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paul_sut

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Hi all i did a fuse board change the other day and when testing i got a ZE reading of 0.43 and a KA of 5.93 why would the KA be so high, also zs failed on all circuits for breakers. This is TNS. what would any one recommend??
 
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Octopus

  • Thread Starter Thread Starter
  • #2
Are you sure your KA is correct?
 
R

Rauer

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  • #3
Try your tester again!
 
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paul_sut

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  • #4
i've just got one of these new megger mft1710 and i pressume i have i think it came up 593A? also why would my zs fail for example for the ring 32a breaker i got 6.29??
 
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Octopus

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  • #5
i've just got one of these new megger mft1710 and i pressume i have i think it came up 593A? also why would my zs fail for example for the ring 32a breaker i got 6.29??
In red - are you sure? Your OP said 5.93Ka - which is it?

Your ring may have a problem, what were your "dead" tests like? Or is it all circuits?
 
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paul_sut

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  • #6
on all circuits
 
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p11jor

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  • #7
Loose connection somewhere in the original install? I had a zS of 4000 once!

There is a few fixes tho, you could add some bonding to create parallel paths, this may bring your zs down

You could rewire it with a bigger CSA cable to lower the r1+r2 value

Or as you will now have rcd protection you can have a maximum of 1667ohms but if it's over 200ohms check it out!
 
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p11jor

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  • #8
Just another thought if you were taking the Ze then the main earth would of been removed from the earth terminal, once that was replaced to start doing zs's could the terminal screw cross threaded and backed off abit?
 
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paul_sut

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  • #9
All rcd protected now but is old house all decorated etc dont want disturbance if you know what i mean. main bonding all done to water and gas 10.0mm, main earth 16.0mm to sheath. Another reading i got was the boiler which was new circuit tight connections etc and the zs was 6.41 on 16a breaker.?
 
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Octopus

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  • #10
Hum, not sure what's going on here. Silly question but you did reconnect the earth to the CU/Met didn't you?
 
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paul_sut

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  • #11
yea i thought this aswell checked all connections all ok and tight
 
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p11jor

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  • #12
Mmmmm wierd one, has the whole wiring been inspected?? I.e is the ring final results ok? It could be a case of a DIY job few jbs spuring of the 2.5 in 1mm Somthing on the lines of that?
 
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Rauer

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  • #13
Check the main earth connection , then get onto the supplier, least that's what I'd do!
 
R

Rauer

  • Thread Starter Thread Starter
  • #15
Hasnt that ze failed too tho? Daily certain a tns is 0.18
 

telectrix

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Mentor
Arms
Esteemed
try measuring Zs at the outgoing of MCB. if its higher than the Ze, then it's the MCB and/or main switch affecting the value.
 
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paul_sut

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  • #18
Thanks for all your help i think im gonna arrange a re visit as this baffled me, yea all r1 r2 and end to end good on ring all good. but again how far do i go as obviosly only priced for fuse board one of these things i suppose.
 
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jumpin jax

  • Thread Starter Thread Starter
  • #19
Ze for tns is max 0.8, have you checked your loop readings on your meter with your known monthly check source to check meter ok
 
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Octopus

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  • #21
With the Zs readings you refer to makes me wonder if the CU is reliant on parallel paths to gas and water for the "earth" path. Are you sure that the main earth is connected from the CU to MET to incoming supply??

What was the Ze for the old board? i.e. before you started?
 
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Rauer

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  • #22
TN-S max. Ze is 0.8.
My bad! Why I have 0.18 in my head I don't know!
As someone said in an earlier post you've got additional protection in (rcd ) so aslong as its not too bad it's ok .........ish
 

Jimmy Boy

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Arms
Hi all i did a fuse board change the other day and when testing i got a ZE reading of 0.43 and a KA of 5.93 why would the KA be so high, also zs failed on all circuits for breakers. This is TNS. what would any one recommend??
Don't suppose you got a Ze and PFC before you whipped the old CU out ?
J
 
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jumpin jax

  • Thread Starter Thread Starter
  • #24
Try a loop test on the mcb at the board, it could be the main RCD or main switch causing bizarre loop readings causing bizarre loop readings
 
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p11jor

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  • #25
Just go as far as you have done and put it in lim on schedule of inspections (I think you will be covered anyway due to building fabric and floor covering) and just make good use of the departures section, cover yourself as much as you can.
 
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Rauer

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  • #27
One of my old employers used to change my measured zs's to calculated ones from r1+r2 as his assessor used to query them if they were different and he told him he would prefer it that way!!
 
