Different continuity R1 and RN readings on ring

[Guest 004]

Changing a CCU and reconnecting existing wiring (twin and earth) = did a N - CPC IR test for earth leaks prior to dismantling the old (BS3036) unit.

cheers

The resistance would need to be a lot higher buddy.

 

[Guest 004]

The IR might be wonderful but I never mentioned it.

A joint with 0.3Ω resistance, if that is the cause of the odd reading in the neutral, is capable of getting quite hot. E.g. passing 10A it would dissipate 30W and get as hot as a soldering iron. At 32A it would be giving out ten times as much heat. As it's a ring, it would cause more or less incorrect sharing of the total load between the two sides instead, depending on where it's located.

The world is not perfect as you say but it is better to find out the imperfections with the MFT than by tracing the burning smell.

That is just bad math

 

[Lucien Nunes]

Eh?

P=I²R
=10 x 10 x 0.3
=30W

or
=32 x 32 x 0.3
=307W

Just to make sure we're on the same page, I'm working in base 10.

 

[Guest 004]

Are you seriously suggesting that a 2.5mm cable cannot handle 307W?

 

[Silly Sausage]

No he's not.
We're talking about a poor joint here, so in few mm[SUP]3[/SUP] of metal, dissipating 100s of Watts, things are going to get HOT!

 

[Guest 004]

Poor joint or poor accessory

 

[Lucien Nunes]

I see we're thinking along similar lines now. Another theory was that one side of the cable had been 'stretched'. For a bit of mental arithmetic practice, try the following:
A single-core cable of length L1 with resistance 0.41Ω is stretched so that its cross-section reduces uniformly until the resistance is increased to 0.72Ω. What is its new length L2, assuming the resistivity of the copper remains constant*?

* it does change somewhat, when you work metals.

 

[Guest 004]

Ok we will have fun with that, but the install is twin and earth.

 

[Lucien Nunes]

the install is twin and earth.

My point exactly. You would have to do bizarre things to the cable to make that difference. When the OP has a chance to do the wander lead test or R1+RN/4, we can discover whether it's a rogue reading, a bad connection, or whatever. In the meantime, L2/L1= ?

 

[Guest 004]

I see we're thinking along similar lines now. Another theory was that one side of the cable had been 'stretched'. For a bit of mental arithmetic practice, try the following:
A single-core cable of length L1 with resistance 0.41Ω is stretched so that its cross-section reduces uniformly until the resistance is increased to 0.72Ω. What is its new length L2, assuming the resistivity of the copper remains constant*?

* it does change somewhat, when you work metals.

2.5mm is 7.41 Milli ohms per metre
So length 1 is 55 metres
And when stretched is 97 metres

Leaving resistivity out of the equation

if you stretch a cable the cross sectional area of that cable will reduce thereby increasing the resistance/metre to an unkonwn value

hence it will not have to be as long!!!

 

[Lucien Nunes]

it will not have to be as long

True

increasing the resistance/metre to an unknown value

False

Hint: You can leave CSA and actual length out of the equation too, to find L2/L1. Volume of copper is constant. Sub an expression for CSA into the expression for R. Finding numerical answer will require taking a square root.

It was just to show the extreme stretchification needed to account for an extra 0.31Ω

 

[Motty]

Really appreciate all the replys, and the different perspectives. Sorry didnt get back til now internet been off for 2 wks thanks to a change in phone provider!

I explored the legs of each socket by stripping all fascias off and plotting the ring layout. However, found that the ring must go to a JB under the floor, because the boiler switched fused isolator and 2 sockets in the 2nd bedroom were all spurs from this inaccessible box.

Went back over what I had checked and after L-L(0.41 ohms) N-N (0.71ohms) CPC-CPC(0.66ohms) testing, the R1 + R2 readings appeared normal (0.27-9) on the 4 sockets or first "half" of the ring, and 0.36 ohms highest at the beroom 2 spurs. Incidentally the R1+RN readings at each outlet varied between 0.21 to 0.30 when cross-connected. Incidentally IR readings came in at >999Mohm L-N, 857Mohm L - CPC, >999Mohm N - CPC.

Customer insists I cant take up the flooring upstairs. For now, all sockets re-terminated, and put a 16A MCB to lower the risk of overload on the circuit. Planning to put on the EIC for the CCU change that IMHO this ring should be rewired and re-configured, and next inspection 5 yrs rather than 10. As rightly stated, I am liable fot the safety of this circuit now i've changed the CCU, and can't seem to stop worrying about it ... A sharp learning curve out in the real world!

 

[Badged01]

What is its new length L2, assuming the resistivity of the copper remains constant*?

Missed this gem of a post originally but couldn't resist an early morning mathematical challenge .... I get the new length ~ 73M.

Correct or are my maths failing me again?