Sure, that too. I've often expressed surprise that some similar practice hasn't been adopted in various safety-critical situations, such as drug doses - when 'missed decimal points' are far from unknown, and can be catastrophic.
Kind Regards, John
Why we pay professional, qualified, registered sparkies
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Kind Regards, John
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That's the version suitable for broadcasting before 9pm
OK, so are you old and/or wise enough to apply it to this:
Kind Regards, John
Ooops I will edit.
I will edit.Sponsored Links
Body and end colour, dot for multiplier
colours still the same
I guess tolerance wasn't invented then..
colours still the same
I guess tolerance wasn't invented then..
Indeed ... so the answer is?
I think it was, actually, but I'm not sure whether what one can see at the 'other end' is a silver marking or just solder and general ageing!
Kind Regards, John
Fair enough (very easy mistake - we've all done it!), but I'm noit sure that you are completing off the hook! Having now got "As PSCC will be higher than or the same as the PEFC ...." correct, can you explain the logic behind "...it is not necessary to record it."?Ooops
If PSCC were appreciably higher than PEFC, I would have thought that would have been a good reason for recording it.Kind Regards, John.
Yes, I have got a bit tangled haven't I?
I started by writing in reply to Eric's point -
As (R1 + Rn) will be lower than or the same as (R1 + R2) it is not necessary to record it.
meaning, of course, that it is bound to be of a value to operate the opd under fault conditions.
Then I thought I would change to 'PSCC' and 'PEFC' but did not remember to alter 'lower' and of course PSCC IS recorded as PFC although (R1 + Rn) is not.
Is that better?
I'd better go to bed.
I started by writing in reply to Eric's point -
As (R1 + Rn) will be lower than or the same as (R1 + R2) it is not necessary to record it.
meaning, of course, that it is bound to be of a value to operate the opd under fault conditions.
Then I thought I would change to 'PSCC' and 'PEFC' but did not remember to alter 'lower' and of course PSCC IS recorded as PFC although (R1 + Rn) is not.
Is that better?
I'd better go to bed.
Sort-of. I think the problem was that your one correction brought about the need for further changes/corrections!
Agreed. You perhaps should have settled for that
Indeed.
Yes
Kind Regards, John
We use lower case r1, r2 and rn to denote end to end resistances of the ring, capital R1, R2 and Rn are used for radials or a completed ring.
For 2.5mm with a 1.5mm CPC the r1 should be the same as rn and the r2 should be 1.67x higher than r1 or r2.
With L and E interconnections in place the R1+R2 value should be (r1+r2)/4 for any socket on the ring, a spur will be higher and the highest is the recorded R1+R2 value.
Similarly, with L and N interconnections in place the resulting resistance at each socket should be (r1+rn)/4.
The PFC (greater of PEFC and PSCC) needs to be recorded for the distribution board, it is more to do with the protective devices ability to safely disconnect under fault conditions. The maximum EFLI (Zs) of each circuit deals with the circuits ability to disconnect in time.
For 2.5mm with a 1.5mm CPC the r1 should be the same as rn and the r2 should be 1.67x higher than r1 or r2.
With L and E interconnections in place the R1+R2 value should be (r1+r2)/4 for any socket on the ring, a spur will be higher and the highest is the recorded R1+R2 value.
Similarly, with L and N interconnections in place the resulting resistance at each socket should be (r1+rn)/4.
The PFC (greater of PEFC and PSCC) needs to be recorded for the distribution board, it is more to do with the protective devices ability to safely disconnect under fault conditions. The maximum EFLI (Zs) of each circuit deals with the circuits ability to disconnect in time.
Afaict phase-neutral fault current will usually be higher than phase-earth fault current because earth wiring is usually higher resistance than neutral wiring but there are a couple of scenarios where PEFC could be higher.
1: A bad neutral connection.
2: An installation with significant parallel earth paths (supplementary bonded structural steelwork, conduit/trunking systems, systems where several submains run paralell to each other and have their earths linked at the far end) .
1: A bad neutral connection.
2: An installation with significant parallel earth paths (supplementary bonded structural steelwork, conduit/trunking systems, systems where several submains run paralell to each other and have their earths linked at the far end) .
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This to me is the problem. Using 1.67 is all well and good when twin and earth of the same type is used throughout and there is no bonding to the earth used on the ring.
However to find a house with full plans showing exactly what is connected to what and what cable is used is rare and we get sheds taken from FCU's of the ring with SWA and bonding taken to pipes etc. So to compare the line or neutral impedance to earth impedance just does not work.
To take a socket and really does not matter too much which one and record the readings for that socket when the installation is in good order means in the future one can compare those readings to see quickly if anything has gone wrong.
However having said that I found with my own house the RCD trips when trying to measure the loop impedance on one ring yet does not trip with same make RCD on the other circuit so although theory is OK in practice with my house it's a case of using low ohm meter.
I do however have the readings for kitchen sockets on the ring the loop tester will test and once a year I retest and compare. At home reasonable vibration free never found a fault but at work mainly of things like batching plants I have high lighted faults before they caused a break down by simply measuring and recording the loop impedance.
Metal framed buildings do seem to have more problems than the domestic house with vibration causing terminals to become lose. However it would be hard to blame any one where they have become lose.
I did have one house with a problem with the telephones where another electrician had failed to find the problem. It did seem rather an odd fault so I started to measure the loop impedance. I got some strange readings so went to the consumer unit where I found every screw lose where the MCB's were connected to the bus bar. It would seem the consumer unit supplier had slotted the MCB's requested into place but not tightened the screws and the electrician fitting it had not read the notice saying check all screws for tightness and had wrongly assumed since already fitted all screws would be tight. With that case clearly the electrician was at fault. But cases where one can show something as plane as that are rare.
But again it was the loop impedance readings which alerted me to problem so had these been required on the paper work then it would have also alerted the other electrician to the problem.
OK, so are you old and/or wise enough to apply it to this:
Kind Regards, John
Hmm, more tricky.
24K 5% If that end band is gold. I think...
Indeed, and the version I was taught <cough> years ago can't now be said after 9pm eitherThat's the version suitable for broadcasting before 9pm
Looks like red-yellow-orange to me, that would make it 27k.OK, so are you old and/or wise enough to apply it to this:
And one that comes immediately to mind in a domestic environment would be a fault between line and external earth (metal pipework) in (say) the kitchen. Where said pipework has just entered the house under the sink and has a 10mm2 bonding cable back to the CU. The resistance of the earth in that case is likely to be significantly lower than the 1.5mm2 CPC in the T&E of the ring final.
Similarly, a line-earth fault in an immersion heater. The supply cable is likely to be longer, and the earth return longer too - but what's the CSA of some 22mm pipework (maybe 2off) leading back to a bonding point ?
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