Oh - that's a bit further than I had expected. If that's the case, then the houses at the other (closer) end presumably must have very low impedance TN-S earths - lower than many a TN-C-S one.
Kind Regards, John
B-curve vs C-curve
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Kind Regards, John
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The idea of the magnetic part of the trip is to disconnect very fast 0.01 seconds I think is memory serves me correct, under fault conditions, with a RCD this will disconnect within 0.03 seconds with a line - earth fault, so in real terms looking at line - neutral faults.
Although line - neutral could be measured in impedance Ω, it is normal to quote as prospective short circuit current A, and a B = 3 to 5 times thermal tripping current, C = 5 to 10 times thermal tripping current and D = 10 to 20 times thermal tripping current. So a C16 has the same tripping current as a B32 MCB. And the PSCC will need to be in excess of 160 amp, not hard to satisfy, but with a C32 looking at 320 amp this gets harder to satisfy.
It really does not matter if TN or TT supply, we want a supply with under the PSCC rated on the MCB's at the MCB's and over the PSCC that the MCB will trip at measured at the end of the circuit. So less than 4.7kA at consumer unit and more that 400A at end of circuit.
I have found some makes of reduced low voltage transformers do have a high in rush, with the same MCB plugged in close to consumer unit they will trip supply, but at centre of ring final or end of radial they will hold, the resistance of the supply cable is enough to reduce inrush current. So normally if they trip the supply then the supply has a high enough PSCC to allow the use of a C or D type. However we still need to measure, and a impedance or PSCC meter is not cheap, also they seem to be the first meter in the set to go out of calibration, I have not had a low impedance ohm meter go far out, or an insulation tester, but have had 3 impedance meters go well out of calibration, not talking about failing, had many meters fail, but talking about calibration.
I had a socket at home I knew the earth loop impedance was 0.21Ω and at start and end of a day where I would be using the impedance meter I would test it in that socket, better than finding out 10 inspections down the line, then one day it altered it was 0.28Ω, my son also did same as me, so went up to his house and checked it, one on his socket and two compared to his meter. Nothing wrong with meter, on retuning to my house noticed the road works, what was being done is anyone's guess, however the impedance is only guaranteed to be better than 0.35Ω with TN-C-S with a 100A supply, it is in real terms the point where they would have problems with volt drop. 230/0.35 = 660 amps approx, a 45A C type 450 amp so could in theory use one. However get it wrong and the supply fuse may pop before the MCB.
Although line - neutral could be measured in impedance Ω, it is normal to quote as prospective short circuit current A, and a B = 3 to 5 times thermal tripping current, C = 5 to 10 times thermal tripping current and D = 10 to 20 times thermal tripping current. So a C16 has the same tripping current as a B32 MCB. And the PSCC will need to be in excess of 160 amp, not hard to satisfy, but with a C32 looking at 320 amp this gets harder to satisfy.
It really does not matter if TN or TT supply, we want a supply with under the PSCC rated on the MCB's at the MCB's and over the PSCC that the MCB will trip at measured at the end of the circuit. So less than 4.7kA at consumer unit and more that 400A at end of circuit.
I have found some makes of reduced low voltage transformers do have a high in rush, with the same MCB plugged in close to consumer unit they will trip supply, but at centre of ring final or end of radial they will hold, the resistance of the supply cable is enough to reduce inrush current. So normally if they trip the supply then the supply has a high enough PSCC to allow the use of a C or D type. However we still need to measure, and a impedance or PSCC meter is not cheap, also they seem to be the first meter in the set to go out of calibration, I have not had a low impedance ohm meter go far out, or an insulation tester, but have had 3 impedance meters go well out of calibration, not talking about failing, had many meters fail, but talking about calibration.
I had a socket at home I knew the earth loop impedance was 0.21Ω and at start and end of a day where I would be using the impedance meter I would test it in that socket, better than finding out 10 inspections down the line, then one day it altered it was 0.28Ω, my son also did same as me, so went up to his house and checked it, one on his socket and two compared to his meter. Nothing wrong with meter, on retuning to my house noticed the road works, what was being done is anyone's guess, however the impedance is only guaranteed to be better than 0.35Ω with TN-C-S with a 100A supply, it is in real terms the point where they would have problems with volt drop. 230/0.35 = 660 amps approx, a 45A C type 450 amp so could in theory use one. However get it wrong and the supply fuse may pop before the MCB.
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I've had some very very low Zs's, on both -S and -C-S supplies.
0.10, 0.11. Even some that were 0.0X. We thought it was an equipment error, so we checked with 3 other testers: same.
0.10, 0.11. Even some that were 0.0X. We thought it was an equipment error, so we checked with 3 other testers: same.
One does hear oif the very low Ze figures ...
