As secure has indicated, that regulation appeared about 35 years ago. As he said, prior to that one could have two single sockets (but still only one double socket) on an unfused spur.
Kind Regards, John
Weird result determining order of sockets on ring circuit
- Thread starter jackhaughton
- Start date
Kind Regards, John
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I got that wrong, didn't I??
The difference between the two readings when joined at the far socket is 0.09Ω (90mΩ) and joined at adjacent socket may be 0.009Ω (9mΩ). Seems too low anyway.
However, when joined at the far socket, the readings from the two adjacent sockets should be the same regardless.
So, I would still put it down to the number of contacts.
Try wiggling the plugs and flicking the switches on and off. See if the readings vary.
I very much doubt that it truncates, rather than rounds. However, the manual of the OP's Di-Log 9083P MFT (which appears to be very expensive!) indicates that the accuracy for resistance ('earth continuity') measurement is ±2% ± 5 digits. That means that a resistance of 0.0090Ω could display as anything between 0.00Ω (negative readings being invalid) and 0.06Ω. Perhaps more to the point a very low resistance (approaching 'zero') could display as anything up to 0.05Ω (i.e 50 mΩ) - or, even more to the point, a resistance as high as 50 mΩ could display as 0.00Ω.
Incidentally, the (much cheaper) Fluke 1652's that you and I have specify an accuracy of ±1.5% ±3 digits for resistance ('continuity') measurement - so that the maximum reading we would expect to get when measuring a (true) 0.0090Ω resistance would be 0.04Ω.
Kind Regards, John
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...and vice versa? Actual 0.06Ω reads as 0.00Ω.
So, all readings could be 'correct'.
Crumbs, I hadn't noticed the price.
Indeed - that's the main thing which seems to be wrong - regardless of consdierations of meter zeroing, meter accuracy/rounding etc.
Possibly - although if switch/pin contacts were introducing a resistance as high at 90 mΩ, that would dissipate about 15W at 13A - which I suspect might well have 'shown itself' (if loads that high had ever been applied).
Kind Regards, John
A more useful way of testing would be to remove both ends of the ring from the circuit breaker, connect L&N together at one end and leave the other end open.
Measure resistance between L&N at each socket outlet, those with higher resistance are further towards the open end.
If the sockets are old and worn, results may vary considerably so use probes on the back of the socket instead.
This avoids using the dubious shorted plug, and it's L&N to eliminate the possibility that some of the ring has a 1mm² CPC rather than the more recent 1.5mm².
Measure resistance between L&N at each socket outlet, those with higher resistance are further towards the open end.
If the sockets are old and worn, results may vary considerably so use probes on the back of the socket instead.
This avoids using the dubious shorted plug, and it's L&N to eliminate the possibility that some of the ring has a 1mm² CPC rather than the more recent 1.5mm².
I've qualified that as OP says they are on a spur.
You seem to have replied before added my edit - but, almost - anything up to actual 0.05Ω could display as 0.00Ω (0.05 minus 2% = 0.049, which rounds to 0.05, minus "5 digits" = 0.00)
Yes, that was my point.
Quite so.
Kind Regards, John
Assuming it really is a ring, then even if that were the case, it did not ought to result in different measurements from a distant point to two 'very close' sockets. Given that we are apparently actually talking about a spur, only a substantial resistance between the two sockets could cause that.
However, as recently discussed, since the OP's meter has a ±5 digits accuracy (i.e. ±50 mΩ), the OP is pushing the capabilities of the test equipment to (if not beyond) its limits by using it for the purpose he is using it for.
Kind Regards, John
I've found that testing resistance like that can be "quite inconsistent" - just pulling the plug out and re-inserting it can make a surprising difference to the value. Previously I;d tried to do that to find a fault in the flat - and the readings just weren't "reliable" enough do what I needed.
In my house, when I was doing the very same exercise, I put around 4A DC through the L wires (L simply because they were the easiest to access and separate in the CU). 4-5A is easy to obtain with a 12V battery and car headlight bulb (or spotlight, or ...).
