Earthing neutral

Sorry IT woes mean this is now out of order.


Aah - why do we rush round sleeving bare earth wires, after marking them as inspection failures and have the earth as an insulated core in flex, when it could just as easily be the conduit or metal trunking that is performing the same function, and we do not have to insulate that.

Nothing to do with neutrals this time, but the rise of voltage during fault is still relevant, as is the double fault to danger principle.

A mains flex or the final circuit wiring  is likely to be the bit with most of the voltage drop during an appliance fault - if L-E voltage is 230V before the fault then at the point of junction it may be 115 volts - with a voltage gradient sloping down the live from 230 to 115, and at the same time an identical voltage gradient slopes up the earth conductor. so to stop other things being 'tingly' for the short time between fault on and ADS kicking it off, we prefer to cover them. This assumes the PE and L are comparable resistance.

But in conduit and trunking, the impedance of the earth path is vastly less than that of the live core, due to the greater metal cross section, so the voltage division and exposed touch voltage issue is less serious.

In places with a TT supply, the earth conductors rise to closer to 230V during fault, so firstly the ADS has to be faster and secondly those of us who think about it try to specify plastic conduit, or if we have to have lots of bare metal earthed stuff, then try to make sure that parts that are within reach of each other are either bonded to each other or are arranged so folk cannot get to it.

(and so the same earth, so it all goes live together, and you do not care - aster all the only time you really would like your kitchen sink and taps to be live not earthed,  is when trying to fill a live handled kettle ... though insulation is better still.)


At the substation end, it is the same sort of thinking but in reverse - can a substation transformer fault cause the voltage on exposed stuff to rise until the ADS gets it, be that HV breakers or expulsion fuses, as mentioned above.

Sorry IT woes mean this is now out of order.


Aah - why do we rush round sleeving bare earth wires, after marking them as inspection failures and have the earth as an insulated core in flex, when it could just as easily be the conduit or metal trunking that is performing the same function, and we do not have to insulate that.

Nothing to do with neutrals this time, but the rise of voltage during fault is still relevant, as is the double fault to danger principle.

A mains flex or the final circuit wiring  is likely to be the bit with most of the voltage drop during an appliance fault - if L-E voltage is 230V before the fault then at the point of junction it may be 115 volts - with a voltage gradient sloping down the live from 230 to 115, and at the same time an identical voltage gradient slopes up the earth conductor. so to stop other things being 'tingly' for the short time between fault on and ADS kicking it off, we prefer to cover them. This assumes the PE and L are comparable resistance.

But in conduit and trunking, the impedance of the earth path is vastly less than that of the live core, due to the greater metal cross section, so the voltage division and exposed touch voltage issue is less serious.

In places with a TT supply, the earth conductors rise to closer to 230V during fault, so firstly the ADS has to be faster and secondly those of us who think about it try to specify plastic conduit, or if we have to have lots of bare metal earthed stuff, then try to make sure that parts that are within reach of each other are either bonded to each other or are arranged so folk cannot get to it.

(and so the same earth, so it all goes live together, and you do not care - aster all the only time you really would like your kitchen sink and taps to be live not earthed,  is when trying to fill a live handled kettle ... though insulation is better still.)


At the substation end, it is the same sort of thinking but in reverse - can a substation transformer fault cause the voltage on exposed stuff to rise until the ADS gets it, be that HV breakers or expulsion fuses, as mentioned above.