ranalli
      Jul 13, 2023

      Equipotential Bonding under the foot path in order to protect wet, barefooted people against transient currents while walking around the Pool.

  10. Eng. Ruqi
    Feb 27, 2014


  11. wmouhammed
    Sep 25, 2012

    what is the differance between the grounding and bonding…?

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