basic bad practice

[Stevesplatto]

Thanks Mr W, yeah it's like that!

 

[PC Electrics]

How would you be able to ascertain the value of the supply earth (Ze) if you can't isolate it from protective bonding?

well, you can't. However as mentioned you can do a Zdb. You can also carry out a visual and physical inspection of the earthing arrangements to ensure they are robust. I'm not convinced that your parallel paths concern is anything to worry over too much. Most gas and water mains are plastic now and so parallel paths are limited. Again, you can inspect for this.

It is called "inspection and test"

To be frank I don't very often come across situations where I can't isolate the supply and carry out a Ze loop test and check continuity of the protective conductors. The situation I illustrated, a malfunction clamp, is a real situation which would not have shown up with a zs loop test with all protective conductors connected because of parallel paths... Where's my bad practice?
cheers
s

I don't believe anyone has accused you of any bad practice. I think you have misunderstood the replies.
Perhaps you should "bone up" on your i&t.

 

[Stevesplatto]

I work in West London mostly and the majority of the services I come across are still copper or lead.
I think I'm very well briefed on procedures, I even look up things occasionally ;-)
To re-cap. the discussion got round to; if you can't isolate the supply then obviously the protective conductors should not be disconnected. To substitute for a Ze a Zs (or 'Zdb' which strangely isnt in the regs P39 where you'd expect or on google..) is recommended. However, with all protective conductors connected there is the issue of parallel paths (unless you have all plastic gas and water) and the fact you don't have an R1/R2 because that's a dead test so calculating Ze is a little hit and miss.
Then there's the enquiry angle which in the real world (or at least in London) is almost impossible to get from a supplier and most of the time will only quote the minimum from the regs anyway..
Bad practice on my behalf was suggested by Mr WW and apparently I need to bone up on Inspection and testing procedures!

All that being said obviously a loop test is going to be the way you check continuity on a live installation. My point was, way back at the beginning is that it's not the default choice as a general procedure for an EICR as illustrated by the site quote that started this. You'd isolate the supply and loop test the Ze at least so the Zs's can be put into context. Or, do people just loop test the Zs's and that's it...

Anyway, interesting to see how this forum works when you stick your head over the parapet with an opinion!

Cheers
s

I'm well aware of the 'agreed limitations' caveat (and also the way it's abused!).

 

[Farmelectrics]

any one ever done ze with earth connected and then retested when disconected never much in the readings and sometimes the same reading anyone had same issues

 

[Silly Sausage]

any one ever done ze with earth connected and then retested when disconected never much in the readings and sometimes the same reading anyone had same issues

I might try that later...do a true Ze then test with all earths connected then just through the water and gas pipes.

 

[Stevesplatto]

Good idea!

 

[malcolmsanford]

There is no actual guide to Zdb testing, but when I did my 2400 many many moons ago this was how I was trained to do it and how it is done

With the equipotential bonding conductors connected we can make a Zdb test at the distribution board, the test is carried out and the value recorded. If there is any difference between the recorded value of Ze and the recorded value Zdb then this difference is the value of the parallel paths associated with the equipotential conductors and this value must then be added to all subsequent measured values of Zs.

 

[mogga]

To be frank I don't very often come across situations where I can't isolate the supply and carry out a Ze loop test and check continuity of the protective conductors. The situation I illustrated, a malfunction clamp, is a real situation which would not have shown up with a zs loop test with all protective conductors connected because of parallel paths... Where's my bad practice?
cheers
s

HOW do you Issolate the supply fully? If its by removing DNO fuse then you are acting Illegaly and by recording this on cert you are admitting your guilt

 

[telectrix]

HOW do you Issolate the supply fully? If its by removing DNO fuse then you are acting Illegaly and by recording this on cert you are admitting your guilt

i thgink he meant isolating the installation by the main switch in the CU , so as to be able to disconnect main earth to measure Ze.

