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Topic Title: Equipotential bonding questions
Topic Summary: Does it increase or decrease safety!?
Created On: 15 April 2010 02:17 PM
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 15 April 2010 02:17 PM
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dtl1606

Posts: 8
Joined: 15 April 2010

I recently had a porch built, but the builder disappeared without giving me the electrical completion certificate needed by the council. To address this, I had a periodic inspection report done.

The inspection reported two issues:
1. No equipotential bonding to the water and gas pipes
2. No RCD protection to socket circuit

The council aren't interested in either because they say these are related to the house as it exists rather than the porch, so everything is okay from that point of view. (its the original 1950s vintage wired fure box with the house wiring upgraded in about 1970)

I didn't know what equipotential bonding was, and this is the explanation I got from the inspector.

"In the event of a fault to earth ocurring in some appliance the earth potential will rise to approximately 240V. Without equipotential bonding, this means there will be a 240V potential difference between any earthed appliance and the gas and water pipes. If anyone was to touch the earthed appliance and, say, a tap, they would receive an electric shock. Equipotental bonding means that there would be no potental difference between the earthed appliance and, e.g., the tap, and so no shock"

I have two problems with this:
1. Surely the impedence of the earth cable will be approximately equal to the live, so at most the earth potential could be about 120V
2. You will receive an electric shock if you are standing in your bare feet in your living room and touch a live wire: surely equipotential bonding means that, under a fault condition, all the exposed metalwork in your house is at an elevated potential. While it might prevent a shock if you are touching both an appliance and, say, a tap, it could cause you to receive a shock if you are touching, the tap alone?

A second question relating to the actual implementation of the equipotential bonding. The inspector commented that it might be sufficient for the gas and water pipes to be bonded to eachother, but that was down to how the electrician doing the work interpreted the regulations. It makes sense that this would achieve the equipotential since the gas pipe is connected to earth by virtue of the earthing of the central heating boiler, but how can this be a matter of individual interpretation - either it is, or isn't acceptable under the regulations!
 15 April 2010 02:19 PM
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dtl1606

Posts: 8
Joined: 15 April 2010

Managed to accidentally post my question without quite finishing it

My questions may seem basic, but I'm a physicist not an electrician

Thanks

David
 15 April 2010 04:07 PM
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gkenyon

Posts: 4923
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The impedance of the earth cable is greater than that of the current-carrying conductors in most UK wiring, because we use a reduced size protective conductor in Twin and Earth.

Depending on the type of earthing, the "external" impedance of the earth conductor (i.e. outside the premises, in the supplier network) may also be relatively large.

For this reason, it's possible for the difference in potential between gas/water and the electrical protective conductor system in the premises to be almost the supply voltage (i.e. 240V) during a fault, depending on what these impedances are.

"Main Equipotential Bonding" is normally taken to be a fundemental element for safety for most types of domestic electrical installation in the UK.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 15 April 2010 04:39 PM
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dtl1606

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Thanks for explaining how it could get to 240V. Doesn't that mean however that my second problem with the equipotential system is valid - I could get a shock if I touch e.g. a tap under a fault condition since the tap will be at 240V?
 15 April 2010 04:53 PM
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perspicacious

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"You will receive an electric shock if you are standing in your bare feet in your living room and touch a live wire: surely equipotential bonding means that, under a fault condition, all the exposed metalwork in your house is at an elevated potential. While it might prevent a shock if you are touching both an appliance and, say, a tap, it could cause you to receive a shock if you are touching, the tap alone?"

I've got carpet over both my wooden and concrete floors

Regards

BOD
 15 April 2010 05:10 PM
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DNOworker

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In short yes, what you are saying in theory is correct, but there is also a second part that "bonding has to play". As pointed out by the previous poster, bonding is a primary protective measure in domestic installations as is the overall protective measure of "Automatic Disconnection of Supply (ADS).

The 10mm diameter bonding cables create a low impedance path back to the main earth and therefore the fault current that will flow when the fault occurrs, should operate the the circuit protectve device (fuse/MCB etc) within a specified time. This should ensure that the tap does not remain live for very long.

Under BS761:2008, 0.4 seconds is generally taken to be the maximum disconnection time in a domestic property, by which time the voltage arrising on the tap may not have got to full potential! - hope this helps explain it a bit more and no doubt others can add or correct me, but this is essentilay how it works.

Regards DNO worker
 15 April 2010 05:17 PM
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OMS

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The touch voltage is a function of the fault current and the impedance of the circuit protective conductor. You will note the fault current is a function of the impedance of the line conductor and the earthing conductor of both the house wiring and the supply cables

If we set the external impedance of the supply system to zero then the limiting condition will be the ratio between the line conductor impedance and the earth conductor impedance within the property.

Graham mentioned the fact we tend to use reduced CSA earths in the UK so you would (for each of your examples) be exposed to a worst case (ie asymptotic) touch voltage of about 160Volts (and usually quite a bit less). That said, for specific supply systems which are usually unearthed it could be higher than that and approach the full supply voltage

Equally, as BOD pointed out we tend not to be wet and naked in all rooms of the house so again the risk is limited by clothing, footwear and the presumed fairly insulating floors (although I accept that the hand to foot shock is about 2.5 times more dangerous than the hand to hand shock).

