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 ???

 

[timbobelfast]

And was the ground wet? Ie in favourable conditions?

If so then you could try coupling two rods together if they are 5/8s.

 

[telectrix]

max. Zs is 1667 ohms if 30mA RCD fitted to all circuits.

 

[IQ Electrical]

I checked the date first but it's not April 1st.

What Zs readings would you expect on such a system with an Ra value of 187 Ohms?

Your bonding will have reduced Zs already but RCD protection will give you much higher max Zs figures depending on RCD ratings (up to 1667 Ohms for 30 mA)

You really need to brush up on the reg's before you undertake such work, clients are paying for your competency and skills...

 

[srl-8]

teletrix. just to confirm, you are saying the my readings were acceptable ? I understood that (on page 103 of osg) my maximum ZS for a type b 40a Breaker is .093 ohms. ?? The reading i got was 2.6 ohms ??

where are you getting the 1667 from ?

thanks

 

[jaresquire]

oh dear..

 

[Des 56]

My ZE reading was 187 ohms

So far so good,its after that you are losing the plot

Clue for research purposes only
Its a TT system with a 30 m/amp rcd

 

[srl-8]

Ok, so on page 50 on the regs, is states that the max ZS is 1667 for TT, so does page 103 of the OSG not contridict this ?

Sorry, but i am a bit confused......

 

[GLENNSPARK]

i think its time the o/p took a look at 411.5.....

 

[Richard]

Sorry and everything, but oh my god! You quite obviously don't really understand what your doing! If you had 180ohm odd as a Ze, then theres no way in hell your going to get 2.6 ohm Zs!! Zs will never be lower than Ze

 

[JONOXNAM]

Hi srl-8 If you cannot get get your Zs down to meet disconnection you can employ an RCD but remember touch voltage 1667 x 30mA = <50V (Touch voltage).

Also remember disconnection in TT is less than a TN system i.e. 0.2s vs 0.4s - 411.3.2.2

Plus are the figures correct - remember Zs = Ze + (r1+r2) so Zs cannot be lower than Ze...

 

[telectrix]

Sorry and everything, but oh my god! You quite obviously don't really understand what your doing! If you had 180ohm odd as a Ze, then theres no way in hell your going to get 2.6 ohm Zs!! Zs will never be lower than Ze

yes it will, due to parallel paths via the bonding of gas and water. lead pipe is a bloody good earth.

 

[Octopus]

Sorry and everything, but oh my god! You quite obviously don't really understand what your doing! If you had 180ohm odd as a Ze, then theres no way in hell your going to get 2.6 ohm Zs!! Zs will never be lower than Ze

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.

 

[Richard]

yes it will, due to parallel paths via the bonding of gas and water. lead pipe is a bloody good earth.

Indeed, the readings obtained don't suprise me atall!

 

[Richard]

To be fair, the OP hasn't mentioned anything about bonding. But yes, i'd aggree that more often than not, a TT system with a lead water pipe will he significantly lower Zs's than the Ze.

 

[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.

??

 

[Simlec]

This is going to be a very long thread me thinks! :rolleyes4:

 

[One_of_them]

if your R1+R2 readings came back as satisfactory then that circuit impedance should be OK. Therefore you need to start looking at other factors relating to this issue i.e your Ze!! Are you earthing conductors connected correctly and secure and in good condition?

Get another rod sank in and see if this is any better to lower your Ze

 

[Specialist]

Discrimination ? i.e is the RCD feeding it in the House a time delayed type.

 

[IQ Electrical]

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.

??

Ok then your maximum allowed Zs reading for each 30mA protected circuit is 1667 Ohms, problem solved in 31 posts.

Your RCD at the supply end should ideally be an 'S' type time delayed to achieve discrimination with the RCD at the remote end but I wouldn't loose sleep over that one.

 

[Richard]

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

ok, first it's not an issue.

When you tested Ze, You disconnected the earthing conductor which went to the earth rod I imagine.

As previosuly mentioned, you may well have bonding in place to the Water service???

Try testing with the earthing conductor re-connected, and you will probibly get a reading of somewhere around 2 ohms. As previsouly stated by telerix, by connecting the water pipe you've effectivly got a much better earth rod!

With regards to the values in the Regs - Being a TT system, because your Ze is not garenteed stable, circuits must be protected via RCD (in your case 30ma), which from what you say they already are.

 

[mechelec]

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.

would that be a 30mA RCD protecting the sub main and another 30mA RCD protecting the final circuits in the outbuilding i.e. 2 RCD's in series

I was too slow in replying to specialist (too slow, specialist beat me) sorry to cause confusion

 

[IQ Electrical]

I'm busy writing an FAQ on TT systems for a certain organisation, judging by some of the questions (and answers) on this thread, I need to write faster!

 

[GLENNSPARK]

and if you are going to start driving more rods in...then go for depth every time ...."depth is the key with TT".....

 

[GLENNSPARK]

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.

would that be a 30mA RCD protecting the sub main and another 30mA RCD protecting the final circuits in the outbuilding i.e. 2 RCD's in series

too slow, specialist beat me

i can feel discrimination entering the fray here!!..lol....

 

[Geordie Spark]

and if you are going to start driving more rods in...then go for depth every time ...."depth is the key with TT".....

Now THAT could have been posted by our good Friend E54 !!

 

[Engineer54]

I refuse to get involved!! lol!!! ....But please, when refering to TT systems it's ''Ra'' NOT Ze....

 

[biggles]

sorry if i read that wrong mechelec but why would 2 rcd`s in series slow the disconnection time

 

[Specialist]

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.

would that be a 30mA RCD protecting the sub main and another 30mA RCD protecting the final circuits in the outbuilding i.e. 2 RCD's in series

too slow, specialist beat me

Hey: I've got to beat someone at posting at least once a year:lol:

 

[Geordie Spark]

I refuse to get involved!! lol!!! ....But please, when refering to TT systems it's ''Ra'' NOT Ze....

Now that's a shame ..... I enjoy reading your posts.

 

[IQ Electrical]

sorry if i read that wrong mechelec but why would 2 rcd`s in series slow the disconnection time

It wouldn't, in practice, both devices would trip under fault conditions because there would be no discrimination without the upstream device being an 'S' type time delayed.

 

[Specialist]

sorry if i read that wrong mechelec but why would 2 rcd`s in series slow the disconnection time

I don't think Mechelec was talking about it slowing Disconnection times actually mate, he was saying he was too slow to beat me posting the question of Discrimination.

 

[biggles]

doh, brain not in gear.

 

[mechelec]

sorry if i read that wrong mechelec but why would 2 rcd`s in series slow the disconnection time

It wouldn't "slow the discrimination time? " but it would mean there was no discrimination between the RCD's and it would be pot luck which would trip first. inconvenient at best, dangerous at its worst. First one should be time delayed or S type

 

[srl-8]

So to confirm,

ZS readings of 2.6 ohms or there abouts is acceptable, providing the system and all circuits are protected by a 30ma RCD. (which they are)

??

Ok then your maximum allowed Zs reading for each 30mA protected circuit is 1667 Ohms, problem solved in 31 posts.

Your RCD at the supply end should ideally be an 'S' type time delayed to achieve discrimination with the RCD at the remote end but I wouldn't loose sleep over that one.

 

[srl-8]

i got confused earlier, because a member said the my ZS reading could never be lower than my ZE...

My ZS of 2.6 etc is lower than 187 omhs.....?

So to confirm,

ZS readings of 2.6 ohms or there abouts is acceptable, providing the system and all circuits are protected by a 30ma RCD. (which they are)

??