Very Hi ZS Reading on TT system? any advice

[srl-8]

Hi guys,

i have changed a fuse board today in a outbuilding/large shed (it has a kitchen and shower and lounge etc.. a )

It is a TT system with earth rod.

My ZE reading was 187 ohms - now the regs state I am ok upto 200, so I assumed this was ok.

All the R1 + R2 and insulation resistance came back normal.

When I carried out the ZS tests, the reading were very high, for example:

Shower circuit on a 40a MCB - 2.6 omhs !
Kitchen Ring on a 32a mcb - 2.7 omhs !

same for lounge and bedroom rings.

I have checked to regs and these figures are over the maximum allowed.


Any advice ???

 

[GLENNSPARK]

Sorry Richard, but I have to disagree with you. From my experience a ZS on a TT system is more often than not lower than the ZE. This is due to parallel paths through water and gas services.

yep...at a house we were at recently.....Zs (at the board) through the gas and water services was in the region of 80 odd ohms....there was no earthing (rods or otherwise) to this installation other than what was being provided by the afore mentioned services.....

 

[Richard]

yep...at a house we were at recently.....Zs (at the board) through the gas and water services was in the region of 80 odd ohms....there was no earthing (rods or otherwise) to this installation other than what was being provided by the afore mentioned services.....

Yes, sorry I was trying to demonstrate a completly different point!! and completly aggree with wwhat you say!

Excluding Earth bonding for a second, the Zs cannot be lower than the Ze..correct? which I think the OP didn't understand.

 

[Specialist]

Unfortunately I think there's a lot the OP does'nt understand or does'nt appear to understand.

 

[GLENNSPARK]

assuming that BONDING not earth bonding..that dont exist mate......anyway assuming that main bonding conductors were not in place then a Zs should be higher than a Ze....but never forget the ability of parrallel paths to bring a measured value down....

 

[telectrix]

agreed, but with bonding connected, your Ze effectively comes down from say 180 ohms to , in this case, around 2 0hms. this also means that the pefc will flow mostly through the bonding rather than the rod.

 

[srl-8]

We i am trying to understand, I have carried out a good few ZS test and up until now they were all fine results... Your right I dont know it all, I want to learn.. I am trying ti understand these results... thats why I have come on to this forum..

 

[GLENNSPARK]

the lowsst impedence path.....

 

[GLENNSPARK]

We i am trying to understand, I have carried out a good few ZS test and up until now they were all fine results... Your right I dont know it all, I want to learn.. I am trying ti understand these results... thats why I have come on to this forum..

basically you have a means of earthing here that can have a max of 1667 ohms....but a Ze of over 200 ohms may not be stable...in worst case conditions.....you aint gonna get a Zs of of any final circuits here anything like a tn system and you may also find that you are relying on an RCD soley for fault protection....

 

[mechelec]

We i am trying to understand, I have carried out a good few ZS test and up until now they were all fine results... Your right I dont know it all, I want to learn.. I am trying ti understand these results... thats why I have come on to this forum..

We never stop learning and NO-ONE knows it all. That said you are being employed by a customer for your knowledge and ability to carry out a safe and reliable installation and as such you need to know what you are doing. You clearly have a basic knowlege of the need to test but no understanding of how to interpret the results.
As a matter of interest you believed that the readings for Zs were above those quoted in the on site guide, what did you do about it? as your belief was it was unsafe. I bet it was left connected until you could get on this forum to find out what to do.

Not meaning to beat the same old drum or insult you but I hazard a guess that you have recently joined a Part P scheme after completing a short course.

 

[Specialist]

srl-8: None of us know it all & that's not what wer'e saying, but at the end of the Day the Public are paying for you to go into their property & carry out a Board change or whatever & expecting you to be fully competent at what your doing. From your post it appears that although you seem to understand basic testing, you don't understand the results you get & what to do about them. Not to put too fine a point on it but you could be risking peoples Lives & Property.

 

[GLENNSPARK]

We never stop learning and NO-ONE knows it all. That said you are being employed by a customer for your knowledge and ability to carry out a safe and reliable installation and as such you need to know what you are doing. You clearly have a basic knowlege of the need to test but no understanding of how to interpret the results.
As a matter of interest you believed that the readings for Zs were above those quoted in the on site guide, what did you do about it? as your belief was it was unsafe. I bet it was left connected until you could get on this forum to find out what to do.

Not meaning to beat the same old drum or insult you but I hazard a guess that you have recently joined a Part P scheme after completing a short course.

yes and if that is the case (a short part p course)...then it need addressing as were talking disconnection times here and these "courses" really should be covering this......

 

[mechelec]

yes and if that is the case (a short part p course)...then it need addressing as were talking disconnection times here and these "courses" really should be covering this......

They should, but in all honesty 5 weeks does not give you any time to learn the theory behind how we get the results we get and the reason for the tests being required. I have no doubt that testing is briefly covered but it takes time to teach it to people who in many cases have had no or very little past electrical experience.
There is no substitute to working with an experienced electrician and learning the ropes over time combined with theory based learning in college before going out alone.
Now what did they call that type of training that had worked for years and was proven to produce (mostly knowlegable) sparks and weeded out the dead wood and chancers (answers on a post card please)

 

[srl-8]

some info for you all.
1) yes I am part P
2) the property is my own, no customer life or money at stake
3) I came here looking for advice and help.
4) system left unconnected until i solve the problem.

So, who wants to answer my question straight? Please..

ZE reading 187ohm

I can determine ZS by enquiry, ZE +R1 +R2 = ZS

However, i tried to test and measure ZS at the end of the circuit and got these high figures of 2.6 2.5 etc etc...

again, any ideas on how to solve this issue???

Thanks

 

[IQ Electrical]

some info for you all.
1) yes I am part P
2) the property is my own, no customer life or money at stake
3) I came here looking for advice and help.
4) system left unconnected until i solve the problem.

So, who wants to answer my question straight? Please..

ZE reading 187ohm

I can determine ZS by enquiry, ZE +R1 +R2 = ZS

However, i tried to test and measure ZS at the end of the circuit and got these high figures of 2.6 2.5 etc etc...

again, any ideas on how to solve this issue???

Thanks

It's Ra, not Ze on a TT system.

Is your TT system protected by an RCD as it should be with that Ra value?

What is the rating of the RCD?

 

[srl-8]

ZE was 187

the system is protected by a 30ma RCD from the main house, and the new board is also protected by a 30 ma RCD.

??