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I just thought of something

ebee
I know it`s many years ago that I queried the age old saying that was taught in college etc as to the r1 & r2 cross connection to form a double loop and the statement was made that this gave the exactly the R1 + R2 reading of the whole ring when taken from any point on the ring.

My statement was that this statement was not quite right and the word "exactly" needs substituting with "substantially" (I think the error was about 6% which as 6% of an already small number was not a great worry and it was still a very good approximation fit for use).


Anyway to add to that,,it just occurred to me . If we leave connected and test at a spur then it adds the spur value to the (nearly) ring value so that`s usually OK too.

However that`s only for spurs near to ring midpoint.!

If we had a spur nearer to one ring end than to midpoint it would therefore give a missleadingly large R1 + R2 value.

Not normally an issue but in extreme cases too pessimistic and causing a headscratch.


Off course field errors and instrument errors give missleading readings too.


I`d say once we done the fig 8 for the ring we should really connect ring ends together then test R1 + R2 from ring origin to each spur end to get as truer reading.


I know, I should get out more


?
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    If we had a spur nearer to one ring end than to midpoint it would therefore give a missleadingly large R1 + R2 value.

    Agreed.

    I`d say once we done the fig 8 for the ring we should really connect ring ends together then test R1 + R2 from ring origin to each spur end to get as truer reading.

    But perhaps only necessary if the original figures were too high to show that it was satisfactory. We don't necessarily have to know a precise R1+R2 figure for every point - just be able to confirm it's within an acceptable range.


       - Andy.
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    Yes, Guidance Note 3 uses the word "substantially". I still struggle to see how a simple ring final can be wired wrongly so as to cause danger.  "The test results show if the ring has been interconnected to create an apparently continuous ring circuit, which in fact is broken or connected as a "figure of eight"." A difficult concept to visualise.


    Z.
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    A cross-connected ring is easy to achieve for a diyer whom, upon lifting a floorboard below a socket, sees 2 x cables going to it and bridges them out in the mistaken belief that by doing so he is maintaining an actual ring. I am doing no more fault finding on stuff like this, but merely dropping the mcb rating from 32 to 20 and have done with it.
  • 0
    whjohnson:

    A cross-connected ring is easy to achieve for a diyer whom, upon lifting a floorboard below a socket, sees 2 x cables going to it and bridges them out in the mistaken belief that by doing so he is maintaining an actual ring. I am doing no more fault finding on stuff like this, but merely dropping the mcb rating from 32 to 20 and have done with it.


    So, if I understand you correctly the two ring cables going to a socket are "bridged out". I can not visualize this situation please explain further.


    Z.


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    "But perhaps only necessary if the original figures were too high to show that it was satisfactory. We don't necessarily have to know a precise R1+R2 figure for every point - just be able to confirm it's within an acceptable range."


    Agreed Andy.

    That was my point. An attempt to cut out the occasional misleadingly high reading. It`s only taken me a few years to even consider it
  • 0
    Zoomup:

    Yes, Guidance Note 3 uses the word "substantially". I still struggle to see how a simple ring final can be wired wrongly so as to cause danger.  "The test results show if the ring has been interconnected to create an apparently continuous ring circuit, which in fact is broken or connected as a "figure of eight"." A difficult concept to visualise.


    Z.


    Yes I noticed the word "substantially" a short while after my post. I`m not claiming they considered my humble observation though.


    Indeed on my point I raised with them about bathroom zones being from finished floor level rather than from bath pan floor level they threw me back, even though I pointed out that I`d seen bath legs raised on plynths a couple of times. Just a humble electricians viewpoint I`m afraid


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    Zoom, I am talking about a ring within a ring - a o being turned into an 8.

    Also, how many of you take the time and trouble to remove the socket fronts in the event of high readings and apply test prods directly to the cable ends?

    How many times have you experienced directly lower readings having done this?.I have lost count of the number of socket outlets which either have worn switches or oxidised plug contacts which have resulted in false positives which suggest high resistance ring faults.
  • 0
    So, if I understand you correctly the two ring cables going to a socket are "bridged out". I can not visualize this situation please explain further.


    Well, imagine a ring feeding a thin corridor - leg A feeds one side, lets call those  sockets 1, 3,, 5 and leg B feeds the other side, lets call those  sockets 2,4,6.  5 and 6 link at the far end.

    All is  well with the world. now imagine that the room is redecorated or in some way renovated and 'plink' some cables get drilled, cut or sockets are moved for wallpaper and the free ends now drop below the floor, pairs from socket 4 and 3 perhaps.

