Unexpected Reading Between Earth And Neutral

[not_shocked_yet]

Here are my measurements for 4 of the circuits so far.  I removed each L, N and E and connected directly with crocodile clips (I removed the L's as one mcb had two circuits and it was just easy to remove the others as well although I know I didn't need to Andy):

All numbers are Mohms

Kitchen lights:  LN  0.98  LE  >999  NE> >999

Back Room Lights:  LN  0.08  LE  >520  NE> 460

Lighting Living Room Circuit 1  LN  >999  LE  >999  NE> 16.0

Lighting Living Room Circuit 2  LN  >999  LE  >999  NE> 0.05

Other pressures elsewhere today meant I wasn't able to continue the testing.  There are 2 ring final circuits and the outdoor lights still to be tested.

Situation I'm in is that the client asked me to sort out a tripping RCD which I fixed, it was loose conductors in the light switch.  She is happy for me to investigate the insulation resistance but doesn't want to pay for it of course.  I have suggested that she should have a periodic survey completed as the building is very damp, has only been used occasionally and the wiring was done in 2002.

Can you guys help me to interpret these readings and let me know if you think I should press the client to agree to a periodic?

I am confident that no equipment was left on during the Kitchen/Back Room tests although the results might suggest there was?  The building is a simple 3 room stable with just a fridge and an oil boiler for heating.  I removed all the light bulbs etc...  there are definitely no neons etc...  I didn't remove the transformers for the lighting circuits in the living room and they have no earth connection at the transformer.

Looking at the LN readings for the Kitchen and Back Room is likely to cause overheating.

The 16Mohm would give a fault current of 560/230 = 0.035A  - is this correct? (560 volts was the voltage I read as delivered during the test)  

Many thanks.

 

[Andy™]

Can you guys help me to interpret these readings and let me know if you think I should press the client to agree to a periodic?

:slap

the circuit most likely to be the problem should be blatantly obvious

another with a test kit but no idea how to understand what its telling you...

 

[NozSpark]

It's quite obvious from your test results that you haven't fixed the fault that is causing the RCD to trip

If you read your regs,, any IR reading over 1Mohm passes, however any reading less than 2Mohms should be investigated

I suggest that you buy a copy of GN3, there are other good guides to testing,,,, I'm sure someone will post their recommendations

Oh.... And I'd also suggest that there is a load still connected on your first circuit

 

[ProDave]

Low L-N readings most likely mean there was something still connected, e.g the timer circuit of a bathroom fan (remember to turn off all fan isolators). That won't cause imbalance and won't trip an rcd.

Low L-E or N-E readings are the ones that you should be suspicious of. there is one reading listed there that would have all of us pulling that particular circuit to pieces to find the fault. Can you guess which one it is.

I would suggest you have NOT fixed the rcd tripping fault, it just hasn't happened for a bit but there is most clearly a fault on one of those circuits you tested.

 

[not_shocked_yet]

Andy, you might be the most experienced electrician on the planet but if you keep up the attitude you will keep getting the same s**** back.  Why don't you try to be helpful for once instead of just being a d***?  I (and I'm sure other posters) would prefer to see an intelligent and helpful response from you - I haven't tried to hide that I'm new in this game and with your help or without it I will learn and I will be a better sparky.

 

[kerching]

Here are my measurements for 4 of the circuits so far.  I removed each L, N and E and connected directly with crocodile clips (I removed the L's as one mcb had two circuits and it was just easy to remove the others as well although I know I didn't need to Andy):

All numbers are Mohms

Kitchen lights:  LN  0.98  LE  >999  NE> >999       WHAT VOLTAGE DID YOU TEST AT

Back Room Lights:  LN  0.08  LE  >520  NE> 460  F low

Lighting Living Room Circuit 1  LN  >999  LE  >999  NE> 16.0

Lighting Living Room Circuit 2  LN  >999  LE  >999  NE> 0.05   F low

Other pressures elsewhere today meant I wasn't able to continue the testing.  There are 2 ring final circuits and the outdoor lights still to be tested.

Situation I'm in is that the client asked me to sort out a tripping RCD which I fixed, it was loose conductors in the light switch.  She is happy for me to investigate the insulation resistance but doesn't want to pay for it of course.  I have suggested that she ?should have a periodic survey completed as the building is very damp, has only been used occasionally and the wiring was done in 2002.   loose or touching earth?

Can you guys help me to interpret these readings and let me know if you think I should press the client to agree to a periodic?

I am confident that no equipment was left on during the Kitchen/Back Room tests although the results might suggest there was?  The building is a simple 3 room stable with just a fridge and an oil boiler for heating.  I removed all the light bulbs etc...  there are definitely no neons etc...  I didn't remove the transformers for the lighting circuits in the living room and they have no earth connection at the transformer.

Looking at the LN readings for the Kitchen and Back Room is likely to cause overheating.?

The 16Mohm would give a fault current of 560/230 = 0.035A  - is this correct? (560 volts was the voltage I read as delivered during the test)  last time i looked 560/230= 2.44 ish

Many thanks.

