Earthing and Bonding
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, 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.
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 which will either blow the fuse in the plug and/or trip the RCD protecting the circuit.
During fault conditions, earthing may also reduce the voltage rise of anything earthed, which in addition to the limiting of the shock duration described above can also reduce the shock risk.
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)
Bonding
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 itself that is introducing the dangerous voltage.
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
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.
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 into the room.
For example suppose an electrically heated towel rail develops a fault which makes it electrically live. (Of course this also supposes that it is not earthed properly: which should never happen but the regulations adopt a belt and braces approach). Without bonding, such a fault would result in the towel rail being at mains voltage, while adjacent basin taps might offer a path to earth via the water pipework. This would be a very dangerous situation since touching both towel rail and a tap 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 with that of the earth of any electrical circuits supplying the room, then the towel rail fault will try to bring both taps up to mains voltage (230V). However touching both rail and tap at the same time 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).
Notes
- For equipotential zones to work all accessible 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 independently to earth unless also included in the bonding. (note that in some buildings (e.g. a greenhouse) this can be very difficult to achieve)
- Electrical appliances with exposed metalwork, may be bonded via their earth (or circuit protective) conductors, and do not necessarily need separate bonding wires or earth clamps.
- It is the conductive pipework (etc) entering the special location 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.
- There is no requirement to explicitly connect supplementary bonding to the main earth terminal in or adjacent to the consumer unit, although 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.
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
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.
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.