Bonding questions

In principle your concern is well founded, but it all hinges on what really is a 'good' connection to what I shall call the  terra-firma earth, not to be confused with the MET earth or the CPC earth.....


This is an area that BS7671 does not handle very clearly, indeed it is fair to say that the best approach to earthing and bonding or not has rumbled on as being a bit contentious since the first edition of the regs in the 1880s, getting worse ~ 100 years later in the 1980s as PME and bonding mania co-incided, and has eased a bit with the RCD


The problem for the PEN fault is there is no sensible totally  safe size, unless you go totally impractical and use the substation fuses as your guide, as the diverted current via your pipes and NE bond may be much larger than the rating of your service cable, in the case of a fault where your house now carries the neutral current of the rest of the street.

However it is almost never that bad - there is not normally a case where only one properly has functioning main bonds and no-one else does, so the current,  which on a typical 3 phase supply is smaller than the phase currents anyway, is divided by many paths.


That's not to say there are not odd things that happen ( like in this US video) but the cable sizes are a compromise as the alternative is  too unwieldy. 

One could consider fuses in the earth path to protect the cables, but there are other very good reasons why we do not do this.


At the same time the standards to not distinguish things that are really loosely earthed, like short lengths of pipe between say an oil tank and a boiler house, or planted 'street furniture'  where the resistance as an electrode cannot be less than several tens of ohms, and a really thin bit of wire would be more than adequate.

Insulated joints (" I J "s to the gas people ) are a thing and slowly becoming more common, but can make other situations worse, as there may be different voltages on either side of that join.

Mike

In principle your concern is well founded, but it all hinges on what really is a 'good' connection to what I shall call the  terra-firma earth, not to be confused with the MET earth or the CPC earth.....


This is an area that BS7671 does not handle very clearly, indeed it is fair to say that the best approach to earthing and bonding or not has rumbled on as being a bit contentious since the first edition of the regs in the 1880s, getting worse ~ 100 years later in the 1980s as PME and bonding mania co-incided, and has eased a bit with the RCD


The problem for the PEN fault is there is no sensible totally  safe size, unless you go totally impractical and use the substation fuses as your guide, as the diverted current via your pipes and NE bond may be much larger than the rating of your service cable, in the case of a fault where your house now carries the neutral current of the rest of the street.

However it is almost never that bad - there is not normally a case where only one properly has functioning main bonds and no-one else does, so the current,  which on a typical 3 phase supply is smaller than the phase currents anyway, is divided by many paths.


That's not to say there are not odd things that happen ( like in this US video) but the cable sizes are a compromise as the alternative is  too unwieldy. 

One could consider fuses in the earth path to protect the cables, but there are other very good reasons why we do not do this.


At the same time the standards to not distinguish things that are really loosely earthed, like short lengths of pipe between say an oil tank and a boiler house, or planted 'street furniture'  where the resistance as an electrode cannot be less than several tens of ohms, and a really thin bit of wire would be more than adequate.

Insulated joints (" I J "s to the gas people ) are a thing and slowly becoming more common, but can make other situations worse, as there may be different voltages on either side of that join.

Mike