    In the faff of fishing to reconnect , as a first go, cables are linked incorrectly and now we have two a short rings, one with no supply.  But the reason may not be realised.

    123 work but 456 are dead, maybe it does not seem to reach and a junction box is added.

    To fix this some re-linking is done, and now we have a ring with a cross-bar in the middle, so all sockets work  most still have 2 wires , but a couple of sockets now have 3 wires, or there is a JB under the floor.

    It will pass a 'ring round' test from each socket, so you may think you have ring. This is where the fig8 test comes in,


    Oddly as I type this I know I have some artefact that  must be something a bit like this in my own house, but not exactly where, and I have simply convinced myself it can wait until the bedroom floor comes up - there is a bed side socket with a sort of bypass via the bedside socket in the other room, in that you can take either one socket off, and the ring remains continuous, take both off, and it does not. Somewhere there is a JB or 2 I have not yet found. I've only lived here 16 and a bit years,...


    Now  a fig 8 is not in the onsite guide,  but like the lolliop or the ring with a fat spur with more than one socket, it may be fine - it is in a sense a lollipop with the feeder radial replaced by another pair,  but to work out if the cable lengths and socket locations means it is OK or not is probably harder work than fitting a 25 A MCB or rewiring it as a proper ring.


    The really bad one is the 'crab claws' or 2 radials where it probably is best to step down to 20A and call it a center fed radial.

    Mike.
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    PS in my opening statement second to last line my ref to a fig 8 might be misleading. I was refering to the r1 r2 crossover test to give R1 + R2 figure rather than the Fig 8 bridged ring situation which it is designed to find (amongst other things).


    Makes me think that one of our more knowlegable members should sketch, rings, fig 8s, crab claws, lollipops, 3 & 4 leaf clovers, trees etc etc.

    Could make a small book in its own right ?


    Do any of us make a habit of doing the crossover test to see if we`ve absent mindedly done a fig 8 or do we just do the test on an existing circuit by virtue of thinking "I would never do anything so daft!" ?
  • 0
    mapj1:
    So, if I understand you correctly the two ring cables going to a socket are "bridged out". I can not visualize this situation please explain further.


    Well, imagine a ring feeding a thin corridor - leg A feeds one side, lets call those  sockets 1, 3,, 5 and leg B feeds the other side, lets call those  sockets 2,4,6.  5 and 6 link at the far end.

    All is  well with the world. now imagine that the room is redecorated or in some way renovated and 'plink' some cables get drilled, cut or sockets are moved for wallpaper and the free ends now drop below the floor, pairs from socket 4 and 3 perhaps.

    In the faff of fishing to reconnect , as a first go, cables are linked incorrectly and now we have two a short rings, one with no supply.  But the reason may not be realised.

    123 work but 456 are dead, maybe it does not seem to reach and a junction box is added.

    To fix this some re-linking is done, and now we have a ring with a cross-bar in the middle, so all sockets work  most still have 2 wires , but a couple of sockets now have 3 wires, or there is a JB under the floor.

    It will pass a 'ring round' test from each socket, so you may think you have ring. This is where the fig8 test comes in,


    Oddly as I type this I know I have some artefact that  must be something a bit like this in my own house, but not exactly where, and I have simply convinced myself it can wait until the bedroom floor comes up - there is a bed side socket with a sort of bypass via the bedside socket in the other room, in that you can take either one socket off, and the ring remains continuous, take both off, and it does not. Somewhere there is a JB or 2 I have not yet found. I've only lived here 16 and a bit years,...


    Now  a fig 8 is not in the onsite guide,  but like the lolliop or the ring with a fat spur with more than one socket, it may be fine - it is in a sense a lollipop with the feeder radial replaced by another pair,  but to work out if the cable lengths and socket locations means it is OK or not is probably harder work than fitting a 25 A MCB or rewiring it as a proper ring.


    The really bad one is the 'crab claws' or 2 radials where it probably is best to step down to 20A and call it a center fed radial.

    Mike.


    The chances of the corridor ring scenario is most unlikely to happen in real life. Almost impossible I would say. And if it did, which I doubt very much, we would have a ring comprising sockets 1, 3, 4 and 2. Also a ring of 3, 5, 6 and 4.


    Electrically this is not unsafe regarding current carrying capacity of conductors or earthing. Also if the 5 and 6 link was left out no problem would exist, they would just become two unfused spurs.


    An experienced electrician would notice that something was wrong and would not link sockets 3 and 4. Even a D.I.Yer having read his Readers Digest book of Home Electrics would not make this silly mistake.


    Z.




     


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