Question on earth loop....

[Subneural]

Maybe I'm being extremely dumb here but I need some advice please as I'm missing something.

I've installed a ring final in a bungalow. 1970s Mcleans job with lots of paramount walls. Service is PME.

Tested the ring and its spot-on at every socket. Connect the power and test earth loop and the readings increase the further from the board I get.

So I go back and test again. R1,-R1, R2-R2, Rn-Rn at the board then R1+R2, R1+Rn, R2+Rn at each socket. Everything matches up.

Test continuity for every cable down the back wall where the readings are highest and everything is fine.

Connect everything up, power up and the earth loop still reads like 2 radials not a ring. I get 0.28ohms at the nearest sockets and 0.56ohms at the furthest.

Can anyone explain this to me ????

 

[bassman]

What is the distance between nearest and furthest also length of cable run, it will be less on the nearest socket.

Take the earths out of Cu and put in a connector then test loop from different points

Or get a peice of damp string and ask them to water it.

 

[cheggers]

there is nothing wrong with your reading, the loop reading will go higher the further you go away from the consumer unit because the resistance increases, take the highest reading and put this on the cert

 

[Evans Electric]

Yes I think you're OK with that, further from board , higher the reading.

Just noticed its a bungalow, cables may all be in the roof space, dropping down wall , long circuit.

Deke

 

[Barx]

As mentioned above, your readings are correct. Thats why we have to record the highest Zs values. This will be the outlet that is electrically furthest from from the cu (not always the furthest physically). Its when you cross connect the legs for R1+R2 that the readings should be the same throughout the ring (spurs increasing).

 

[rainydays]

Have you cross connected the conductors correctly. When this is done and it is a true ring main the readings should all be substantially the same, at all sockets. Higher values at spurs.

If you have not cross connected correctly you will get values which increase towards the centre point and then decrease when you get futher away from the mid point

 

[paulh]

What "rainydays" says is correct if you are doing ring final continuity checks. This is a loop impedance check and as the others say the further away from the cu you get the higher the readings will get.

 

[kme]

Maybe I'm being extremely dumb here but I need some advice please as I'm missing something.not dumb. not asking is dumb!

I've installed a ring final in a bungalow. 1970s Mcleans job with lots of paramount walls. Service is PME.

Good, useful info!

Tested the ring and its spot-on at every socket. Connect the power and test earth loop and the readings increase the further from the board I get.

Y....e.......s.....?

So I go back and test again. R1,-R1, R2-R2, Rn-Rn at the board then R1+R2, R1+Rn, R2+Rn at each socket. Everything matches up.

Test continuity for every cable down the back wall where the readings are highest and everything is fine.

Then you`re doing things correctly!

Connect everything up, power up and the earth loop still reads like 2 radials not a ring. I get 0.28ohms at the nearest sockets and 0.56ohms at the furthest.

Can anyone explain this to me ????

Following on from my colleagues, the readings will remain the same when the loop is cross connected, except for spurs & bridges.

try this:

ring is such that ALL conductors are 2.5mm (inc. cpc)

Each conductor, end to end resistance is 100ohms

There are 9 sockets, equally spaced around the circuit.

For calculation purposes, the Ze is 0.00ohms

So.

Ring - end to end will give 3 readings of 100ohms - okay?

cross connect, and test at sockets will give 50ohms ( 2 paths 100ohms in parallel) - at any point!

When you do earth loop tests, at first socket.

cpc - 10ohms and 90ohms to end of ring. = 9ohms (1/rt=1/r1+1/r2)

Phase - 10ohms and 90ohms to end of ring. =9ohms

so earth loop = 9 +Ze + 9 = 18ohms

at middle socket:

cpc - 50ohms and 50ohms = 25ohms

phase - 50ohms and 50ohms = 25ohms

so earth loop would be 25 +Ze + 25 = 50ohms

Does that help mate?

 

[Subneural]

Many thanks for all your replies.

I was sure the circuit was fine but I've never had the earth loop readings appear to rise and fall so definately - most of my previous work has been on existing circuits which by their nature have a bit of variable in them due to age.

 

[SPECIAL LOCATION]

Maybe I'm being extremely dumb here but I need some advice please as I'm missing something.

I have just got to re-emphasise one VERY VERY important t point here.........

listen carefully...

I will zey siz only onze!!!

This forum is the ABSOLUTE AND UTTER COMPLETE DOGS WOTTISTS at answering this sort of question...

We ALL had to learn somewhere...

and this is as good a place as any IMHO?

unless of course i have missed a basic point...

in which case Admin .. or the Don or his boys .. or the slightly tamer moderators..

will no doubt correct my missunderstanding...

Oh and by the way...

Your readings sound good to me...

KME has explained the basics of the maths..

but if you want it from another angle...

just ask!

another thought...

o.28ohms nearest to CU

0.56ohm furthest socket..

you could also back check...

I assume 0.28 is within a gnats whisker of the Ze?

so R1+R2= 0.56 - 0.28 = 0.28

as R1+R2=(r1+r2)/4 on a ring..

you can work backwards which I work it that r1=0.42 & r2=0.70

{e.g. 0.42+0.70/4=0.28}

from tables 9a OSG

so if r1=0.42 length is approx 0.42/7.41*1000=56.68m

and r2=0.70 Lent is approx 0.70/12.10*1000=57.85m

so I guess your ring is around 57m in length???

HTH

 

[Apache]

I have just got to re-emphasise one VERY VERY important t point here.........listen carefully...

I will zey siz only onze!!!

This forum is the ABSOLUTE AND UTTER COMPLETE DOGS WOTTISTS at answering this sort of question...

We ALL had to learn somewhere...

and this is as good a place as any IMHO?

unless of course i have missed a basic point...

in which case Admin .. or the Don or his boys .. or the slightly tamer moderators..

will no doubt correct my missunderstanding...

Oh and by the way...

Your readings sound good to me...

KME has explained the basics of the maths..

but if you want it from another angle...

just ask!

another thought...

o.28ohms nearest to CU

0.56ohm furthest socket..

you could also back check...

I assume 0.28 is within a gnats whisker of the Ze?

so R1+R2= 0.56 - 0.28 = 0.28

as R1+R2=(r1+r2)/4 on a ring..

you can work backwards which I work it that r1=0.42 & r2=0.70

{e.g. 0.42+0.70/4=0.28}

from tables 9a OSG

so if r1=0.42 length is approx 0.42/7.41*1000=56.68m

and r2=0.70 Lent is approx 0.70/12.10*1000=57.85m

so I guess your ring is around 57m in length???

HTH

Do you read palms and predict the future too

 

[seajayess]

You must remember when doing R1+R2 you are doing a cross connected reading, like one big loop, when you do a Zs it is like doing a radial with two legs of 2.5 cable so the further you go away from the supply the higher the readings will be until you start coming back from the mid point then it should start to decrease again CJS