Moving an earth bonding cable

Tom99

keith969 wrote: »

It means you are less likely to be electrocuted if there is a fault.

[FONT=Verdana, sans-serif]But in layman's terms how is that bonding reducing the risk of electrocution?[/FONT]
[FONT=Verdana, sans-serif]I am particularly interested because in my daughter's house this bonding is missing, but the gas meter is in an outbuilding detached from the house.[/FONT]
[FONT=Verdana, sans-serif]She has a quote but since it involves a long cable run, both through the house then bridging the gap to the outbuilding it, is quite expensive.[/FONT]
[FONT=Verdana, sans-serif]The fuse box is in the kitchen and the gas pipe runs right next to the fuse box so why can't a connection be made there with 0.5m of cable rather than 30m?[/FONT]

TheCyclingProgrammer

Tom99 wrote: »

[FONT=Verdana, sans-serif]But in layman's terms how is that bonding reducing the risk of electrocution?[/FONT]
[FONT=Verdana, sans-serif]I am particularly interested because in my daughter's house this bonding is missing, but the gas meter is in an outbuilding detached from the house.[/FONT]
[FONT=Verdana, sans-serif]She has a quote but since it involves a long cable run, both through the house then bridging the gap to the outbuilding it, is quite expensive.[/FONT]
[FONT=Verdana, sans-serif]The fuse box is in the kitchen and the gas pipe runs right next to the fuse box so why can't a connection be made there with 0.5m of cable rather than 30m?[/FONT]

Wikipedia has a fairly simple laymen explanation:
https://en.wikipedia.org/wiki/Electrical_bonding

Tom99

TheCyclingProgrammer wrote: »

Wikipedia has a fairly simple laymen explanation:
https://en.wikipedia.org/wiki/Electrical_bonding

Excellent, thank you.

Homersimpson

In simple terms the reason for bonding the incoming services (gas, water, oil etc.) is to ensure that everything remains at (approximately) the same potential.

Say you have a momentary electrical fault to earth on your metal toaster, the case become lives, the sink adjacent is effectively earthed via the incoming water pipe and therefore a potential difference exists between the two. If you are touching both you would get a shock.

If the incoming water pipe is bonded to the electrical system it all rises and falls together.

Interestingly in the new 18th edition of the wiring regs incoming plastic services wont need to be bonded which is a long overdue change.

With the OP's question, I would just rewire the main equipotential bond cable, to be extra safe I would turn off the mains supply while you do it although that shouldn't be necessary.

Make sure the bond is within 600mm of where the pipe enters the building.

In terms of testing, in 20 years of the electrical industry I have never seen anyone test the earth bonding in a non medical location, thats all contractors i've worked with from one man bands to the large multi-million pound turnover national contractors. Main bonding testing just isn't done.

Tom99

Homersimpson wrote: »

In simple terms the reason for bonding the incoming services (gas, water, oil etc.) is to ensure that everything remains at (approximately) the same potential.
Say you have a momentary electrical fault to earth on your metal toaster, the case become lives, the sink adjacent is effectively earthed via the incoming water pipe and therefore a potential difference exists between the two. If you are touching both you would get a shock.
If the incoming water pipe is bonded to the electrical system it all rises and falls together.

But the sink/water pipe 'bonding' is a wire joined to the main earth terminal, so why would the "live toaster+you+effectively earthed sink" give you a shock but the "live toaster+you+sink wired to main earth cable" not give you a shock?

Homersimpson

Tom99 wrote: »

But the sink/water pipe 'bonding' is a wire joined to the main earth terminal, so why would the "live toaster+you+effectively earthed sink" give you a shock but the "live toaster+you+sink wired to main earth cable" not give you a shock?

So looking at the two scenarios;

Live toaster + you + sink earthed via pipework back to incoming pipe into ground = potential difference between the electrical system earth (which is in a fault scenario) and the sink earth which is at earth potential.

Live toaster + you + sink earthed via pipework back to incoming pipe into ground and earth connection to electrical system (which is in a fault scenario) = very little potential difference between the electrical system earth and the sink earth which is now at the same potential as the electrical system less the minor negligible resistance of the cables and pipes between the two.