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p11jor

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  • #29
I would do another Ze too....could be the armoured knocked when brushing up cable clippings from the meter cupboard, loosened it off abit
 
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Octopus

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  • #30
but again how far do i go as obviosly only priced for fuse board one of these things i suppose.
Next fuseboard change estimate you do you, should include a caveat, stating that if you identify issues with the installation, which is very likely, that additional costs are to be expected.
 
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paul_sut

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  • #31
No unfortunatly i didnt do test on old board. ring reading for excample were l=0.36 n=0.36 e=0.56 r1 r2=0.93 then zs 6.29
Main earth def connected straight to sheath of cable and connections all tight.
 
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paul_sut

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  • #32
Il try the zs on outgoing of mcb next time thank you again
 
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Rauer

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  • #33
Sounds like a main earth problem to me
 
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p11jor

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  • #34
What reading was you getting from the main earth to the gas and water?
 
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paul_sut

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  • #35
never tested between earths i will try this on nxt visit to what do you think may have happened?
 

Jimmy Boy

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Arms
If there is a fault with a mcb/rcbo or a fault thats fair enough but the Zs is an impedance test so thats why I don't agree with calculated Zs values,you should be measuring the total impedance including any protective devices, and comparing with the disc times after all what if you have nipped up the screw on the insulation instead of the copper ? it would /could give you an iffy reading ?

J
 
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p11jor

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  • #37
Just thinking of parallel paths, water / gas being the main earth......every tt system I have been to has just had messily rod in and some with non at all i get readings of around 6 and that's the earthing coming from the gas & water so that's seaming the most likely to me.

PS I know you are on a tns system but used the tt as a example
 
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Snapester

  • Thread Starter Thread Starter
  • #38
Is the earth solder joint on to the main cable or has it been clamped with an earth clamp? Seen theses before gone loose?
What did you get with the main earth disconnected and supply off? 0.43ohms? Which seems a good reading and you should have a PFC of around 534.9A?
Did you test between main bonding and the board for a low ohm reading?
 
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paul_sut

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  • #39
Thank you for all your help il try and arrange date to go back and will let you all know what i did and found. Also has any one got any advice on what i can write on cert to cover my arse in good wording if you know what i mean? thanks again p
 
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Octopus

  • Thread Starter Thread Starter
  • #40
Thank you for all your help il try and arrange date to go back and will let you all know what i did and found. Also has any one got any advice on what i can write on cert to cover my arse in good wording if you know what i mean? thanks again p
I wouldn't be issueing a cert without some more investigation.........
 

Jimmy Boy

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Arms
Paul to be blunt..paperwork isn't an 'Arse covering exercise' you need to get to the bottom of what is WRONG this is potentially dangerous, investigate it and sort it
J
 
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p11jor

  • Thread Starter Thread Starter
  • #42
I would write, board only fitted by myself, high zs readings upon testing, unable to carry out full inspection due to building fabric, all connections at db have been checked and are correct, r1+r2 readings are not enough when added to this Ze to cause this high zs.......additional protection by RCD is in place.
 
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Snapester

  • Thread Starter Thread Starter
  • #44
Did you perform r1+r2 testing on circuits? If so what were the results?
 
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p11jor

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  • #45
Paul to be blunt..paperwork isn't an 'Arse covering exercise' you need to get to the bottom of what is WRONG this is potentially dangerous, investigate it and sort it
J
Problem we have unlike the gas we don't have the power to turn off.
 
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paul_sut

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  • #46
I just said on my post i was arranging a re visit for more investigation!!
 
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p11jor

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  • #48
Wouldnt it go on the length of run?
Oh you mean the the line & earth together not the end to ends!!


Yer it's r1 + r2 / 4 iisnt it
 
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Snapester

  • Thread Starter Thread Starter
  • #51
Well no your readings should be 0.23 if you did a figure 8 at the board and tested at each point on the ring?
Something don't add up to me and you need to have good results and thorough tests to prove the main earth is at fault, especially if you are getting good readings of .43ohms with the Ze?
 
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Snapester

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  • #52
:77: I have this feeling on this thread
 

Jimmy Boy

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Arms
I think as for the ring reading, you should go to each point of use, and take the highest reading, now if there was a spur of a reasonable length off the ring the ohms would be higher on this than a pure ring and this would be the value entered on the cert as proof of meeting the disc time.

J
 
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p11jor

  • Thread Starter Thread Starter
  • #54
l=0.36 n=0.36 e=0.56

0.36 + 0.56 = 0.92

0.92/4 = 0.23 ohm

Yer I put that underneath I miss read the first one!
 
You're too diplomatic today Murdoch:)
 
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p11jor

  • Thread Starter Thread Starter
  • #58
But you have the choice as to whether you should power it up, this whole thread worries me.

J
With additional proTection by rcd is in place then 1667 ohms is the highest zs, we have 6ohms here, that's 1661 ohms less that the maxium.....although that is totally unrealistic zs and should be investigated but even then the regs states anything over 200ohms. So he has done everything to bs7671 here.
 