FRICK Boatey boy beat ME this time! Now THERE IS a first

just too slow

 

[Andy™]

Andy, you might be the most experienced electrician on the planet but if you keep up the attitude you will keep getting the same shite back.  Why don't you try to be helpful for once instead of just being a dick?  I (and I'm sure other posters) would prefer to see an intelligent and helpful response from you - I haven't tried to hide that I'm new in this game and with your help or without it I will learn and I will be a better sparky.

im not the most experienced electrician on the planet, and never claimed to be. but the simple fact is, your going to a job, probably charging your customer a fortune, then coming here for advice on the very basics. i do not expect you to know everything, no-one does, and thats what these forums are about, but at your stage of training, you shouldnt be working alone

 

[ProDave]

Andy, you might be the most experienced electrician on the planet but if you keep up the attitude you will keep getting the same shite back.  Why don't you try to be helpful for once instead of just being a dick?  I (and I'm sure other posters) would prefer to see an intelligent and helpful response from you - I haven't tried to hide that I'm new in this game and with your help or without it I will learn and I will be a better sparky.

The point is, if you do not understand that 0.05M hms  from N-E is a problem, then perhaps you are in the wrong profession?

Why do you think such a low N-E reading might give issues and might cause an RCD to trip?

Or do you think it's still high enough not to worry about? After all there's not much voltage between N and E is there?

I sometimes think the old fashioned meggers, with a mechaincal dial and a needle were better. With those, if the needle stayed well over to the left all was well in the world. If the needle made a bee line for the right hand end of the scale, you knew you had a problem. it was obvious.  These days it just tells you some numbers, and it would appear some people just can't visualise what those numbers mean.

 

[not_shocked_yet]

Hi kerching, Opps, the cal at the bottom should be 560/16000 = 0.035A

Test was at selected tester voltage of 500V but the tester screen gave me an actual reading of 560V during the test.

I just noticed that 2 of the line conductors were not connected when I visually checked the light switch - this was a 4 gang switch with the 2 circuits split between the four switches (2 switches per circuit).  I'm assuming that the connections were loose and they came adrift as I pulled the face plate away.

What do you mean by 'F low' ?

Andy, client is a friend of a friend and I am not charging for my work.

 

[ProDave]

What do you mean by 'F low' ?

I would think he means Flipping low, or another word beginning with F

 

[not_shocked_yet]

Hi Pro Dave,

Reading should be 1 M ohm or greater however it is accepted that in the field a measurement above but close to this number should still be considered a fault.  The lower the Mohm reading the less resistance is being found - the resistance of the conductor if insulation is good should be high as it is not a good conductor of electricity.  So, a low reading tells me that the insulation has deteriorated to an extent that more current is leaking from the L conductor to E conductor than would be the case if the insulation was good (accepted that there will always be some current leakage as all materials are conductors - some just really not very good).  Or a conductor(s) may have become loose and are closer to each other than they should be.  

The lower the reading value the worse the condition(s) above will be.

The imbalance registered at the RCD will cause it to operate.

 

[kerching]

Hi kerching, Opps, the cal at the bottom should be 560/16000 = 0.035Aerrrrrrrrrrrrrrrr.........in that case the calc should be , according to your theory, 560/16000000 since Mohm is MILLION ohms not 1000

Test was at selected tester voltage of 500V but the tester screen gave me an actual reading of 560V during the test.

I just noticed that 2 of the line conductors were not connected when I visually checked the light switch - this was a 4 gang switch with the 2 circuits split between the four switches (2 switches per circuit).  I'm assuming that the connections were loose and they came adrift as I pulled the face plate away.

What do you mean by 'F low' ?    flipping, fairly, fricking, freaking   take your pick!...other expletives are available

Andy, client is a friend of a friend and I am not charging for my work.

 

[ProDave]

Hi Pro Dave,

Reading should be 1 M ohm or greater however it is accepted that in the field a measurement above but close to this number should still be considered a fault.  The lower the Mohm reading the less resistance is being found - the resistance of the conductor if insulation is good should be high as it is not a good conductor of electricity.  So, a low reading tells me that the insulation has deteriorated to an extent that more current is leaking from the L conductor to E conductor than would be the case if the insulation was good (accepted that there will always be some current leakage as all materials are conductors - some just really not very good).  Or a conductor(s) may have become loose and are closer to each other than they should be.  

The lower the reading value the worse the condition(s) above will be.

The imbalance registered at the RCD will cause it to operate.

But do you not think the 0.05 M hms  is even beyond "poor insulation" and might signify an actual fault like a short circuit, a pinched wire, L and E the wrong way round in a light fitting etc etc.

that's a long way passed "not ideal"

 

[not_shocked_yet]

Yes I recognise it is very low.  Why doesn't the rcd trip then?

 

[kerching]

Yes I recognise it is very low.  Why doesn't the rcd trip then?

calculate the fault current to see why

you have the resistance....ok it is an impedance but I for one do not really care

you have the voltage

you have the frequency

you have Zs

you have Ze

You have Ip

you have the tripping current

some of the above put into the correct formula will give you an answer

then see if this is <= tripping current=>

remember also that some RCDs weaken over time so a ramp test may possibly be required to prove integrity

 

[ProDave]

But that's not the end of it.

A N-E fault can cause an rcd to trip when a heavy load is drawn on a different circuit. Can you work out why or how?

 

[not_shocked_yet]

Hi Pro Dave,

Is it because the high current draw is inducing a current in a neighbouring circuit?

 

[kerching]

i have the DiLog version [well i think it is DiLog?]

very handy...got me out of the mire on a few occasions

 

[Andy™]

Hi Pro Dave,

Is it because the high current draw is inducing a current in a neighbouring circuit?

no. think about voltage difference between neutral & earth

 

[apprentice87]

Were you looking the other way when they explained Ohms law???

By the way. IR testing is done with the cpc connected to the earthing bar.

john...