Discuss Interconnected ring final circuits. in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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My head's wasted having spent two hours trying to trace an interconnection between two ring final circuits. I started nice and methodical and went downhill from there:bigcry::bigcry:....under pressure to get to the next job, I gave up, but am back on Thursday to nail it on the head.

So, how would you do it???
 
Well, yes, if all else fails, all into one mcb is possible, but not ideal. I'm trying to think of the best/most effective way of tracing the fault ( always assuming it's not a connection under the floor!).
 
Tricky!

Do you know which particular sockets are supposed to be connected to each circuit?


I'd start with removing one socket and splitting the conductors there, then identify which cable at the DB becomes 'free' from the interconnected rings. You should be left with three interconnected cables and one lonely one at the DB.
Then identify all of the sockets which are fed by the 'free' cable, mark them all with a bit of tape of one colour along with the cable at the DB and at the split socket.

Pick another socket from the ones not marked by tape to take off and split the conductors there, you should hopefully get another cable free of the interconnection and then identify all of the sockets which that cable feeds and mark with a different colour of tape.

You should start to build up a picture of where each leg of each ring runs, and start to narrow down the location of the interconnection.

It might even be helpful to draw a plan of the areas and mark the socket locations in different colours to identify which leg they are connected to.


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You could try doing a Zs at all the sockets on "both" circuits and start looking at the ones with "lower" readings.......
 
If you identify 1 outgoing conductor (leg) and connected this to the DB, are all 3 remaining disconnected conductors still live?
 
I'd start at the distribution board (if possible) isolated (safely, of course) and with every load unplugged (or FCU switched off).

I'd draw up a list of sockets per room (I usually number them clockwise from the entrance nearest the front door, to save confusion later, e.g. Bed1-1, Office-2, etc.). Plus a plan with these marked on.

At the DB, take one outgoing cable, connect L & N together on that cable only, then with a carefully nulled MFT, measure (R1+Rn) around the ring. Making sure you are getting a good connection to the cable, and not adding anything for a dirty switch or socket contacts. If you've time, open the face plates and measure directly onto the terminals.

With care, you can map out the electrical order of the sockets, generally they will flow sensibly round the floor plan. Note any that are spurs (if you open them up).

I'm avoiding measuring (R1+R2), in case of parallel paths (boiler, hob ignition connections etc).

Repeat for the other cables out of the DB.

Sometimes a couple of sockets are so close together electrically that it is hard to determine which order they are in. But you should be able to get a moderately accurate picture of the cabling, and narrow down the area of cross connection.

Of course the cross connection could be in a junction box under floorboards (is this a domestic property?) rather than at a socket - e.g. a later alteration by someone who didn't understand the wiring properly (or at all).

Edit: If you have a "spider"-type ring where every socket is essentially a spur from a junction box under the floor boards, this method won't be so useful
 
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I would start by splitting each ring circuit in half, one circuit at at time. too see witch half of the leg is okay.Then concentrate on the half/ 2 half's that's affected. Murdoch's suggestion is worth a try too.
 
Here is a possible way for identifying the points on each ring which are bridged together without first removing all the front plates and disconnecting socket connections.
FIRST TURN OFF ALL POWER AND OPEN ALL CIRCUIT BREAKERS AND PROVE DEAD ON ALL SOCKETS TO BE TESTED.
You will need:
a. Digital voltmeter(DVM).
b. 5 x 50Watt halogen lamps connected in parallel.
c. 32Amp circuit breaker connected in series with the 5 lamps
d. 12Volt dc supply from a car battery.
e. Connector strip, paper and pen.
f. A 13Amp plug with short length of flex from L and N to a connector strip into which are inserted the probes of the DVM.
Connect items b, c and d in series and check illumination of lamps and control by circuit breaker. Now sever one conductor of this circuit and put a connector on each of the free ends.
My terminology: Ring1 and Ring2. Ring1Brown1 and Ring1Brown2 are the line conductors of Ring1. So, Ring2Brown1 and Ring2Brown2 are the line conductors of Ring2. Mark up the conductors somehow.
I assume you have completed the standard RFC checks and the only fault is a link between Ring1 and Ring2.
Procedure
1. Disconnect and separate Ring1 and Ring2 line and neutral conductors.
2. Connect the lamp rig in series with Ring1Brown1 and Ring1Brown2.
3. Connect Ring1Blue1and Ring2Blue1 with Ring2Brown1.
4. Turn on circuit breaker to circulate circa 20Amps through Ring1 line conductors. A voltage gradient will exist around this path. No current is flowing in Ring2's conductors nor along the bridge between Ring1 and Ring2. Thus, Ring2 is at an equipotential with Ring1 at the point of the bridge with Ring1.
5. Insert item f, the plug and DVM, into each socket in turn. Procedure step 3 above provides a convenient way of connecting the DVM to the potential of Ring2's line conductor through the plug.
6. Using the paper and pen mark up for each socket the polarity and voltage of the DVM reading eg: +, - or 0 and voltage reading.
7. Disconnect and now repeat the procedure steps 5 and 6; this time put the lamp test rig in series with Ring2's line conductors and connect Ring1Brown1 with Ring1Blue1 and Ring2Blue1.
8. If there is only one bridge between Ring1 and Ring2 only two sockets can be at equipotential (and of course along any spur from each of them). So those sockets with a 'double 0Volts' are at equipotential and thus bridged, all the others with be a marked either 0 and + or 0 and - with a voltage figure.
This method might work to help you home in on the problem and then decide on what to do about it.
 
Well, another two hours testing, and.......the interconnection is under the floor. It's a mid floor flat with laminate flooring throughout and the landlord is not for it being lifted.
Marconi, I didn't have time to try out your suggestion (but I'm going to try it in a "test rig" scenario, as I'm intrigued).
Thanks all for your advice :clap:
 
Hi folks, sorry, could not access the site for the past couple of days. Final solution was to create one "ring final circuit...with a twist" and down-rate to 20A. Not ideal, but all the landlord was willing to go for.
Interconnection noted on the EICR.
 
Hi folks, sorry, could not access the site for the past couple of days. Final solution was to create one "ring final circuit...with a twist" and down-rate to 20A. Not ideal, but all the landlord was willing to go for.
Interconnection noted on the EICR.

If its 2 interconnected 32A rings, why was it necessary to down rate to 20A or have I missed something?
 
If its 2 interconnected 32A rings, why was it necessary to down rate to 20A or have I missed something?
If there is an interconnection then one or more legs may potentially be overloaded as it is no longer truly a ring designed not to have excessive load near either end. I wouldn't be happy leaving it on a 32A device in those circumstanes even though it is unlikely to present a problem.
 
If its 2 interconnected 32A rings, why was it necessary to down rate to 20A or have I missed something?

That would be one hell of a mathematical headache to try and prove that it was safe to put on a 32A supply.
It is a deviation from the standard circuits of bs7671 whatever you do with it so you need to be able to prove that it is safe.

If interconnected rings (within a single circuit, so a figure of 8) were safe on a 32A supply then we wouldn't need to carry out tests to ensure that they don't exist.
 
Risteard & Dave have both set out my reasoning for down-rating the ocpd. Not particularly happy to have all the sockets on a 20A, but it was that or a Code 2:sad_smile:
 
Code 3 and the interconnection (and the need for 20A ocpd) recorded in the comments section. Did think about no Code, but felt it could (ideally) be improved.
No arguments please - I'm the inspector and coding is at my discretion :boxing_smiley:
 

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