Don't be so pompous.
Main earth bonding
- Thread starter Cabbyfan
- Start date
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That's obviously true. However, I don't understand your concentrating on the need to bond internal gas and water pipes (when supply enters in plastic) when, as I have just written, these are probably far less likely to have or develop paths to true earth than many other metal objects within a building.
In any event, as we know, virtually all internal pipes are earthed (incidentally), even if the CPs doing that earthing will have smaller CSAs than one would use for main bonding conductors - so, provided there are not earth currents high enough to melt those CPCs, there is no problem, anyway.
Kind Regards, John
And you consider that a good safety practice - jeez
Hmmm. I certainly agree that neutral will never rise to a dangerous potential above earth under those circumstances - even in the worst imbalance situation (zero current in two of the phases), the neutral current will only be as high as the greatest current in any of the phase conductors - and, AFAIAA, the neutral supply conductor is usually of the same CSA as the phase ones - so there will not be a large rise in potential.
As for your 'very high current' hypothesis, that surely will not arise other than with 'real' extraneous-c-ps (e.g. metal supply pipes), which will be properly bonded with conductors of adequate CSA to carry the possible currents. All these 'incidental' paths to true earth (through wet walls or whatever) we're talking about are going to have such (relatively) high resistance/impedance that only a very small current would flow, even if the neutral (aka CPC with TN-C-S) potential became very high. Nothing would even get slightly warm, let along catch on fire or melt CPCs.
Kind Regards, John
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http://www.bbc.co.uk/news/uk-england-leeds-14075618
This was small fires in three or four kitchens as bonding over heated and then a gas pipe failed. Not known if the gas was via a metal service pipe that failed when carrying current or the fire affected the pipe.
This was small fires in three or four kitchens as bonding over heated and then a gas pipe failed. Not known if the gas was via a metal service pipe that failed when carrying current or the fire affected the pipe.
Mine is £22.50
My comment was perhaps a little facetious. As I've just written to bernard, it just isn't going to happen. Provided only that 'real' extraneous-c-pcs (e.g underground metal service pipes which enter the property, or structural metal which enters the earth) are properly bonded, any of these other 'possible incidental paths to earth' we are talking about are going to be of such (relatively) high impedance than the currents through them will never even be 'appreciable' (no more than a handful of amps at the very very worst, more likely mA), let alone high enough to melt any CPC, even if one applied 230V to them.
Kind Regards, John
You are presumably talking about (inadequate) bonding of real extraneous-c-ps (e.g. underground metal service pipes which enter properties) - and no-one denies the crucial importance of adequately (or, what is deemed to be 'adequately') bonding them.
However, as I've just written several times, you are never going to get enough current though any of the 'incidental' paths to earth (through walls etc) to even warm anything up significantly, let alone start a fire.
Kind Regards, John
Seriously, never say never!
Possibly the oddest of ones I came across was an open circuit neutral on the meter tail, which caused a partial melt down in a consumer unit.
This could have resulted in a fire.
The cause, the open circuit neutral connected with a neutral to earth fault on the lighting circuit so all the load current was flowing through a 1mm2 cable core.
Not directly connected to this discussion but an example
Possibly the oddest of ones I came across was an open circuit neutral on the meter tail, which caused a partial melt down in a consumer unit.
This could have resulted in a fire.
The cause, the open circuit neutral connected with a neutral to earth fault on the lighting circuit so all the load current was flowing through a 1mm2 cable core.
Not directly connected to this discussion but an example
There is probably somewhat of an unfair comparison going on there. The 'charge-out rate' of a member of a large organisation is always going to be much higher than that of a one-man-band or small company, but the amounts ending up 'in the pockets' of the two (for comparable jobs) may be much the same.
Kind Regards, John
Going back to
The need is to ensure that you have a good Faraday cage affecting the property to ensure the safety of the occupants in the case of a lost neutral on a TNCS supply.
It saves, I suppose, it saves putting a mesh in all the floors and walls.
Look at it from that other view
The need is to ensure that you have a good Faraday cage affecting the property to ensure the safety of the occupants in the case of a lost neutral on a TNCS supply.
It saves, I suppose, it saves putting a mesh in all the floors and walls.
Look at it from that other view
Yes this was metal water supply pipes and metal water mains in the street. If the network was fused at 200 amps what CSA of bonding would be needed to carry the worst case Neutral current onto the water pipes and back to the substation without the bond over heating ?
Voltage gradients in a damp wall can create some "interesting" and sometimes hazardous situations that do not include fire.
In general, I would agree. One of the first things I was taught at uni was never to say 'never', 'always', 'all', none' etc.!
However, in the context of what I was saying, I think one is as close to 'never' as makes no difference. The smallest CPC used in fixed domestic wiring is 1mm² which is deemed to be able to carry around 15A continuously, and undoubtedly at least double that before any harm (or risk of fire) could arise. Even if one applied 230V to the 'wet wall' (or whatever incidental path to earth) that would require an path to earth of less that 8Ω before the CPC was at any risk. If, more realistically, we were talking 50V, it would have to be less than 2Ω. That just isn't ('ever'??
) going to happen with a wall, or anything like it. Even a 'deliberate' domestic TT electrode stuck in the ground would not achieve that! Kind Regards, John
For a start, it would surely be extremely unlikley that 200A would flow through the bonding conductor of a single property. To get a high current would require several properties to be involved, and the neutral-earth current would presumably be shared between the bonding conductors in each of those properties. The 'usual' (minimum for TN-C-S) 10mm² main bonding conductor can carry 70A continuously in free air, so I would imagine at least double that (maybe even 200A) before it would come to any harm or seriously overheat. If several properties were involved, each equally sharing the N-E current through 10mm² main bonding conductors, I would therefore expect that, between them, they could probably cope with a total N-E current of 'several hundred amps' before any significant harm (or serious overheating) resulted to any of the conductors.
Kind Regards, John
And ask them if they have done a risk assessment associated with making isolated metal pipes part of the electrical installation.
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