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PME and TT earthing

Postby griffo » Tue Sep 23, 2008 5:31 pm

Hi I have worked in a few properties (usually electricians houses) which have a combination of PME and TT earthing.

How does this fit in with the regs and were you to carry out minor works how would you catogorise the earhing method?

Are there any implications by having a combination of the two?

In the one property I carried out a test of the Zs accross the incoming supply and it was a very Low reading 18 ohms rings a bell, but still a rod as well?
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Postby ericmark » Wed Sep 24, 2008 9:47 am

If you have an earth rod and PME the earth rod is considered as an extraneous-conductive-part in the same way as water pipes, gas pipes etc. so needing minimum of 10mm² cable not 6mm² as in a TT system and the whole system is regarded as TN-C-S i.e. the earth rod is not considered as an earth rod. Often this happens where an electrician has found there is no earth phoned up the supply authority and also fitted an earth rod and RCD and considered it as TT which is correct then the supply authority have come and supplied an earth from their system also which they are required to do if possible. The only “implications by having a combination of the two” is the cable has to be 10mm² min to earth rod.
At the supply it is called Ze at the end of a circuit it is called Zs on a TN-C-S system Ze should be 0.35Ω or better outside the consumers installation with a TN-S system it should be 0.8Ω or better outside the consumers installation and with a TT it should be 21Ω or better outside the consumers installation as it enters the installation with exception of TT the reading may be slightly higher I realise the TT figure may seem odd what it means in real terms even if you have earth rods all around your house and earth mat between them you still may not get more than 21Ω due to suppliers earth not being good enough and where suppliers change supply transformers this could change so however good your earth you still must use an RCD as you can’t guarantee the suppliers earth is better than 21Ω.
If Ze is 18Ω then it is not good enough to be PME but for TT we can go up to 200Ω at the rod, but this is the rod only, not with all other extraneous-conductive-parts connected. I have seen paperwork with 0.5Ω entered as the earth rod impedance invariably the tester has failed to remove all the extraneous-conductive-parts before testing which where he is unable to switch off supply may be accepted but he must put a note to explain this and it would attract a code 4 further investigation required.
This of course should all be taught when doing C&G 2391 course and exam and is why it does need training. To disconnect an earth rod to test with a earth loop impedance tester can be very dangerous and using an earth rod test kit also needs skill and unless the tester holds a C&G 2391 the employer may have a hard time in a court of Law convincing people he had employed someone with enough skill if anything went wrong. So any employer allowing employees without a C&G 2391 to inspect and test is being a lot braver than I would be or very fool hardy.

Postby TOPSPARK » Wed Sep 24, 2008 6:52 pm

In the case you are describing the house should be using only 1 of the two earthing methods you have said exist. In the terms of the tt system under the regulations now and this was pointed out to me at my last annual inspection only 2 months ago that if the supply tails are 16mm squared then the main earth supplying the property must also be run in 16mm.if it is supplied with 25 mm tails then it stills has to be supplied in 16mm earth.this is written in bs7671 as well as the assessor showed me but alas i cannot remember the page or reg at present.But only one earthing method should be used at any one time
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Postby ericmark » Thu Sep 25, 2008 1:57 am

Please remember some of the Electrical organizations impose extra rules on their members. 542.4.1 requires any lighting conductor to be connected to main earth system. In view of that I can't see how any organization would require an earth rod connection to be removed. Also considering where the building has exposed metallic structural parts 411.3.1.2 (v) these have also to be bonded and 542.2.1 says how (iv) Underground Structural metalwork embedded in foundations can be used instead of an earth rod so there will be building where it is due to building design impossible not to have an equivalent to an earth rod connected to the main earth as well as the TN-C-S earth provided by the supply authority. So as to being able to have both TT and TN-C-S at the same time this must be allowed but as I have already said the rod would only be considered as an extraneous-conductive-part.
The cable sizes to earth rods can of course be greater than the regulations require and there is nothing to stop any electrical organizations imposing extra rules on their members in regard to this so there could well be an implication if you are in some way connected to such an organization. The same goes for main tails and some supply authorities insist on tails bigger than the cable they supply the house with. Regulation 544.1.1 gives min size as 6mm for TT supply but as I say if both systems are used the it is not an earth rod as far as regulations go so would need to be at least 10mm sounds odd I know but what Topspark says is in a way correct as if there is a TN-C-S supply the earth rod is not counted so it is just a TN-C-S supply. And with a PIR the rod would not be tested in the same way as water pipes are not tested to see if they are a good earth.

Postby griffo » Thu Sep 25, 2008 7:18 am

Thanks for the replies, Eric sorry to confuse Ze and Zs for some reason the two wont stick in my mind, in the TNCS system I mentioned the reading must have been 0.18 not 18 as I stated, as I recall checking the reading against the OSG, your point about the rod becoming extraneous metalwork is interesting, I hadnt thought of it that way before.
Regarding further training I would welcome it, when part P was introduced I needed to update my skills to carry on doing the work I had allready been doing, looked around and found a local college doing a part time course, after asking the principle tutor a lot of questions and being assured that attending this one course would satisfy my needs I paid the £1200 fee.
On the first day of the course we were all informed that this was a low level course to get us some electrical experience and pass the part p exam on bldng regs and we would also need to undertake 2381 and 2391, both of which we could carry on and do at the same place, so two years later after spending in the region of £5000 in course fees, literature, test instrements and lost wages, paid and turned up for the 2391 course only to be told on the first evening that we were not eligable to do this course unless fully qualified electricians and wa-----rs like me shouldnt be there, this from the same tutor who told us this was the route to take.
Hence why I try to gather as much knowlege as I can from forums like this and coleages in the industry and far from being brave or foolhardy, I have tried to follow the correct route only to have been lied to and decieved out of money by a local college, I dont know anyone one else in my industry who has invested anything like as much as me into appropriate training, most still carry on adding fused spurs for boilers, connecting jacuzzi baths or putting long extension leads on appliances, I take a pride in trying to do carry out my work in best, safest most profesional way with my skills, knowlege and experiance.
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Postby ericmark » Thu Sep 25, 2008 1:02 pm

