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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

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  • #2
Are you sure your KA is correct?
 
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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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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!
 
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Rauer

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  • #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

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  • #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

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  • #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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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

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  • #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

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  • #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.........
 

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