230v or 240v

[bernardgreen]

No, not unless we start talking about ridiculously high voltages, high enough to still cause enough current to flow through the coil despite the low-resistance shunt across it.

Ohms law will dictate that the current required to trip the mechanism will being flowing if the potential difference between the E and F terminals is 50 volts or more. There is no need to have "ridiculously high voltages" across the coil. The current through the fault will also be determined by ohms law with 50 volts applied to the resistance ( impedance ) of the fault circuit.

Yes, if there is anything else (connected directly to the "F" terminal) placed within the resistance area of that electrode,

The fault current will raise the potential of the ground in the "resistance area" and thus reduce the voltage difference between E and F thus reducing the current in the coil. The raising of the potential of the "resistance area" compared to ground outside the "resistance area" means the CPC could be considerable more than 50 volts above any ground that is outside the "resistance area".

In short any parallel path that connects to ground inside the "resistance area" will result in the CPC reaching voltages higher than 50 volts before the trip operates.

 

[Paul_C]

Ohms law will dictate that the current required to trip the mechanism will being flowing if the potential difference between the E and F terminals is 50 volts or more. There is no need to have "ridiculously high voltages" across the coil. The current through the fault will also be determined by ohms law with 50 volts applied to the resistance ( impedance ) of the fault circuit.

What I meant was that if you have a short of negligible resistance across the coil, then due to the resistance of the earth connection it will take a much higher voltage than is available in order to develop sufficient voltage across the resistance of the short. For example, if you had a 500-ohm coil which trips when there is 20V applied to it, with a shunt of 1 ohm across it you would need a current of almost 20A through that shunt. If the earth electrode were, say, 50 ohms, then straight away you would need a potential of almost 1000V. If the earth electrode resistance were the maximum of 500 ohms which was specified for the common Crabtree E50, then with that 1-ohm shunt in place you'd need a potential of about 10kV.

The fault current will raise the potential of the ground in the "resistance area" and thus reduce the voltage difference between E and F thus reducing the current in the coil. The raising of the potential of the "resistance area" compared to ground outside the "resistance area" means the CPC could be considerable more than 50 volts above any ground that is outside the "resistance area".

Are you talking about a correctly installed system here? The specifications of the ELCB for maximum earth rod resistance took into account the rise in potential (relative to true earth) that would occur at the "E" terminal during a fault in order to trip before the "F" terminal would reach 50V relative to true earth potential.

In short any parallel path that connects to ground inside the "resistance area" will result in the CPC reaching voltages higher than 50 volts before the trip operates.

Which is why I said that operation depends upon the integrity of the earth electrode (including surrounding area) and the connection to the "E" terminal of the ELCB. So long as nothing else provides a parallel path to the electrode conductor or to within the resistance area of the reference electrode, then the ELCB will do its intended job properly. But there seems to be a lot of confusion these days with people thinking that a parallel earth path on the "F" side of ELCB will result in the circuit-breaker not functioning correctly. That's why I said I feel that the voltage-operated ELCB seems to be regarded in an unfairly bad light these days.

 

[JohnW2]

But there seems to be a lot of confusion these days with people thinking that a parallel earth path on the "F" side of ELCB will result in the circuit-breaker not functioning correctly.

Indeed, and as I wrote to Bernard earlier, that's because most 'these days' people don't seem to undertsnad what "functioning correctly" means/meant for a voltage-operated ELCB, because they are thinking of it as a device (like an RCD) designed to respond to a certain fault current - which it obviously never was.

In fact, something which probably confuses '21st century people' more than most things is that in a TT installation with a VOELCB, it was the ELCB coil itself which represented by far the largest component of the Zs of any circuit in the installation - in the absence of any parallel paths limiting the 'L-E' fault current to around 0.5A. I can but presume that Zs measurements in such installations were not undertaken in those 'old days', since they would have been pretty unhelpful!

Kind Regards, John

 

[EFLImpudence]

Like the csa of 80A tinned copper fuse wire is about 2.5mm²...

Very interesting.

I think that's good enough for me.
So, If we derate the CCC by 50% for buried in insulation, that gives a fair idea of the conductor's true capability.

That'll mystify the young ones.

 

[Paul_C]

I can but presume that Zs measurements in such installations were not undertaken in those 'old days', since they would have been pretty unhelpful!

Indeed, there were two distinct tests specified by the 14th edition:

E.4 Where earth-leakage protection relies on the operation of fuses or excess-current circuit-breakers the contractor or other person responsible for the work shall test the effectiveness of the earthing of each completed installation, or major alteration, by means of an earth-loop-impedance test in accordance with Item (2) of Appendix 6.

