If anyone wonders why I go to the expense of fitting Easi-joists, then here is the reason why. It's because I care about my fellow tradesmen.
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- Thread starter noseall
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I remember a housing estate in Rhyl, as normal at that time an electrician wired the first house in the estate, he decided where cables would run and what was required. The joists with the knock out holes were used, which meant the cables did not need to go 1/3 into the room before being threaded beam to beam.
After the design was ratified a semi-skilled guy was put to wire the houses and the electrician only returned to inspect and test.
The builder then decided the joists were too expensive and changed the design to use standard wood joists, the semi-skilled guy knew the 1/3 rule, but even if he did know the loop impedance requirements he had no way to measure the loop impedance, three houses were wired before one was tested with the new routing and it was found the loop impedance was exceeded.
Now came the arguments:-
Can the electrical firm alter their quote for the work now the work required has changed?
And what do you do with the three houses already wired?
At that time ELI for a 32A ring was 1.44Ω and the extended ring was showing 1.50Ω was that enough (pre-2008) to worry about anyway?
I think because the builder did not want to pay and the electrical firm was some what miffed by the extra time it takes to drill holes so profit margins were reduced they opted for fitting 20A MCB's instead of 32A and told the builder it was his fault.
I seem to remember the remaining houses had a side to side split rather than up/down stairs split to reduce the wiring required. This also at the time caused some arguments as to if people would expect the side to side rather than up/down split.
It was not long after that my son was laid off, not sure if due to general down size in building or due to arguments between builders and electricians. But the problem is still around, the builder saves money on joists, but then every other trade costs more.
After the design was ratified a semi-skilled guy was put to wire the houses and the electrician only returned to inspect and test.
The builder then decided the joists were too expensive and changed the design to use standard wood joists, the semi-skilled guy knew the 1/3 rule, but even if he did know the loop impedance requirements he had no way to measure the loop impedance, three houses were wired before one was tested with the new routing and it was found the loop impedance was exceeded.
Now came the arguments:-
Can the electrical firm alter their quote for the work now the work required has changed?
And what do you do with the three houses already wired?
At that time ELI for a 32A ring was 1.44Ω and the extended ring was showing 1.50Ω was that enough (pre-2008) to worry about anyway?
I think because the builder did not want to pay and the electrical firm was some what miffed by the extra time it takes to drill holes so profit margins were reduced they opted for fitting 20A MCB's instead of 32A and told the builder it was his fault.
I seem to remember the remaining houses had a side to side split rather than up/down stairs split to reduce the wiring required. This also at the time caused some arguments as to if people would expect the side to side rather than up/down split.
It was not long after that my son was laid off, not sure if due to general down size in building or due to arguments between builders and electricians. But the problem is still around, the builder saves money on joists, but then every other trade costs more.
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Well that would be the builder's fault for changing the design without agreeing it with other trades. As for the solution of having different layout of rings, sounds sensible to be as long as it's clear which circuits are which.
What was the voltage?
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I did a job with those knock out hole joists. The Bulgarian chippies threw up the joists, not knowing about the importance of the knock out holes. So they did not line up.
I didn't care, my cables can wiggle about, but the plumber was right pi55ed off!!
I didn't care, my cables can wiggle about, but the plumber was right pi55ed off!!
When we used to do a lot of houses, the knockout holes almost never lined up and when they did they wouldn't knock out so had to be drilled to avoid taking out a huge chunk.
That's surely not relevant, is it? Prior to 2015 (Amd3), the maximum Zs was calculated on the basis of the nominal 230V supply voltage (hence 1.44Ω for a B32), and since then has effectively been calculated on the basis of 218.5 supply voltage (230V x 0.95). At no time that I'm aware of has it been permissible to determine maximum Zs on the basis of what the actual supply voltage happened to be at some point in time (hence 1.37Ω for a B32).
... or am I perhaps missing the point of your question?
Kind Regards, John
Probably not.
I was just implying, with relation to Eric's figures, that 1.50Ω would be alright if the actual voltage was 240V or above although not compliant with the printed maximum figures for a nominal 230V (prior to Cmin.).
A remarkable coincidence that it was exactly 1.50Ω, though.
