external earth loop impedence to high


Postby danny001 » Sat Jun 07, 2008 7:56 am

hi,
i have a report which has a few problems listed
1 being external earth loop impedence to high - can any body say how this can be brought down

also on several mcbs the report says

zs reading above 100% of max zs so to high for disconection in required time

can anybody tell me what that means and how it can be fix

thanks
danny001
Posts: 47
Joined: Fri Feb 22, 2008 11:35 pm

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Postby ericmark » Sat Jun 07, 2008 3:10 pm

The fitting of earth leakage trips (or if already fitted smaller ones in ma ratting) will increase the Zs and Ze required so rather than bringing Ze and Zs down you take the required level up. Zs can be reduced by reducing cable length or increasing cable size but Ze is in a TN system set by the supplier not the consumer in which case the supplier should be contacted with TT system i.e. you have an earth rod. Then adding more metal should improve the reading. It you give us some more information as to TT or TN system is used and/or the Ze reading which will tell us any way plus what earth leakage is already fitted. Then we will be better able to advise.

Eric
ericmark


Postby danny001 » Sat Jun 07, 2008 4:30 pm

hi,
thanks for what you have said it is interesting,
the system is a TN-S

the ZE reading is 3.62

no sure if this info will help but in one of the sections that says

Zs reading above 100% of max Zs so too high for disconection in the req time -
this reading is for one of manyand is for some shop lights which is a 10amp mcb and is 2.5mm with cpc 1.5mm and 60898 mcb type c

3 phase
400volt

thanks if you need any more info i have the full report
thanks again
danny001
Posts: 47
Joined: Fri Feb 22, 2008 11:35 pm


Postby danny001 » Sat Jun 07, 2008 5:44 pm

hi.
what are earth leakage trips
are they a type of mcb that can be use
thanks
danny001
Posts: 47
Joined: Fri Feb 22, 2008 11:35 pm


Postby ericmark » Sat Jun 07, 2008 8:11 pm

First the report is quite right 3.62Ω is too high this is controlled by the supplier who should be informed as there may be a fault which they can rectify. This is first move and likely they will give you a site visit and correct the fault. Ze has a bearing on Zs so possibly this will cure all your problems.
Out of interest only as I don’t think it will be required you could fit a 300ma RCD which would add to fire protection too and would replace the main switch this would allow the Ze reading to rise to 150Ω and still pass. There are a number of names for the same thing RCCD, RCD, Earth Leakage trip, and they measure how much power goes out and how much power comes in which should be the same and if it varies by the set amount for example 300ma then it will trip as if there is a difference then some electricity must be going to earth. The 300ma is the largest which is considered to protect from fire. And since it is big it is unlikely to trip for silly things. There are smaller versions set to 30ma and these will protect personnel but you would not want to fit these on whole supply. As I say all that is needed is a phone call to supplier and I am sure they will cure it and supplier not billing agent.
It is considered that a shop etc will have an instructed or responsible person either directly employed or on a retainer and as a result it is common not to provide an explanation as it is assumed that you will understand.
Some one needs to manage your electrical safety, I don’t mean do the work but they would be responsible to call in electricians to do PAT testing as well as the PIR which has just been completed. Unlike the domestic you come under the Electricity at Work act and every electrical item has to be on a register and any faults recorded etc. Whoever is employed either directly or indirectly to manage the electrical system in your shop should be advising you. I don’t mind giving advice but “Watch your backâ€
ericmark


Postby danny001 » Sun Jun 08, 2008 8:01 am

Hi,
i will contact the supplier as this seems the best option at the moment,

someone else mentioned the 300ma rcd to replace the main switch but the tenent in the shop does not want to have the tills tripping out and that would cause them problems

someone said to me you could increase the size of the in coming earth and that may help - is this true

thanks again
danny001
Posts: 47
Joined: Fri Feb 22, 2008 11:35 pm


Postby ericmark » Sun Jun 08, 2008 10:27 am

It is very unlikely a 300ma S type will trip with no fault. S means there is a delay so thunder storms will not affect it. You can get RCD's that can be set to 1 amp and 1 minuet delay but they have to be only settable by skilled persons so did not seem a good option. But again as I said it was only for interest as I am sure your fault is down to your supplier. And yes increasing the size of incoming earth would normally reduce the Ze but it should not be that high anyway and is more likely due to a rusty bolt, oxidization or loose connection. So even if a RCD would allow you to pass it would not remove the root problem so must be referred to supplier first.

All best Eric
ericmark


Postby danny001 » Sun Jun 08, 2008 4:08 pm

hi,
iam going to ring the supplier on monday and try to get them in so they can have a look

when they do there own reading which i think they will.
i know the present reading is 3.62 but what is a reading which is accepable
for the tn-s system is there a web site i can look at or something i can print off

thanks again
danny001
Posts: 47
Joined: Fri Feb 22, 2008 11:35 pm


Postby ericmark » Sun Jun 08, 2008 9:05 pm

Sorry to say not that easy. For B type divide 46 by rated amps for C type divide 23 by rated amps for D type divide 11.5 by rated amps.
So for a B6 we get 7.7 and for C32 we get 0.72Ω these are for Zs values which = Ze + (R1 + R2) so if for example if your largest MCB is a B32 than Zs must be less than 1.4Ω if at the moment it is 4.62Ω and Ze is reduced to 0.4 then it will pass. There is a table 41.3 for easy reference but the book with it in costs £65 since you did not give the Zs values I can't help. If there are just one or two failing than using RCBO's instead or RCD's would be an option. A RCBO is a RCD and MCB combined. All faults should have a code against them. Code 1 means it has to be done but Code 4 just tells you it does not comply with latest edition of regulations and before any alterations or additions are done it will need upgrading. Even if it still fails the code number may mean you don't need to do anything in a hurry and you can plan how and when any upgrades are completed.

All best Eric
ericmark


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