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?

 

[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.

 

[VoltzElectrical]

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

They'll have to if they want their insurance to remain valid! What is it really going to cost them for you to go back and do a quick Zs at a socket/ light. No trip setting won't even disturb a thing?

What is this nonsense?

 

[ArseyDee]

I'm confused.

You have R2 reading (conduit).
Presumably, you have the measured RFC end to end resistances (r1,rn).
You have Ze following DNO fix.

All the ingredients are there for a calculated Zs.

Or have I misunderstood?

 

[telectrix]

still wants a visit to measure Zs. calculations are for the design stage and wimps who can't or won't perform live testing.

 

[VoltzElectrical]

I need help and some advice on how to wipe my ---, and then tie my shoelaces.

Anybody?

 

[MDJ]

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

Of course they won't, they expected a professional electrician to visit their premises and install an electrical system and to issue test certificates based on testing with instruments in regard to the works carried out. You didn't do your job properly and you need to eat humble pie and get back there and do the Job right, absolutely ridiculous, appalling in fact.

 

[Darkwood]

Go to the school talk to whoever you liaise with tell them you will be needing to take sample zs tests, i assume prior the DNO upgrade a majority of the circuits failed on Zs ..even if you calculate them all some may have other issues not related to the original poor earth and may be now borderline pass/fail even with your new calcs ... its circuits like these that need retesting .... your only going around doing random zs tests and ensuring the ones that are borderline by calc actually do pass.... a morning or a days work depending on school... go in on a sat' you charge for it job done where's is the problem.

Its seeming more like it not actually been a problem for a school to find a couple of hundred extras pounds and more like you hate doing it and trying to find excuses... even in these tight buckle times a school won't refuse if it stop them getting insurance .... i spent yrs working in them during hols and acted as Electrical Foreman and i cannot understand a school refusing to pay under your circumstances the extra time to do some sample tests.

Are you a company boss / self employed or is someone else pulling your strings and telling you you can't go back and to just calculate it? If the latter tell your boss from me he's a $%^$£^$&^ XXXX%$^^& and to (*&^% > ***&^%$$ &^%% ---and if it gets stuck up there then tough titty.

 

[Deleted member 72666]

I need help and some advice on how to wipe my ---, and then tie my shoelaces.

Anybody?

Can't help you there voltz, as the wife does me mine!

 

[VoltzElectrical]

Of course they won't, they expected a professional electrician to visit their premises and install an electrical system and to issue test certificates based on testing with instruments in regard to the works carried out. You didn't do your job properly and you need to eat humble pie and get back there and do the Job right, absolutely ridiculous, appalling in fact.

Way too harsh and well off the mark there based on my reading of the OP.

Sorry if I'm wrong.

Sounds like you should spend less time hanging out with Glenn!

 

[R G]

How can you give out electrical certs that demand an Earth Loop test carried out with a live supply , should have factored in a possible re visit within your pricing , if thats the issue , whatever that does not change anything your name is on them certs

 

[MDJ]

Way too harsh and well off the mark there based on my reading of the OP.

Sorry if I'm wrong.

Sounds like you should spend less time hanging out with Glenn!

I don't think so