In the case of a phase to cpc fault in an appliance e.g. a washing machine, the exposed conductive part i.e. the casing of the washing machine would be at about 80v with respect to the m.e.t. (80v = fault current times cpc resistance). If some unfortunate person were to touch both the washing machine & an extraneous conductive part e.g. cold water pipe or tap, they would be exposed to this 80v potential difference - nasty but not fatal. The only current that main bonding conductor would carry is the shock current in the person's body. Supplementary bonding would complicate things a bit, but does anyone agree with me so far?
they would be exposed to this 80v potential difference - nasty but not fatal. < Under the right circumstances, that voltage could be fatal..
I HAVE that nasty washing machine. It came with the house we restored. I was sure that a live cable had been accidentally connected to earth because of the severity of the shock when touching taps or the metal parts of sockets. It only happened when in bare feet on an un-insulated tiled floor. In the rest of the house there is no problem, presumably because of the chipboard, floating floors insulated with polystyrene. I spent ages checking for damaged wires or miswired terminals until I tracked it down. Obviously the machine has been unplugged and will probably have to be scrapped. The fact remains that the same thing could happen again and all the cross-bonding does nothing to help the matter. The electric shock was quite unpleasant. How can this be avoided?
In the case of a phase to cpc fault in an appliance e.g. a washing machine, the exposed conductive part i.e. the casing of the washing machine would be at about 80v with respect to the m.e.t. (80v = fault current times cpc resistance). If some unfortunate person were to touch both the washing machine & an extraneous conductive part e.g. cold water pipe or tap, they would be exposed to this 80v potential difference - nasty but not fatal. The only current that main bonding conductor would carry is the shock current in the person's body. Supplementary bonding would complicate things a bit, but does anyone agree with me so far? No.
surely, all the bonded pipework would rise to the same potential, I cannot see how you could get 80 volts on the cpc of the washing machine & 0 volts on the met when they are connected together
Farthrh's point is that the washing machine is not bonded to,for example the pipework. Then under certain conditions when a fault phase to E occurs we get a touch voltagethis is due to the difference in resistance between the met earth and the washing machine cpc . I think farthhhyr is advocating bonding in kitchens not only bathrooms
Bonding is required to all metal services and any other metal that could become live - that includes kitchens.
Bonding is required to all metal services and any other metal that could become live - that includes kitchens. That's not quite the whole story, is it?
How do you arrive at your figure of 80V? What fault current are you supposing? What CPC resistance are you supposing? Who are you really?
Ok Fatrajah, following your philosophy, why don't we just not earth any equipment, nor bond anything, nor earth down extraneous conductive parts? That way, nobody will feel the Pd, because it wont be there! You are a genius, I wish I'd met you 20 years ago. Life would have been so much easier.
a quick point has anybody else noticed that in the osg page 29 it says that 6mm should be used for main bonding on a TT supply but on page 33 of the electrician's guide to the building regs it says use 10mm they are both producd by the IEE so why should they be different
In the case of a phase to cpc fault in an appliance e.g. a washing machine, the exposed conductive part i.e. the casing of the washing machine would be at about 80v with respect to the m.e.t. (80v = fault current times cpc resistance). If some unfortunate person were to touch both the washing machine & an extraneous conductive part e.g. cold water pipe or tap, they would be exposed to this 80v potential difference - nasty but not fatal. The only current that main bonding conductor would carry is the shock current in the person's body. Supplementary bonding would complicate things a bit, but does anyone agree with me so far? Fatrajah, You are probably working and haven`t had a chance to respond to my previous queries - so I will speculate. Shock voltage of 80V, assume CPC resistance of .05 ohms and that phase is able to deliver the necessary current of 1600 Amps. Ignoring the fact that this voltage will only last for long as the fuse doesn`t rupture/RCD (or other device) doesn`t trip, as you say, this voltage is wrt to the MET. Except where the plumbing has been interrupted by plastic, the main earth bonding will effectively link cold water pipe (as the MET is/should be bonded to the mains water inlet). Therefore, there will be a hypothetical 80V between the cold water tap and the washing machine case. Agreed. The situation is slightly more complicated due to parallel earth paths. However, what is your point?
a quick point has anybody else noticed that in the osg page 29 it says that 6mm should be used for main bonding on a TT supply but on page 33 of the electrician's guide to the building regs it says use 10mm they are both produced by the IEE so why should they be different I spotted this last week as I don't encounter TT systems very often. Phoned NIC, first reaction was should be 16mm main bond to CU (the installation had 6mm)but advisor backed down when I pointed out 6mm was in the guide. In answer to the actual question I believe 6mm is adequate for TT installations because of having a direct point of earthing ie the rod, and an incoming RCD (100mA), provided that all appropriate specs are met. Don't know about you guys but I really hate these inconsistencies, that basically point out the NIC are trying to push towards super-safe rather than does the job in the real world. For instance, according to the standard circuits you need 10mm cable for a 40A circuit - so why hasn't virtually every house with an electric cooker burnt down then??? It also causes huge amount of confusion and detracts from the work that flipping desk-jockeys keep trying to make harder!!
Have queried this with NIC again. Apparently there is an equation on page 114 of the regs that given a 25 Ohm Ze would allow a bonding conductor of only 0.2mm, however regs state minimum size is 6mm, hence 6mm it is. He didn't answer reason for discrepancies, perhaps someone else could phone this time.
TNS - Main bonding and equip bonding can be based on size of neutral conductor or on adiabatic equation SO LONG as Zs is compliant. PME - Main bonding and equip bonding must be based on size of neutral conductor or min size of 16mm and 10mm respectively. Can't remember why exactly but I think something to do with a broken pen conductor. Zs must comply in any case. TT - I think it doesn't matter since the Zs won't comply anyway, eg, shock protection provided by RCD instead of MCB.
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