Fair enough, and I agree - but as you acknowledge, reg-wise it has never been permissible to have a Zs which would not be 'alright' at 230V (now 218.5V), even if it was 'alright' with the supply voltage as it was when measured.
Maybe, but that might suggest a degree of honesty! It would arguably be even more of a "remarkable coincidence" had the Zs been recorded as exactly 1.44Ω !
Kind Regards, John
I perhaps should have added that if one is being 'pragmatic', rather than strictly complying with regs, then it's worth remembering that (contrary to what some people seem to imply) nothing dreadful suddenly happens if the Zs of a circuit slightly exceeds the "printed maximum". If the Zs does slightly exceed that maximum, then the only consequence would be that the 'worst case' disconnection time would slightly exceed the (essentially arbitrary) 'required value'.
Kind Regards, John
I'm not convinced that there is not a lot of difference between "not quite arbitrary" and "essentially arbitrary"
Don't forget about the context. I was talking about disconnection times which very slightly exceeded the required figures because the Zs of the circuit slightly exceeded the calculated/tabulated 'maximum'. In other words, I was not talking about the difference between 0.4s and 0.6s, 0.8s or 2sec but, rather the difference between 0.4s and, say, 0.41s or 0.42.
I realise that there is a physiological basis underlying the required disconnection times, but how much precision do you think there is in the statement "0.4 seconds is roughly the maximum time that 95% of people should be exposed in a left hand to feet shock for a voltage of around 240 V a.c., with a low risk of irreversible physiological side effects, given average "dry" impedances of the human body, operating in a "TN" system" ?? Except, perhaps, in WWII Germany, there has never been any way of obtaining such data empirically in human beings, and even what data we have from animals (mainly pigs) is fairly limited. As for real-world experience of electric shocks, no matter what the outcome, we never have much of a clue as to what was the precise duration of the shock.
Even the above statement includes the word "roughly". Given the paucity of hard data, I would be extremely surprised if anyone could put their hand on their heart and say they were confident that the "maximum duration for 95% of people" (to avoid irreversible physiological effects) was closer to 0.4s than to, say, 0.3s or 0.5s (or probably even 0.2s or 0.6s). Hence, although it attempts to be physiologically based, I wouldn't personally say that the precise magnitudes of the adopted 'max disconnection times' are not much more than "essentially arbitrary"!
Although a side issue, one also has to remember that none of this is of any relevance (in terms of electric shocks) other than in the extremely rare event ('coincidence') of someone being unlucky enough to be in contact with a part at the very moment it becomes live due to a 'negligible impedance' fault. In reality, virtually all electric shocks due to such faults would be prevented by a disconnection time of 4 seconds (or even 4 minutes, or longer), let alone 0.4s !
I also must be missing something, because I've never understood the reason for the shorter permitted disconnection times in TT installations. I can't see how that can be physiologically-based. Even if it were precisely true that 95% of people would not suffer lasting effects from a shock whose duration were less than 0.4s, their body would know nothing about the nature of the earthing of the electrical installation, so why does the limit have to be 0.2s in a TT installation??
Ironically, given that TT installations are usually reliant on RCDs for fault protection, the disconnection times in the face of a 'negligible impedance L-E fault are likely to be very much shorter than with a not-RCD-protected circuit in a TN installation. Even at a current of 'just 5 x IΔn, the required operation time of an RCD is 0.04 seconds, so goodness only knows how rapidly it would operate at the ≥5,333 x IΔn it would experience with a negligible-impedance fault in a Zs-compliant 32A circuit!!
Kind Regards, John
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Maybe the 0.4 seconds is arbitrary in the first place, but for aB circuit breaker that's how quick the magnetic trip operates under 5x rated current. Any lower and the magnetic part is not guaranteed to trip, therefore waiting for the overload trip to kick in, which would be a lot later.
So once you're using an MCB, the decision on impedence is made for you, so don't think that slightly higher loop resistance would necessarily only give slightly slower disconnection.
So once you're using an MCB, the decision on impedence is made for you, so don't think that slightly higher loop resistance would necessarily only give slightly slower disconnection.
According to the time current graphs, anything less than 5x results in at least 12 seconds disconnection time.
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