 

[Stevesplatto]

I use the red switch ;-) Seriously, the main switch, no need to pull the DNO fuse in this case.

Malcolm:
Found the Zdb procedure written out:
Ze; Zdb and Zs

 

[Deleted member 9648]

Found this on a large testing companies website:

'Where the mains supply is connected, as will be the case for periodic testing, Perhaps 'will' should be replaced with 'may'.....the protective and equipotential conductors must not be disconnected because if a fault occurs these conductors may rise to a high potential above earth. In this case, an earth-fault loop tester can be used to verify the integrity of the protective system.'

So wrong, I wonder how many wrongly measured Ze readings they've reported (parallel paths). I emailed them suggesting they might consider turning of the main isolator to reduce the risk of the conductors rising to 'a high potential above earth' before disconnecting the main earth for a real Ze test..
Big company as well..

Ride em cowboy!..rather harsh....I dont really think where an install cannot reasonably be isolated for testing a Ze inludiing parallel paths is a cowboy practice...more of an operational necessity on occasion.

.................

 

[Stevesplatto]

.................

I didnt paste the whole page but this was part of a description for general practice in periodic inspection and testing. My point way back then was good practice would be to isolate the installation in the first instance. The cowboy thing was more in jest really, I'm sure they're a good company. Discussion moved on a bit to how satisfactory using a loop test for Zs would be. Although Ze can be arrived at by enquiry supposing there was say a problem with the DNO's strap on a TN-S which a Zs may not p/u because of parallel paths/ and so on..
Good to question these things from time to time I think...
Cheers
s

 

[Des 56]

It seems his point has been lost, with the replies giving the alternative and regular situations where procedure has to be adapted

The way I read the post,the company are making their own real world experiences a set procedure in deference to the IEE, rather than a substitute for procedures which often have to be adapted because of necessity

 

[ackbarthestar]

Yeah right! The regs do say 'by enquiry'. Ever tried to get a Ze spec in writing out of a supplier? And what if you suspect a problem with say a suppliers TN-S strap clamp (which I did just recently) they will ask you what the Ze is before they'll come out!

I think having just achieved the C&G2391 is just the beginning of the journey.
Ze, by inquiry is already available to you since the DNO will quote their maximum standard value. In fact they probably won't even bother to reply to you, because you should know this. However, the Ze will rarely be the maximum quoted value. and would be better measured.
Now smaller installations where complete isolation is possible then a Ze can be performed by disconnecting the earthing conductor from the MET. But anything larger, where total isolation is inconvenient or impossible then you must not disconnect any of the main protective conductors.
A Zs close to, or at, the main intake is perfectly acceptable method of verifying that there is a protective system in place and to verify that the TNS sheath is still providing a reasonable result

 

[Stevesplatto]

I think having just achieved the C&G2391 is just the beginning of the journey.
Ze, by inquiry is already available to you since the DNO will quote their maximum standard value. In fact they probably won't even bother to reply to you, because you should know this. However, the Ze will rarely be the maximum quoted value. and would be better measured.
Now smaller installations where complete isolation is possible then a Ze can be performed by disconnecting the earthing conductor from the MET. But anything larger, where total isolation is inconvenient or impossible then you must not disconnect any of the main protective conductors.
A Zs close to, or at, the main intake is perfectly acceptable method of verifying that there is a protective system in place and to verify that the TNS sheath is still providing a reasonable result

(When did 2391 come into this?)
I think I said that didnt I? DNOs don't give real world values and just quote regs. My point was that the procedure of NOT isolating the supply, disconnecting the main earth and measuring Ze was cited as the default method. The example you give of taking a Zdb while accepted practice when the supply cannot be isolated (not 'convenient' is not a reason) isnt ideal because of bonding to gas/water etc which in many cases gives a better reading than the Distributors! So it won't give a true measure of the condition of the distributors earth. it's a compromise...

Cheers
s