Difference between revisions of "Earthing and Bonding"

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''In a discussion on the uk.d-i-y newsgroup John Rumm gave [http://groups.google.co.uk/group/uk.d-i-y/msg/a0854fb0707b63a7 this explanation] of Earthing and Bonding, which is the basis for this article.''
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'''Earthing''' and '''bonding''' are often confused (sometimes even erroneously referred to as "earth bonding"). This article attempts to clarify the differences and requirements for each.
  
−
----
+
==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.
 +
 
 +
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)
  
−
There is no such thing as "earth bonding": there is earthing, and there is
+
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.
−
equipotential bonding (main and supplementary), but they serve different purposes.
 
  
−
==Earthing==
+
==Bonding==
  
−
Good earthing will ensure that in the event of a fault the supply can be disconnected quickly by providing a path for a large fault current to flow, hence causing rapid operation of a Circuit Protective Device. It will also limit the voltage rise of anything earthed that one can touch to a safe level (under 50V typically) during the fault. So if a live wire falls off the inside a washing machine and makes contact with the case, the big fault current will open the fuse in the plug or circuit breaker on the circuit quickly. Should a person happen to be touching the machine at the time, the voltage they are exposed to should be low enough to not place them at significant shock risk.
+
During fault conditions, bonding is designed to limit the voltage difference between any accessible electrically conductive parts. This is 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.
  
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== Bonding ==
+
''(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)''
  
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Two types of bonding are recognised: '''Main''' and '''Supplementary'''
+
Two categories of bonding are recognised: '''Main''' and '''Supplementary'''
  
 
===Main Bonding===
 
===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.
+
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.
 +
 
 +
Where the incoming supply itself is plastic (increasingly common) and the property has internal plastic piping, no main bond is required to the service.  
  
−
=== Supplementary bonding ===
+
If there is a plastic supply, but all internal pipework is metal, then the pipework should be treated as one would other "extraneous" metalwork like the CH system's pipework, and include it in the main EQ bonding.
  
−
Supplementary (equipotential) bonding is not designed to clear a fault condition, or limit the absolute touch voltage. What it is supposed to do is electrically tie together any  conductive parts (pipe or other services etc) that could under fault conditions introduce a dangerous potential into the room. Thus anything and everything that one can touch will be at the same electrical potential<sup>[1]</sup> as everything else - even if it is elevated way above true ground. So for example, without bonding a fault could result in hot taps being at 230V (say, due to being connected to a faulty inline water heater) while the cold taps are earthed via the rising main: this would pose a serious shock hazard since touching both taps would expose one to a 230V potential difference, causing electrocution. However if the taps are bonded together then both taps might be at mains voltage (230V) under fault conditions, but touching both exposes one to zero volts of potential '''difference'''. Equipotential zones only work where all "extraneous-conductive" parts (such as the earth wires of any circuits feeding power into the room as well as metal plumbing) are securely connected together and there are no conductive items or surfaces connected to earth.
+
===Supplementary bonding===
  
−
Note that there is no requirement to explicitly connect supplementary bonding to the main earth terminal in or adjacent to the consumer unit. In reality you will often have a connection by default via the circuit protective conductors of any circuits that are included in the equipotential bonding.
+
''Supplementary'', or ''cross'' bonding is usually found in ''special locations'' (i.e. ones containing a bath, shower, sauna, or pool since here water and electricity may be in close proximity '''and''' its more likely that the users may be undressed. These factors combined make injury from shock more likely). 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. (Rooms such cloakrooms or toilets without a bath or shower, are not treated as special locations since although there is water, its less likely users will be undressed).
 +
 
 +
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 short period of time. 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) risking 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 either due to a fault elsewhere, or by being in contact with earth elsewhere. So if the building has structural metalwork in contact with earth that runs through it and some is accessible in the room (without the need for tools to get at it).
 +
 
 +
==== Supplementary bonding: Things that don't need bonding ====
 +
Metalwork that exists in isolation (like window frames, or baths, basins, metal furniture etc) 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.
 +
 
 +
{| class="wikitable" style="text-align:center" cellpadding="3" border="1"
 +
|+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.
 +
 
 +
*See [http://s428.photobucket.com/albums/qq6/adamko2020/?action=view&current=img104.jpg correct fitting]
 +
 
 +
==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]].
  
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[1] in reality the bonding may fail to tie all elements together at exactly the same potential, however it must limit any potential difference to 50V or less.
 
  
 
===Plastic Pipework Installations===
 
===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)).
+
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.
+
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.  
  
−
=== Installations not in "Special Locations"===
+
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.
  
−
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).
+
===Appliances===
  
−
=== Change to protection for Special Locations in 17th Edition ===
+
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.
 
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==
 +
 +
*[[Earthing Types]]
 +
*[[TT Earthing]]
 +
*[[Ufer Earthing]]
 +
*[[Electrical Circuit Faults]]
 +
 +
[http://groups.google.co.uk/group/uk.d-i-y/msg/14a0b8a90fca226b Previous discussion]
 +
 +
[[Category:Electrical]]
 +
[[Category:Safety]]

Revision as of 15:10, 23 June 2022

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

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.

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.

Bonding

During fault conditions, bonding is designed to limit the voltage difference between any accessible electrically conductive parts. This is 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 categories 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.

Where the incoming supply itself is plastic (increasingly common) and the property has internal plastic piping, no main bond is required to the service.

If there is a plastic supply, but all internal pipework is metal, then the pipework should be treated as one would other "extraneous" metalwork like the CH system's pipework, and include it in the main EQ bonding.

Supplementary bonding

Supplementary, or cross bonding is usually found in special locations (i.e. ones containing a bath, shower, sauna, or pool since here water and electricity may be in close proximity and its more likely that the users may be undressed. These factors combined make injury from shock more likely). 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. (Rooms such cloakrooms or toilets without a bath or shower, are not treated as special locations since although there is water, its less likely users will be undressed).

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 short period of time. 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) risking 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 either due to a fault elsewhere, or by being in contact with earth elsewhere. So if the building has structural metalwork in contact with earth that runs through it and some is accessible in the room (without the need for tools to get at it).

Supplementary bonding: Things that don't need bonding

Metalwork that exists in isolation (like window frames, or baths, basins, metal furniture etc) 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.

Appliances

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

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