Earthing and Bonding

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In a discussion on the uk.d-i-y newsgroup John Rumm gave this explanation of Earthing and Bonding, which is the basis for this article.

Earthing and bonding are often confused (sometimes even erroneously referred to as "earth bonding"). This article attempts to clarify the subject.


Earthing ensures that in the event of a fault, adequate fault current will flow causing rapid operation of a Circuit Protective Device (fuse, circuit breaker, or RCD) promptly disconnecting the supply. This limits the duration of any shock that one might receive, dramatically reducing the risk of serious injury or death.

On general purpose socket circuits, the size of earthing conductors, and the circuit protective devices used are chosen to ensure that a fault is cleared within 0.4 seconds (or 0.2 seconds if the installation uses TT Earthing). For submains or higher power circuits feeding fixed equipment the time limit is 5 seconds (or 1 with TT)

For example, suppose a poorly positioned live wire in a washing machine becomes abraded by a sharp metal edge when the machine is running and this has the effect of making the casework of the machine "live". Since the case is connected (via its 13A plug) to mains earth, a high current will flow to earth which will either blow the fuse in the plug and/or trip the RCD protecting the circuit.


During fault conditions, bonding is designed to limit the independent voltage rise of any accessible electrically conductive parts so as to reduce the magnitude of the shock risk. (i.e. even if for a short while the tap in the bathroom is at Mains Voltage due to a fault in your immersion heater, the equipotential bonding will ensure that anything else you can touch at the same time as the tap is electrically tied to it, and hence at (much) the same voltage - eliminating the possibility of shock.

(it is worth noting that with both earthing and bonding there can be "fortuitous" effects - i.e. earthing probably will also reduce the magnitude of the likely touch voltages in the way that bonding does, and bonding connections may act as additional parallel earthing paths. While these are of general benefit it is important to understand that these are not the intended actions and hence they may not be relied upon for protection, and each form of protection must function adequately in isolation from the other)

Two types of bonding are recognised: Main and Supplementary

Main Bonding

Main bonding is the electrical interconnection of incoming (metallic) services (e.g. water, gas, and oil pipes) plus any extraneous conductive parts of a building (like the metal framework used in some buildings, or the central heating pipework), to the main electrical earth. This ensures that under fault conditions things like pipework running through a building are not able to take on a dramatically different electrical potential to that of the installation's earth connection. Note this also covers the fault situation where it is the installations electrical earth that is itself introducing the dangerous voltage.

Supplementary bonding

Supplementary, or cross bonding is usually found in special locations containing a bath or shower. Unlike earthing it is not designed to clear a fault. What it does is electrically tie together all accessible conductive parts (pipes, taps, electrical appliances etc) that could under fault conditions introduce a dangerous potential (voltage) into the room.

For example suppose an electrically heated towel rail in a bathroom develops a fault which makes it electrically live. Hopefully the towel rail's heating element will be earthed, and this in conjunction with its fuse etc will limit the duration of that shock risk. Without bonding however, such a fault may result in the towel rail being held at Mains Voltage for a period of time, while adjacent basin taps might offer a path to earth via the water pipework. This would be a very dangerous situation if someone were touching both towel rail and a tap when the fault occurred, since they would be exposed to a Mains Voltage potential difference across the arms and chest (including heart) probably causing severe injury or death.

However if the pipework feeding both hot and cold taps is bonded together with that of the earth of any electrical circuits supplying the room, then the towel rail fault will try to bring not only the towel rail to mains voltage, but also the taps and any other touchable metalwork up to the same voltage. So, someone touching both rail and tap at the same time is exposed to a potential difference of zero volts.

(Actually the bonding may fail to tie all elements together at exactly the same potential, but it is designed to limit any potential difference to 50V or less).

Supplementary bonding: What should be bonded?

  • Metal pipes (water, gas, central heating, drainage) that enter and leave the room (so short stubs of metal that connect to concealed plastic can be ignored if they are only connecting to passive devices like taps)
  • The protective conductors ("earth wire" / CPC) of any circuits that enter the room - so the lighting circuit almost certainly, and possibly a power circuit if there are fused connection units feeding heaters, pumps etc. Note there is no need to bond each and every lamp fitting or appliance - just a single bond to the supplying circuits CPC is usually adequate.
  • Other metalwork that could bring a voltage into the room due to a fault elsewhere - so if the building has structural metalwork that runs through it and some is accessible in the room (without the need for tools to get at it).