Personally I would get the bonding done - a single cable to gas and then looped to water will suffice. The advice to get some 30mA RCD protection on at least the sockets is worth considering as well

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 15 April 2010 05:35 PM
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stureid

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The inspection reported two issues:
1. No equipotential bonding to the water and gas pipes
2. No RCD protection to socket circuit

The council aren't interested in either because they say these are related to the house as it exists rather than the porch, so everything is okay from that point of view. (its the original 1950s vintage wired fure box with the house wiring upgraded in about 1970)


I'll not comment on equipotential bonding because it's been covered already, but to quote regulation 131.8 of BS7671:2008...

"No addition or alteration, temporary or permanent, shall be made to an existing installation, unless it has been ascertained that the rating and the condition of any existing equipment, including that of the distributor, will be adequate for the altered circumstances. Furthermore, the earthing and bonding arrangements, if necessary for the protective measure applied for the safety of the addition or alteration, shall be adequate."

i.e. the BCO is wrong to dismiss the lack of bonding as not applicable to the new work.

-------------------------
Regards
Stuart Reid
RED Electrical
http://www.redelectrical.co.uk
 15 April 2010 05:36 PM
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perspicacious

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" we tend not to be wet and naked in all rooms of the house "

You weren't dangling a carrot for me to respond there, were you OMS?.....................

Regards

BOD
 15 April 2010 05:51 PM
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OMS

Posts: 22349
Joined: 23 March 2004

I suspect we'll hear the cat like tread of the mod shortly if we have a post that contains the phrases "wet and naked" and "dangling carrots"

OMS

-------------------------
Let the wind blow you, across a big floor.
 15 April 2010 06:24 PM
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jcm256

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The electrician should have first checked that the existing earthing and bonding arrangements were adequate because the safety of new work, however minor will depend on the existing earthing arrangements. Was it a cowboy electrician did your new work, if you live in England or Wales how did you square it with the Building Control that they excepted a third party inspection certificate for the work. Was it the BC who said they don't give a monkeys about your existing earthing. This is only a scowling post I am sure in the job you have cowboys would not be allowed.
Regards
jcm.
 16 April 2010 12:28 AM
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AJJewsbury

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Joined: 13 August 2003

Furthermore, the earthing and bonding arrangements, if necessary for the protective measure applied for the safety of the addition or alteration, shall be adequate."

i.e. the BCO is wrong to dismiss the lack of bonding as not applicable to the new work.

But it could be argued that if there aren't any extraneous-conductive-parts in (or within reach of) the new porch, then the lack of bonding doesn't compromise the safety of the addition.

it could cause you to receive a shock if you are touching, the tap alone?

Generally you can't get a shock from just touching a single item, whatever voltage it's at. To receive a shock you need current to flow through your body and that needs a potential difference. Usually the other potential is earth, or something connected with earth (hence the danger of a potential difference between say the kettle and the kitchen tap), but indoors in a dry area, you're pretty isolated from earth, so touch a tap at 240V and your body just becomes charged to 240V, no current flows and you don't feel a thing. Just like a bird perched on a bare high voltage wire.

The 10mm diameter bonding cables create a low impedance path back to the main earth and therefore the fault current that will flow when the fault occurrs, should operate the the circuit protectve device (fuse/MCB etc) within a specified time. This should ensure that the tap does not remain live for very long.

Slight confusion between bonding and earthing (and between diameter and CSA) I fear. CPCs and earthing conductors are intended to carry fault currents, while bonding conductors are intended to keep things at similar potentials (from potential differences could originate outside the installation as well as inside). There are circumstances (e.g. submains) where correctly sized bonding conductors would be too small to carry fault currents all by themselves.

- Andy.
 16 April 2010 09:14 AM
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briandoherty

Posts: 313
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If you play around with the values in the blue cells of this spreadsheet, you may get a better feel for the benefits of equipotential bonding...

Shock Current Spreadsheet

-------------------------
Regards,

Brian
 16 April 2010 11:29 AM
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Inrush

Posts: 741
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If you play around with the values in the blue cells of this spreadsheet, you may get a better feel for the benefits of equipotential bonding...


Interesting Brian.
Why have you calculated for both an internal and external equipotential bond to the gas supply?
 16 April 2010 02:28 PM
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briandoherty

Posts: 313
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I originally made this spreadsheet when there was some debate over whether it was OK (or not...) to connect Gas MEB to point adjacent to external Gas Meter (where Gas Fitter expects to find it) rather than to point where Gas installation pipework emerged / became visible / accessible within the building (where Electrician expects to find it).

If you set R_G_INT_BOND to a very low value and R_G_EXT_BOND to a very, very high value, then that simulates internal Gas MEB, and vice-versa to simulate external Gas MEB.

-------------------------
Regards,

Brian
 16 April 2010 11:45 PM
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dtl1606

Posts: 8
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Thanks for all your input. From what's been said, it sounds like the use of equipotential bonding is about the lesser of two evils - without it, you could receive a shock in the relatively unlikely event that you happen to be touching the casing of an earthed appliance and a metal tap in the 0.4 seconds it takes the circuit breaker to trip during a fault to earth, but with it there is a (probably smaller) chance you could receive a shock it you happen to be touching a tap during a fault to earth in the 0.4s it takes the circuit breaker to trip.

As for what the BCO told me, I live in Scotland and I do know from a friend who worked in the council planning department that the rules are different here than England and Wales. BS7671 obviously is the same, but building regulations are different. One electrician I spoke to (and I have every reason to believe him) said I could actually sign off the completion certificate myself since the regulations say it must be a competent person, not necessarily a qualified person. Since we're planning to move within a year I decided not to go down that route, but I have a colleague at work who is going down that route, but making sure he's documenting everything he's done to back it up.
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