Sorry to here about your experience. Must have been a bad college. I trained on leaving college as a motor vehicle technician and bridge builder. I continued training to become an auto electrician and was trained in the logic circuits used in traffic lights. As a result I was able to change jobs and worked on pumps and traffic lights which was my first introduction to 3 phase systems. From that I went to work abroad where there was no demarcation between Auto and Mains electricians we were all electricians. Here I gained a lot of training in generation working on 1.5 to 750kVA generators many synchronised with each other. Returning to the UK the 16th Edition had just become BS7671:1992 and for the first time regulations were being followed by run of the mill electricians but by this time I was involved with heavy plant concrete batching plants, concrete pumps, placing booms as well as general site electrical systems. Next I moved to large generators 750 Mega Watt each. And then Marine and heavy plant on the River Seven and a Belfast Ship Yard. Then at last back home on a petrol chemical gas receiving plant. And another large generator this time four 600 MW units. From this I went to Hong Kong working on a Tunnel boring machine. In all this time I had twice tried to do 16th Edition and 2391 but both times job moves cut short the course and I never took exam. After moving around the country fitting operation building access and security systems I was employed as an Electrical engineer for second time and thought it was about time I took some exams and I paid about £80, £70, £90 for three courses and exams 16th Edition, Pat Testing, and 2391. Except for one or two mistakes in form filling I had been making I learn very little. After an accident in T5 in 2004 I returned to college and took a full time IT course and from that went to Uni and got my degree in electrical and electronic engineering then had to take upgrade to 17th Edition which cost £175 I wish my wages had risen by same amount between 2002 and 2008! So as yourself I did not do a formal apprenticeship as an electrician mine says engineering craftsman. And I had no problem being accepted by the college or University to sit the course and exams.
I did go to take the 2330 course and exam but it was very apparent this was well under the level I required and I was learning nothing and making it hard for lecturer because I did not fit in with the rest who were also in same class often wondered if they ever made it as electricians! Not sure why your courses were so long all three courses and 4 exams I took for 2381, 2391 etc were one night a week and completed within the one year. Biggest problem with regulations is Chinese whispers where instead of reading the book people tell others what they think is says which invariably is not quite what it says and added to that are the extra rules that some of the Part P registration bodies also want to impose. Plus of course in real life one has to bend the rules best example is core colours for 110 volt cable where most if not all leads have a blue rather than black core for the second phase. On forums like this one has to be careful and quote the regulation number where possible as if one strays for the book answer some one will quote what we have said and tell some poor electrician how he has done it all wrong.

Postby dacstar » Sat Nov 29, 2008 12:57 am

This may upset a few people!
I have started in a new area and need to get used to the local supply companies and what you can get away with such as removing service head fuses etc etc
I have started working in a "barn conversion" and have found a new service head and an earth spike. No mention of PME or any visible MET .... tested the earth spike, ok. Cut a long story short the new service head seems to be doing its stuff and the earth rod is good, i have put in an MET for the bonding and for some other stuff that would benefit an MET and run 16mm2 to the rod in case of any PME issues there is enough meat on the 16mm2 to hold the load until the fuse/rcd clears

as for the training issues i had a tough time as i am a "convert" from Electronics. as we deal with wigglyvolts and sillyamps we arn't sparkies! i dont consider 660v and watercooled thyristors to about 1200A either wiggly or silly.
I got on the C&G 2360 parts 1 and 2 using my HNC, then 2391 at middlesborough(Faraday training centre) 2 attempts! then Prt P with Elecsa as they seemed to want to help the one man band, and not take your money and perscribe more courses, then this year a 1 day conversion to 17th edition thanks to Clarkson evans in Gloustershireish area.
Yes there are certain areas of the building regs to adhere to for the part p especially now the carbon footprint and fire detection are being looked at but the documents you need are available on tinternet such as L1a and L1b in new and existing buildings. Down load for free and demonstrate a working knowledge to the part P assessor like low energy light fittings are 1 per 25m2(dont quote me on that its what Stella is telling me) and he will be happy or guide you to where youve missed the point.
Test equipment is important and expensive, hire it when needed until you can afford it. I did/do when mines in for cal or repair or i needed it and couldnt afford it. The hire cost me about £70ish for a week, plus the usual vat and carriage on top but two or three good jobs to test in the week and you've got that back i can do PIR's with the 2391 qualification for landlords at say £150 a time- 3 in a week!
Griffo, if your still there, keep going, look for the good guys Faraday training, Clarkson Evans, Elecsa, napit. they are there to help, i dont get a hand out for mentioning these people and i would ask for one but they have helped me get to where i am today and that aint stuck in a factory reparing busted circuit boards, there will probably be criticism on this post as someone will say i cant do this or i cant say that or i am wrong for my thoughts on earthing

rant over, the tin is empty and i have to go to work in 7 hours!
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