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Purpose of Equipotential bonding
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.

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:

SAFETY OF ELECTRICAL CONNECTION – DO NOT REMOVE!

Resource: Grounding, bonding, shielding and surge protection – G. Vijayaraghavan
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About Author //

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

12 Comments

  1. […] Bonding jumpers must be used around any insulating joints, pipe, or meters […]


  2. Nhorman Walls
    May 18, 2015

    Thanks for this video, I really appreciate it.


  3. Chris Waters
    Apr 14, 2015

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

  4. […] with a conducting medium prevents charge accumulation by providing a leakage path. This is called bonding and can be achieved by using a bare or insulated conductor of adequate mechanical strength. […]


  5. Ramdas Bagul
    Dec 03, 2014

    Article is very useful ! Thanks all of you!


  6. 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


  7. Eng. Ruqi
    Feb 27, 2014

    Thanks

  8. […] mm2. The shields of the coaxial cables going in or out of the building have to be connected with an equipotential bonding conductor and by the shortest route with a common equipotential bonding bar.An earthing conductor […]

  9. […] installation, the foundation earth electrode is an optimal and effective complement of the equipotential bonding.The design of a foundation earth electrode is governed in Germany by DIN 18014, which, for example […]

  10. […] resulting in safety too peration and maintenance personnel.The earthing system also guarantees equipotential bonding such that there are no dangerous potential gradients developed in the substation.In designing the […]

  11. […] the same Purpose. Let’s try to understand this terminology one by one.Go to Content ↑ BondingBonding is simply the act of joining two electrical conductors together. These may be two wires, a wire and […]


  12. wmouhammed
    Sep 25, 2012

    what is the differance between the grounding and bonding…?

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