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 referred to as "earth bonding"). This article attempts to clarify the subject.
Earthing
Earthing ensures that in the event of a fault a large fault current will flow causing rapid operation of a Circuit Protective Device (fuse or circuit breaker) promptly disconnecting the supply. For example suppose that a live wire in a washing machine has been positioned incorrectly so that it rubs on a sharp metal edge of the casing when the machine is running, and the insulation has now worn away enough that the wire makes contact with the metal of the case. Since the case is connected (via its 13A plug) to mains earth a high current will flow which will blow the fuse in the plug.
Earthing is also designed to limit the voltage rise of anything earthed that one can touch to a safe level (specified as being under 50V AC) during the fault. Thus if someone is touching the washing machine case when the fault occurs the voltage they are exposed to, and the time for which they are exposed, are small enough that they should not be at significant risk of shock injury.
The size of earthing conductors and characteristics of fuses and other protective devices are designed to ensure that a fault is cleared within 5 seconds for all faults, and within 0.4 seconds for circuits feeding socket outlets (to which our washing machines etc, which may be touched, are connected).
Bonding
Two types of bonding are recognised: Main and Supplementary
Main Bonding
Main bonding is the electrical connection together of incoming service pipes (usually water and gas or oil) to the main electrical earth (at the incoming electricity cable and meter). This ensures that (metallic) gas and water installation pipework running through a building are at the electrical earth potential.
Supplementary bonding
Supplementary, equipotential or cross bonding is usually found in special locations containing a bath or shower. Unlike earthing it is not designed to clear a fault or limit the absolute touch voltage. What it does is electrically tie together all extraneous-conductive parts (pipe or other services etc) that could under fault conditions introduce a dangerous potential into the room. For example suppose an immersion heater or inline instantaneous electric heater develops a fault which makes the hot water pipework electrically live. (Of course this also supposes that the pipework is not earthed and the appliance is not properly earthed: which should never happen but the regulations adopt a belt and braces approach.) Without bonding such a fault would result in the hot taps being at 230V while the cold taps might offer a path to earth via the rising main: this would be a very dangerous situation since touching both hot and cold taps would expose one to a 230V 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 then both taps might be at mains voltage (230V) under fault conditions, but touching both exposes one 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.)
For equipotential zones to work all extraneous-conductive parts (such as the earth wires of any circuits feeding power into the room as well as metal plumbing - central heating as well as hot and cold water) must be securely electrically connected together, and there must be no conductive items or surfaces connected to earth.
Note that it is the conductive pipework (etc) entering the special location (bathroom etc) which must be bonded: if there is a mixture of metal and plastic pipework, plastic plumbing connectors etc, then it is the pipework coming into the room, not any downstream of plastic sections, which must be bonded.
Note also that there is no requirement to explicitly connect supplementary bonding to the main earth terminal in or adjacent to the consumer unit, though in practice there will often be a connection by default via the circuit protective conductors of any circuits that are included in the equipotential bonding.
Plastic Pipework Installations
Generally with plastic pipe installations, supplementary bonding of the pipes is not required (although it may be required between the earths of, say, lighting and power circuits if they are both accessible in the room, plus any other extraneous metallic parts (metal Central Heating pipes for example)).
However if a bathroom is plumbed in a mixture of plastics pipe joined to exposed copper or steel pipework, the metal pipework does have to be bonded if it runs between the bathroom and some other part of the building, since a fault outside the bathroom could result in the metal pipework inside becoming live.
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).
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.