If the cable run from transformer to installation were 25 mm, then, by my calculations, for TN-C-S, 0.10Ω would correspond to a cable length (from transformer to installation) of about 57 metres, 0.05Ω to a cable length of about 29 metres and 0.02Ω to a distance of about 11.4 metres - Hence an Re of 0.1 or less is quite credible if the transformer is very close to the property.(all those figures assume a conductor temp of 70°C).
One certainly hears of PFCs of 1,500 A or more, which implies an Re of ~0.15Ω or less.
As for TN-S, I could only guess, since I have no accurate idea of the resistance of the cable sheaths (does anyone know?) - but it's not impossible that it might approach the TN-C-S figures.
Kind Regards, John
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I think that's a bit misleading. The specifications of Type B, C and D MCBs require that they trip at 3-5, 5-10 and 10-20 times their In, respectively. That's different from "3-5, 5-10 and 10-20 times their 'thermal tripping current' " (whatever you mean by 'thermal tripping current') - since the specification also requires that they do NOT trip thermally at currents <1.15 x In.
I'm not sure exactly what you mean by a "reduced low voltage transformer" (230V->110V, perhaps?) - but the 'inrush situation' would obviously have to 'look' pretty close to a ('zero impedance') 'short circuit' for the R1_Rn, even at the furthest point of a ring/radial circuit, to have appreciable impact on the inrush current.
Kind Regards, John
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OK I should have said "In" however was trying not to be too technical.
Yes talking about 110 volt well in case you pick me up on that 55 - 0 - 55 volt, we had some old ones when working for GEC large steam turbines and at first I thought they were faulty, within a few meters of the 11kv to 400 volt transformer they would trip the MCB, but with 100 meters of cable they did not trip. Even close try a few times and you would likely hit the null point and they would work.
Seen motors with resistor start, but not transformers.
Yes talking about 110 volt well in case you pick me up on that 55 - 0 - 55 volt, we had some old ones when working for GEC large steam turbines and at first I thought they were faulty, within a few meters of the 11kv to 400 volt transformer they would trip the MCB, but with 100 meters of cable they did not trip. Even close try a few times and you would likely hit the null point and they would work.
Seen motors with resistor start, but not transformers.
100 metres is, of course, quite a long cable by domestic installation standards but, even so, I remain a bit surprised - since, as I said, what you report implies that the (step-down to 110V) transformer must have bee presenting something approaching a 'sort circuit' to the supply at switch on. I would have thought that even the resistance of the tranny's primary would have avoided such a situation.
Kind Regards, John
But what would its resistance be? I would expect most of the impedance of a transformer to be reactance.
Of course, and that was really my point - that, although resistance is (presumably) a very small component of the impedance, if the situation is such that a difference in R1+Rn at different points in the circuit is able to produce appreciable difference in current (at start-up), that implies that the total effective impedance of the load during start-up would have to be lower than I would expect even the resistive component of the load to be.
As I said, I would have thought that the load would have to present itself as something approaching a ('zero impedance') 'short-circuit' at start-up to facilitate what eric was reporting - and I was suggesting that even 'the resistance alone' of the transformer's primary ought to be enough to preclude that - but maybe I'm wrong!
Somebody must have a yellow lump handy which they could measure. Mine's in the garage, and CBA is an apposite term.
I'm not sure that it is any 'yellow lump' or, rather, just the ones that eric encountered.
I suppose one has to accept that if one of those just (i.e. by the skin of it's teeth) tripped a B32 on switch-on when it was only 6 inches from the CU, then any increase in the loop impedance (no matter how little) could prevent that tripping. However, if it were not 'just' tripping, but appreciably over the trip threshold, then 100 metres of, say, 4mm² T+E between CU and lump would add only about 1.1Ω to the loop resistance, and therefore would only have an 'appreciable' effect on switch-on current if the impedance present by the 'lump' at that time was extremely small. However, as per my previous sentence, that extra 1.1Ω could be enough to prevent tripping if the switch-on current was originally only fractionally above the trip threshold.
Kind Regards, John.
Likewise here. I have not had an incandescent fail for maybe 6 years, but it was common for the B6 to trip when they did. I have also replaced all of the often on (mood lighting) lights with LED, but left brighter CFL's in the less used centre main lights. The rarely used ones, one between our two rear doors and an under stair cupboard are incandescents, so really not worth the effort of changing until they blow in thirty years. I cannot comment on whether a CFL or LED might cause a trip, I have yet to have any fail.
Had lots of CFLs give up the ghost - never took out the breaker.
Have had incandescents do that far more often than not, and I once had one take out a dimmer switch.
The most catastrophic failure though was of a magnetic induction lamp, which exploded, showering glass and molten solder.
Have had incandescents do that far more often than not, and I once had one take out a dimmer switch.
The most catastrophic failure though was of a magnetic induction lamp, which exploded, showering glass and molten solder.
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