Metalwork that exists in isolation (like window frames, or baths) does not need bonding unless there is some special circumstance.

Bonding practice and technical data

Connections to service pipework should be made as close as possible to the point of entry into the building (but on the consumers side of any meters etc), using the correct BS 951 clamps. Main bonding conductors can be run independently to each service, or a single one may be looped through several so long as the conductor is unbroken at each termination point.

Main bonding: typical wire size requirements

Earthing system (see Earthing Types) Wire CSA (mm²)
TN-S 10
TN-C-S 10
TT 6

Note in these values apply where the size of the neutral of the supply is of 35mm² CSA or less, and where the supplier has not specified another size be used

Supplementary bonding: typical wire size requirements

Where a mechanically unprotected single wire is used to effect the bonding (e.g. between BS 951 clamps) the minimum CSA is 4mm²

Earth Clamps

Earth clamps come in three common types and conform to BS 951.

  • E14 clamps are for dry conditions only and are often coloured red.
  • E15 clamps are for dry or damp conditions and are often coloured blue.
  • E16 clamps are for dry or damp conditions and are often coloured green.
  • E16 clamps allow a larger conductor (usually 16mm or greater) to be connected than E14 and E15 clamps.

Earth clamps come in various sizes to fit different sized pipes. Typical sizes are

E1X for 12mm to 32mm pipes.

E1X/2 for 32mm to 50mm pipes.

E1X/3 for 50mm to 70mm pipes.

Fitting Earth Clamps

While simple enough in theory, confusion can arise with these due to the warning label supplied with them. This should not be left on the main tang of the clamp, but should be removed and refitted under the tensioning screw, otherwise it can make it difficult to get the clamp correctly fitted and tight enough.

Common Questions

Legal Requirements

If main or supplementary bonding is not present (or it is substandard), a householder is not legally obliged to upgrade it in line with current best practice. However it is considered to be a serious electrical fault (one which would be recorded as a level 1 failure on a periodic inspection report). As a result, anyone carrying out Part P notifiable modifications to any part of an electrical installation should also check and rectify any bonding faults at the same time. Note that any work that is part P notifiable cannot legally be carried out (either DIY or paid for) without the correct bonding being in place.

Note that if the main bonding is not to current standards, many professional electricians will refuse to undertake any electrical work (whether covered by part P or not) unless they are also instructed to perform the remedial work required. Contrary to popular opinion, this is not a revenue raising exercise, and is actually to avoid them getting into a very questionable legal position.

A similar situation exists where serious earthing faults are present in an installation such as Lighting Circuits Without an Earth.

Plastic Pipework Installations

Generally with plastic pipe installations, supplementary bonding of the pipes is not required. Note however it may still be required between the earths of, say, lighting and power circuits if they are both accessible in the room.

For rooms plumbed in plastic, where the only metal pipe used is for "show work" (i.e. the visible rises of chrome or copper pipe to taps and fittings), there is no requirement to bond the short sections of visible metal.

However if a bathroom is plumbed in a mixture of plastic and metal, and there is some exposed metal pipework connected to other metal pipework that runs between the bathroom and some other part of the building, this should be bonded since this could result in a potential being introduced into the room from elsewhere.


Electrical appliances with exposed metalwork, may be adequately bonded just via the earth (aka circuit protective) conductor in their supply flex, and do not need separate bonding wires or earth clamps.

Installations not in "Special Locations"

In rooms which are not "special locations" (i.e. places likely to put you at increased danger from electric shock, typically because you might be wet), there is also no requirement. So for example a cloakroom with WC and basin would not require it (although it is commonly seen in such circumstances). Kitchens are also not special location for the purposes of supplementary bonding.

Change to protection for Special Locations in 17th Edition

Under the 17th edition of the IEE Regs (BS7671:2008) it is permitted to have no (supplementary) equipotential bonding in a room containing a bath or shower providing that the main equipotential bonding is in place, and that all the circuits feeding the room have additional protection from a RCD with 30mA (or lower) trip threshold.

See Also

Previous discussion