Calculating Zs

[sparx northeast]

I need to calculate the Zs for a ring main that uses the metal conduit as CPC. I know that Zs = Ze + R1R2 but I only have an R2 reading as there is no wired CPC. Anyone got a clue?

 

[Plonker 3]

Re: Calculating Ze

Why do you need to calculate it? Have you not got a ELI meter?

 

[sparx northeast]

Yes I have a multi function Megger. The Ze at the time of testing was way over the max allowed. So the local supply company have eventually repaired the supply 2 weeks later and PME'd it to get the reading down. I could therefore only do dead testing at the time and have no intention of going back to re-test every circuit now. I need to calculate all Zs readings from the new Ze and R1R2s

 

[Plonker 3]

Looks like a revisit I am afraid, did you not think of this before you left?

 

[Bobster]

No wired cpc an yet you have an R2 reading?

 

[Pete999]

No wired cpc an yet you have an R2 reading?

It took me a couple of reads but yes you are right

 

[sparx northeast]

Yes I did think about it before I left but with a Ze reading in excess of 2 ohms, what else could I do?

 

[Plonker 3]

What works have you actually carried out on this job?

 

[sparx northeast]

R2 is the resistance of the conduit

 

[sparx northeast]

A periodic inspection

 

[Darkwood]

new/converted supply...new connected mains terminations ... how do you know the DNO hasn't got a bad connection?... you need to revisit to ensure new setup is correct.

You went to test a job ...it failed you called in DNO to rectify issue.

You now need to go retest .... cant see issue here - 2 separate invoices 1 for initial testing 2nd one for testing following supply upgrade.

Obviously its only the live tests to re-do and you can discuss with the customer the second testing will be a reduced cost visit.

 

[sparx northeast]

Nice thought but it's a school and they ain't gonna pay twice

 

[Plonker 3]

So why are you altering your results, any remedial works are surely chargeable?

The DNO have done there bit, it's not upto the customer to ask for a retest and pay for it surely?

- - - Updated - - -

You issue cert saying not suitable for continued use, that's all they have paid for unless the remedially work is included I the price they are paying you.

 

[sparx northeast]

Nice thought but it's a school and they ain't gonna pay twice

 

[Plonker 3]

We'll tell them you issued cert as requested and will,have to pay again if they want results altering due to upgrade on DNO cable. Not your problem.

 

[netblindpaul]

If they ain't gonna pay, then they don't get the work done, else you'll have to do it for nowt.
You did your bit the first time round, if they want any more work then they should pay for it.

 

[sparx northeast]

We are getting away from the point here. The school was closed for the holiday so was not in use and safety wasn't an issue. I dead tested R1R2s on all circuits with a view to calculating Ze's as is allowed in Guidance note 3. The Ze is now acceptable and I just need to know how to calculate the only circuit that doesn't have a cpc

 

[netblindpaul]

It does have a cpc, it's the conduit.

 

[Plonker 3]

You can't can you, you proved that in your opening post. Just in case you missed it

Zs=R1+R2+Ze

No you only have Ze

R2 only exists in your head.

So looks like a revisit at your expense if you want to reissue a cert with out actully proving anything yourself, which leaves you wide open for all sorts of problems in the future

 

[Darkwood]

Nice thought but it's a school and they ain't gonna pay twice

Either you haven't included or discussed your need to revisit with the school thus not quoted for it but in light of your findings and the issue of a failed cert' you now need to now confirm the new set-up is compliant and issue a post upgrade cert' whether the school likes it or not they will not have a satisfactory cert' unless they pay for a second testing ....as i have mentioned in my edit of my last post ... you should be liaising with the school about the this and explain that 2 thirds of the testing will not be required but you will need to redue the rest as the earthing system has been changed meaning all live previous values have now changed.


I don't see a problem here its standard practice I can only see an issue if you haven't informed the school throughout the testing and remedial about the change to any quoted work that the supply upgrade will have...
If they want a passed cert' you will need to do some retesting how will calculations show any issues that may be present regarding DNO work done?


If you have been back and got the Ze and guidance notes express you can now calculate all Zs values (not got my copy to hand and never needed to go down this route) then id still be inclined to take a few random tests of various circuits to back up your calcs are close to actual measured values ... if not then something could be amiss.