Think of it like a bird sitting on 132kV power line over a field, the bird is fine because there is no potential difference to anywhere and hence they don't explode in a cloud of smoke and feathers.

If they could touch the line and earth then bang!

You might then say well the water pipe is earthed via the mass of soil outside and the electrical system is connected to earth so what is the difference.

The difference is that the mass of soil is not a very good conductor and the electrical earth could be a considerable distance from the house so you rely upon the mass of soil between the water pipe in the ground and the electrical earth which will vary massively between installations, with time of year (i.e. wet or dry etc.).

You might be fine without the earth cable but its very hard to guarantee it.

The other issue is that most smaller properties (i.e. domestic and small to medium commercial/industrial) have whats called a TNC-S supply (this is not alway PME which confuses some people). In a TNC-S system the neutral and earth share the same conductor until they reach the property where they split (terra neutral combined then separated).

This means that the building earth back to the substation will be above 0V as the combined neutral earth conductor carried current and therefore has a voltage across it.

Now that we put RCD's on everything the risks are becoming less which is why supplementary bonding (i.e. cross boding at sinks between taps, radiators etc.) is almost non existent in normal installation these days.

Earthing is a very simple but misunderstood subject, I can count on one hand the number of electrical engineers I have met who understand it fully.

Having worked on earthing systems for medical installations for many years I learnt an awful lot about the subject.

Just as an edit to this, this won't definitely prevent you getting a shock but will prevent it going in one arm, across you heart and out the other, it should also reduce the severity of the shock and coupled with an RCD the supply disconnection should be less than 0.4 of a second.

Ectophile

Tom99 wrote: »

[FONT=Verdana, sans-serif]But in layman's terms how is that bonding reducing the risk of electrocution?[/FONT]
[FONT=Verdana, sans-serif]I am particularly interested because in my daughter's house this bonding is missing, but the gas meter is in an outbuilding detached from the house.[/FONT]
[FONT=Verdana, sans-serif]She has a quote but since it involves a long cable run, both through the house then bridging the gap to the outbuilding it, is quite expensive.[/FONT]
[FONT=Verdana, sans-serif]The fuse box is in the kitchen and the gas pipe runs right next to the fuse box so why can't a connection be made there with 0.5m of cable rather than 30m?[/FONT]

The bonding is supposed to be within 600mm of where the pipe enters the house. Why would someone want to run a long earth wire all the way into the outbuilding?

Homersimpson

Ectophile wrote: »

The bonding is supposed to be within 600mm of where the pipe enters the house. Why would someone want to run a long earth wire all the way into the outbuilding?

Quite right, however, as an addition to this, if the outbuilding has power fed from the house it should be converted to a TT supply (terra terra) where you have an incoming live and neutral from the house but no earth (the supply cable should have an earth but only earthed at one end so its not continuous).

You then have to put and RCD on the main supply in the outbuilding and provide its own earth rod which needs to be installed and tested to ensure that the RCD disconnects in the required 0.4 seconds. This needs someone with an earth rod tester so its not really something a DIY person can do.

Risteard

Homersimpson wrote: »

Quite right, however, as an addition to this, if the outbuilding has power fed from the house it should be converted to a TT supply (terra terra) where you have an incoming live and neutral from the house but no earth (the supply cable should have an earth but only earthed at one end so its not continuous).

You then have to put and RCD on the main supply in the outbuilding and provide its own earth rod which needs to be installed and tested to ensure that the RCD disconnects in the required 0.4 seconds. This needs someone with an earth rod tester so its not really something a DIY person can do.

An outbuilding does not need to be divorced from a TN-C-S Earthing arrangement, even if it contains extraneous-conductive-parts, so long as the cpc brought to the outbuilding is also adequate to be used as a main protective bonding conductor. This often means that a considerably larger conductor is required for the combined cpc/main protective bonding conductor as would otherwise be required simply for a cpc. So often the TT islanding approach is adopted.

Tom99

Ectophile wrote: »

The bonding is supposed to be within 600mm of where the pipe enters the house. Why would someone want to run a long earth wire all the way into the outbuilding?

The outbuilding is a garage and utility room with the main gas boiler, sink, power points etc. There is a small old fashioned fuse board inside the outbuilding. All pipes and cables run through a wooden box type conduit about 2.5m above ground between the house and the outbuilding.