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Richard

  • Thread Starter Thread Starter
  • #59
With additional proTection by rcd is in place then 1667 ohms is the highest zs, we have 6ohms here, that's 1661 ohms less that the maxium.....although that is totally unrealistic zs and should be investigated but even then the regs states anything over 200ohms. So he has done everything to bs7671 here.
Sorry in advance! Thats a poor attitude to take. I can honestly say that i'v never left an installation like that. Something is obviously wrong so it needs sorting end of.
 
So he has done everything to bs7671 here.
Except testing it properly
 
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Snapester

  • Thread Starter Thread Starter
  • #61
over 200ohms for a TT? This is a TNS so anything over 0.8 would have me checking it out. As for relying solely on the RCD and leaving the installation in that state i think is unacceptable. What were the readings on the RCD testing, did they satisfy?
 
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Octopus

  • Thread Starter Thread Starter
  • #62
With additional proTection by rcd is in place then 1667 ohms is the highest zs, we have 6ohms here, that's 1661 ohms less that the maxium.....although that is totally unrealistic zs and should be investigated but even then the regs states anything over 200ohms. So he has done everything to bs7671 here.
Yes, but....... the OP, to his credit is questioning the live test results and isn't happy with the, I'd be investigating them too, if it was me. With a Ze of 0.43 and high Zs on all circuits it shouldn't take too long to track down the issue.

Relying on the 1667 value is not ideal, especially when the calculated values indicate that the live tests should be within normal readings........
 
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Octopus

  • Thread Starter Thread Starter
  • #63
You're too diplomatic today Murdoch:)

Being careful.

The mods are probably watching this thread and input is subdued and generally constructive as yet, it wouldn't take much for one or two posters to stir up trouble and for the OP retaliate.........
 
Being careful.

The mods are probably watching this thread and input is subdued and generally constructive as yet, it wouldn't take much for one or two posters to stir up trouble and for the OP retaliate.........
Hmmmm, with hindsight maybe I should edit my last contribution
 
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p11jor

  • Thread Starter Thread Starter
  • #65
That's not my attitude to leave it, I have suggested many things to test for but untill he gets back there he can't just turn & lock off the customers electric,

I wouldn't use the 1667 myself if I had a good Ze like he does but what I am saying is lets not panic and start telling them kids will burn, the zs's are withing a figure stated in bs7671 for disc times there is now additional protection via rcd do a improvement had been made, then as stated by OP he will return to investigate with so e guidance of our selfs on here who may of experienced the same problems....let's not be booking them into a hotel for the week due to a slightly high zs that If it was a tt system would problem be much greater than that and not even flinched at.
 
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Snapester

  • Thread Starter Thread Starter
  • #66
No we aren't having ago and i think its even good he came on here for advice, we are mainly saying from my point of view that his Ze which he states was 0.43 which is a respectable reading on a TNS which i think he has read the PFC wrong as if you do the math 230/0.43 = 534.8 which looks close to what he had only decimal in wrong place?
So therefore the fault obviously lies within the installation loose connection with earthing system?
 
Snapester, there is no problem with his reading. He has written the result down wrong, he's said somewhere in here that his tester read 543A which obviously is .543KA not 5.43
He's probably looked at this for so long and got himself going round and round in circles till he's ended up disappearing up his own back end.
It's easily done, we've all been there
 
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Snapester

  • Thread Starter Thread Starter
  • #68
No you misunderstand his Ze is fine thats what im saying but his Zs test results are coming in high hence must be within the installation?
 
Yeah mate but one of the confusions is over his pfc, and I reckon he's got himself screwed up somewhere. I did it once using a mate's meter, he came in while I was tearing my hair out and said "It'll help if you set the range properly"
2 minutes later job done and we're in the van having a fag.
I'd bet if he goes back with a clear head it'll be jobs a goodun
 
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p11jor

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  • #70
Zs = Ze + (r1+r2) so the r1+r2 are low so I don't think it's within the installation I'm defo going for something at the intake.....sheathing of armoured or Somthing on the lines of that ..... Looking forward to finding out now
 
Zs = Ze + (r1+r2) so the r1+r2 are low so I don't think it's within the installation I'm defo going for something at the intake.....sheathing of armoured or Somthing on the lines of that ..... Looking forward to finding out now
Ze = .43+(R1+R2).23=.66 so we should be looking at something around that (disregarding parallel paths for now) but often an RCD will introduce a huge increase in the measured value hence the allowance of 1667 ohms. Maybe that's where he should be looking
 
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Rauer

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  • #72
How have you tested r1+r2 on the ring? Have you tested the resistance through the meter?
 
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