Poor Crimping?

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Mad Inventor™
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I've been tidying up a ring circuit. 7 sockets run in 2.5mm T&E all in black and red. Various obvious issues to start with:

- Mixture of singles and twins - various makes and mix of metal and plastic face plates

- Back boxes loose, as in NOT screwed in, sat in massive holes in the wall, some packed with CARDBOARD

- No grommets

- No fly leads

- Wires short

- M3.5 threads stripped - some face plates on one screw only

- Wood screws in place of some of the M3.5s

I tested the circuit initially (calibrated Megger BM221 used) and end to end r2 was open circuit - one cpc was so SHORT in the back box it had not been connected to the faceplate. r1 and rn did NOT match anywhere near - probably as most L and N terminals were loose to some degree.

I have to date; replaced ALL the back boxes with doubles, properly set and screwed in. I've renewed 5No sockets to new 2G MK DP and kept 2No metal 2G metal ones. I have also extended a number of the short leads. This was done using a ratchet crimper with heat shrink butt crimps with another layer of heatshrink over the top as per the photos below:

Crimps:

Photo0237.jpg


Crimps insulated over:

Photo0303.jpg


Now going back and doing my end to end tests and I get:

r1 = 0.25ohms

rn = 0.25ohms

r2 = 0.61ohms

I was expecting something nearer 40ohms for r2 (as in cpc being x 1.67 the resistance). I suspect I have messed up on one or more of the cpc crimps somewhere. Does that look most likely?

Cross connecting the opposite ends of the ring (as per steps 2 & 3 in GN3) and then taking readings at each socket I get (all in ohms). Please note this was done direct to the terminals on the back on some and using a modded plug top "college special" on others. (I've just ordered a proper one.

           L/N            L/CPC

Skt 1 - 0.11           0.21

Skt 2 - 0.14           0.22

Skt 3 - 0.16           0.22

Skt 4 - 0.14           0.22

Skt 5 - 0.11           0.22

Skt 6 - 0.14           0.22

Skt 7 - 0.11           0.24

IR readings btw are all aok.

Is there an "easy way" of pinpointing which crimp could be at fault, do any of the L/N, L/CPC readings stand out?

I guess at the start I SHOULD have addressed the cpc continuity in that one socket and re-tested. Thanks in advance for looking at no doubt what is a newbie question!

 
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That would be very close to what tou'd expect for 2.5/1.0
You know that did half cross my mind (honest) but then forgot about it, having had x 1.67 hammered in for 3 years ...........was there once then a 2.5/1.0 T&E? I'll vernier the diameter, work out the CSA and report back!

 
You know that did half cross my mind (honest) but then forgot about it, having had x 1.67 hammered in for 3 years ...........was there once then a 2.5/1.0 T&E? I'll vernier the diameter, work out the CSA and report back!
1.67 for 1.5 and 2.5 for 1.0 that would give R2 at 0.62

 
Forgot to mention: I did pretty much the same "making good" exercise on the upstairs ring as in new boxes, crimped some extensions etc, etc. Same crimps, same crimper and so on. Just looked at the results I got there, these were:

r1 = 0.21

rn = 0.21

r2 = 0.36

Those figures, correct me if I'm wrong are bang on the money for 2.5/1.5. I suppose they might have done the upstairs ring in 2.5/1.5 and the "problem" ring downstairs in 2.5/1? I'll vernier to be sure.

 
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a parallel path will not increase resistance, only decrease or stay the same.

you can over worry yourself.  You have great readings, your 0.62 is slightly out but as said before it could be 2.5/1.0, or it could be a JB under the floor with a slightly dirty connection.  If your testing 0.62 everytime you test I wouldn't worry.

 
You can over worry about this.

Have you improved the installation? - yes - big style

After you did r1 r2 rn readings how can you be sure you reconnected properly - ans - you can't be sure - rely on your skills!

Is the Zs reading at each socket OK - This is the acid test. If all OK then you have done the maintenance job safely.

You need to be sure that you leave the installation safe - but as an electrician you also need to be sure your business is profitable and hunting down decimal points of ohms can take a lot of time - especially where you can get that variance with croc clips.

 
Maybe I'm being pedantic. I haven't re-energised this circuit as yet so have NOT taken Zs at each socket yet. I'll carry on with a bit more dead testing. My thinking is this:

1) Measure the diameter of the cpc - if it's 1mm csa then leave as is. If 1.5mm carry on investigating.

2) Run round quickly with the crimper again - can't really see how that'll make a difference (unless dirty wire end) as it doesn't release until fully "made". Plus on the upstairs ring where the same work was done all readings and the Zs at each socket is AOK.

3) There's only 7 sockets on the "problem ring" - 2 rooms so I'm thinking some long lead work like this: r1 and rn appear by testing to be good as in connections are properly made. If I then test r1,rn & r2 between say sockets 1 & 3 i.e. with socket 2 in the middle r1, rn should match and r2 be slightly higher - I should be able to calculate an expected figure for r2 depending on whether it's 1 or 1.5mm. This will let me maybe pinpoint the cpc crimps in socket 2. Of course it all falls down a bit    if the crimps at socket 1 and 3 are bad! Or something like that...................Then move on and do the same between 2 and 4 looking at 3 etc. Long winded but "dog with a bone" etc!

Cheers for all the input. Will report back.

 
That would be very close to what tou'd expect for 2.5/1.0
Cheers for that, SCOOBED. Verniered the ends at the cu tonight and found for L & N, 1.75mm dia core, CPC 1.14mm DIA CORE. That seems to fit then with it being a 1mm cpc. That'll teach me to ASSUME that just because the upstairs ring is 2.5/1.5 it doesn't follow the downstairs will be off the same roll etc!

Lesson learnt. Thanks again. Re-did the dead tests AOK, Will get onto Zs etc tomorrow. 

 
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