I then used a multimeter to measure the voltage at each socket, using the E or N (without current passing through it) as a reference and linked to one end of the L ring.
Doing it this way means that you get large enough readings to be fairly reliable, and the voltage read by a typical DMM is going to be pretty well unaffected by slight resistance in the socket.
For spurs, you will get a reading identical to one of the sockets if it's spurred from the back of the socket. If you get a reading that's in between two sockets, then either you've missed one or there's a JB somewhere - but unless you also do visual inspection you'd not know you were on a spur. You'll need to do other tests (such as your resistance test) to determine which socket is the spur, and which is on the ring.
In my house, when I was doing the very same exercise, I put around 4A DC through the L wires (L simply because they were the easiest to access and separate in the CU). 4-5A is easy to obtain with a 12V battery and car headlight bulb (or spotlight, or ...).
I then used a multimeter to measure the voltage at each socket, using the E or N (without current passing through it) as a reference and linked to one end of the L ring.
Doing it this way means that you get large enough readings to be fairly reliable, and the voltage read by a typical DMM is going to be pretty well unaffected by slight resistance in the socket.
For spurs, you will get a reading identical to one of the sockets if it's spurred from the back of the socket. If you get a reading that's in between two sockets, then either you've missed one or there's a JB somewhere - but unless you also do visual inspection you'd not know you were on a spur. You'll need to do other tests (such as your resistance test) to determine which socket is the spur, and which is on the ring.
I have to say that when, after decades of measuring usually much higher resistances, I was initially rather amazed when I first discovered that electricians were routinely measuring resistance from an ohm or three, and down to well under an ohm, and were making decisions based on differences of less than one ohm. My experience had been (and essentially remains) that when I was measuring low or very low resistances, just 'wiggling around or re-adjusting the test probes, clips or whatever could easily result in the 'measured' resistance varying by 'an ohm or three'.
Kind Regards, John
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Would it not be better to measure line - neutral rather than use the earth which may be used to bond so may give odd reading?
I used the loop impedance tester measuring line - neutral with some it auto measured line - neutral when set to the prospective short circuit current. Cheap ones you have to make up a test lead.
Be it loop or resistance the readings will not tell you which side of the centre point the socket is, but normally you can work that out by layout of the house.
I seem to remember there was an instruction set in book 4 saying how to test sockets to ensure you did not have a figure of 8. I also remember on this forum arguing about if there was any reason not to have a figure of 8. From what I remember it's down to where on the ring the figure of 8 joins, at the centre of the ring it is not really a problem, but near to the consumer unit it can cause too much current to be drawn on one leg.
I used the loop impedance tester measuring line - neutral with some it auto measured line - neutral when set to the prospective short circuit current. Cheap ones you have to make up a test lead.
Be it loop or resistance the readings will not tell you which side of the centre point the socket is, but normally you can work that out by layout of the house.
I seem to remember there was an instruction set in book 4 saying how to test sockets to ensure you did not have a figure of 8. I also remember on this forum arguing about if there was any reason not to have a figure of 8. From what I remember it's down to where on the ring the figure of 8 joins, at the centre of the ring it is not really a problem, but near to the consumer unit it can cause too much current to be drawn on one leg.
Surely the biggest problem with a figure of 8 circuit is that unless you know it is there and where to disconnect the middle link, it's much much harder to get meaningful test results - or put another way, faults could be harder to spot.
As to currents, in the absence of faults, I would have thought a figure of 8 would reduce the ability to overload one leg of the ring. With a normal ring, you need a lot of load very close to one end - so that the resistance of "most of the ring" results in the current mostly going down the short leg. With a figure of 8, the additional link will reduce the resistance "the rest of the ring" resulting in less imbalance in current flows when there is a large load close to one end.
As to currents, in the absence of faults, I would have thought a figure of 8 would reduce the ability to overload one leg of the ring. With a normal ring, you need a lot of load very close to one end - so that the resistance of "most of the ring" results in the current mostly going down the short leg. With a figure of 8, the additional link will reduce the resistance "the rest of the ring" resulting in less imbalance in current flows when there is a large load close to one end.
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