E.5 Where earth-leakage protection relies on the operation of of an earth-leakage circuit-breaker, its effectiveness shall be tested in accordance with Item (3) of Appendix 6.

NOTE.- Where it is desired to measure the resistance of the earth electrode the test given in Item (4) of Appendix 6 should be adopted.

The item (4) earth electrode test is the normal three-electrode method.

The test in item (3) referred to consists of applying a voltage not exceeding 45V to the earth to check that the ELCB trips. A voltage-operated ELCB tester was a common instrument, either as a standalone tester or incorporated with a loop-impedance tester. It typically allowed selection of the voltage in steps of 10, 20, 30, 40V etc.

The notes for this test even include the following comments along the lines of what's already been stated above:

When, in accordance with the requirements of Regulations D.10-13, cross-bonding to other services is carried out in an installation where a voltage-operated earth-leakage circuit-breaker is provided, this may result in a direct earth connection of low impedance being introduced in parallel with the path through the earth-leakage circuit-breaker. Thus an increased value of earth-leakage current would need to flow before the circuit-breaker came into operation, and in some instances the cross-bonding may well remedy the deficiency which originally gave rise to the need for the earth-leakage circuit-breaker. These effects are not harmful; it is common practice to use the best solid earth available (e.g. suitably spaced earth electrodes) in parallel with an earth-leakage circuit-breaker. The circuit-breaker would still be effective in preventing the exposed metalwork of the installation from rising to a dangerous voltage.

 

[JohnW2]

Like the csa of 80A tinned copper fuse wire is about 2.5mm²...

Very interesting. I think that's good enough for me.
So, If we derate the CCC by 50% for buried in insulation, that gives a fair idea of the conductor's true capability. That'll mystify the young ones.

Do you mean 'insulation' (and sheathing) of the conductors or thermal 'building' insulation in which the cable might be buried?

It's quite a long way from a bare single fuse wire in air to T&E cable - I wouldn't be surprised if we needed to de-rate by more than 50%, even for the least demanding of installation methods. However, in the other direction, as we all know (and many benefit from), an 80A fuse can sustain far more than 80A for long periods without harm (although I don't know what temperature it attains under such circumstances). With unknowns in both directions, I'm not sure that we are really 'there' yet

I'll stick my neck out and hazard a wild guess that 2.5mm² T&E, clipped direct to something fairly solid, could probably sustain at least 50-60A indefinitely without getting seriously hot or coming to any particular harm. I may, of course, be woefully wrong. Is the eperimentally-minded man with the welder listening?

Kind Regards, John.

 

[EFLImpudence]

Do you mean 'insulation' (and sheathing) of the conductors

Yes. I was thinking that an insulated and sheathed conductor woul be no more adversely affected than a cable surrounded by house insulation.

 

[JohnW2]

Do you mean 'insulation' (and sheathing) of the conductors

Yes. I was thinking that an insulated and sheathed conductor woul be no more adversely affected than a cable surrounded by house insulation.

Fair enough; I imagine that's a good a guess as any. Now where's that man with the welder?

Kind Regards, John.

 

[Stoday]

I'll stick my neck out and hazard a wild guess that 2.5mm² T&E, clipped direct to something fairly solid, could probably sustain at least 50-60A indefinitely without getting seriously hot or coming to any particular harm.

I agree with your guess, JW. As BAS pointed out, 70°C is seriously hot — burning hot indeed.

Which calls into question which table to use for voltage drop calculations? I think your guess supports my view that the 20°C table is more appropriate than the 70°C one. If the cable's run at full load, the conductor temperature is not going to be anywhere near 70°C.

 

[JohnW2]

I agree with your guess, JW. As BAS pointed out, 70°C is seriously hot — burning hot indeed.

Agreeing with guesses is, well, somewhat guessy -)), but it's nice to know that we're thinking in similar ballparks.

Which calls into question which table to use for voltage drop calculations? I think your guess supports my view that the 20°C table is more appropriate than the 70°C one. If the cable's run at full load, the conductor temperature is not going to be anywhere near 70°C.

I think you're right. Indeed, I don't think I disagreed when you raised this before - but, rather, pointed out that when one sees voltage drop discussions going on (in this and other forums/fora), it nearly always seems that people are using the Table 4D2B figures, perhaps not realising that they relate to a conductor temperature of 70°C.

Mind you, as I did say before, whilst I would agree that 70°C is nearly always going to be too high, I also think that 20°C is probably too low (if one wants to be conservative/cautious), since ambient temperature will often be constantly at least 20°C and sometimes well over 30°C. Given that, and the fact that the current is bound to cause some temperature rise, I think I'd be inclined to utilise a figure of at least 40°C.

Kind Regards, John.