Ring final end to end test

[Maxstone89]

As doing a cu change ,I have a ring final in a house with an end to end reading on r1-r2-Rn are so low that somewhere very close under a floorboard the ring must have been connected together somewhere, customer refused to pay more, so i have put in a 20amp mcb. To comply with current rating of a 2.5mm cable for a radial circuit,

What would you have done.

Regards Kurtis Randle

 

[Maxstone89]

Thanks for all your replies, nice to know the thinking of other spark's.

 

[HappyHippyDad]

If r1=0.14Ω and the the resistance for 1m of 2.5mm is 7.41mΩ then the run would be around 19m (approx) assuming all terminations are made well. I guess this could be a ring for a small kitchen.

I haven't got an exact figure for r1 in my head for when I would decrease the rating of the MCB but I think you did the right thing Kurtis. It's a shame they didn't let you do a few basic checks! But I guess they'll have to get you back if the 20A MCB keeps tripping :smiley2:.

 

[Paul.M]

If r1=0.14Ω and the the resistance for 1m of 2.5mm is 7.41mΩ then the run would be around 19m (approx) assuming all terminations are made well. I guess this could be a ring for a small kitchen.

If you have a r1 reading you can calculate the expected r2:-

2.5mm divided by 1.5mm = 1.67 (rounded up)

0.14 X 1.67 = 0.23 ohms. This way you dont have to estimate the length of the run and a more accurate "estimation" can be produced.

If this is a "true" RFC and you are happy that it is safe to energize. Carry out a plug in Zs at all points, deduct the Ze and compare the readings from the dead tests. This should give an indication and more information of what is going on after Kevin kitchen fitter did a JB under the tiled floor lol. (had this problem loads of times).

 

[camerabloke]

Was the testing carried out prior to the CU change?

I guess not if he found the fault after the CU change[emoji41]

 

[Maxstone89]

The old fuse cu was such a mess and muddle it was easier to test after due to lack of assesibility.

 

[Paul.M]

The old fuse cu was such a mess and muddle it was easier to test after due to lack of assesibility.

That can be the case so it is understandable but it is also a gamble.

 

[Englishman]

Is this an old house, e.g. 1960's? Sometimes the RFC is wired so all the socket drops are spurs from junction boxes (under floor or in ceiling) connected off what is then electrically quite a short ring. Then for a single floor, r1=0.14 could be quite a reasonable value.

Complete nightmare to fault find they are.

 

[Maxstone89]

Does anyone do full tests before and after?

 

[Englishman]

Does anyone do full tests before and after?

I only check ze, pfc, bonding and IR in all honesty.

I do the rest of the dead tests while constructing the board and then all live tests once it's all connected.

 

[Paul.M]

Does anyone do full tests before and after?

Quick tests if the board is not a rats nest. Ze, PFC, global IR LN-E, RFC end to end or Zs to check the readings against the Ze.