PME Earth Impedance and RCT times - advice ;-)

[T0ny]

Hi all, great site and sorry for the long post - I'm after some advice as I don't have total confidence in the 4 sparkies I've currently used with regards to the test results on my cert. That said I am not a sparky and if I'm being a total muppet please tell me - I can take it


Basically I bought an old property and it was renovated. The property is 1940's It had lead wiring left over, tin wiring left over, some red n black left over and looks like it was re-wired around about the time the colours changed from what the sparks told me.



Everything has been rewired again by 2 sparks. The CU was a 10 way split load board and has been changed to a 17th board, 12 way 2 rcd (30ma) all breakers type b.



The meter has a PME (series 7) was in when I bought it, no prior certs - overhead cables. Meter is internal ie not in meter box outside.



The install and cert were completed by a qualified spark, he is returning at some point to upgrade one circuit which is about 7yr old, this was not accessible at the time. I have read the 17th and osg and in all honesty it seems he's done everything mint and even above ie 16mm earth bonds to gas n water, and all pipes cross bonded. LV in bathroom, board is balance well and well though out. The only junction boxes are on lights these are MF. Oven and Hob on separate circuits. All cables in oval and basket trays in ceiling. All terminations in MK boxes. To my layman eyes the design and work looks mint. Also another lad I know is a spark and he works away, he called over at the end of the first fix and second and said fair play the work is great. So I was happy.


However when it came to testing and the cert is where I've lost confidence. Here is my problems.

First of all I'm not 100% sure of the PME. It has a PME sticker from supplier and a section for the earth cutout by fuse), the tags are missing from the fuse section and tails. I spoke to a western power guy when I was out and about, just picking brains and he said - mate that's the norm as any sparky working on a job will often cut the tag and take the fuse with them so its not just isolated but some clown doesn't come and hit the power back on for their trade to see. If he had his way he'd leave that tag off he said. Which was what the sparks said to me as well, and the lad I know who works away. Makes sense.


However when I seen on the cert that the Ze is 0.45 then I thought well it should be lower for pme, even though I read here there is no legal limit. So part of me is thinking when the place had the split load board they, just clipped the tags and installed the pme because both the tag on the fuse and on the tails section is missing, but the meter (dial) one is still there.


From what I gather the street looks like most are on TT or worse they've just ripped the rod out and thrown down a patio or drive! Going to try and nose in some houses meet the neighbours and see if they are connected to PME. I can't confirm yet but will get nosey


So my first question is this, is this safe. The sparks view is that the reading is higher than the so called 0.35 but is still far lower than what I would get on a TT system. So regardless of if some person or the supplier has connected up the earth to pme, is this ok or should it be disconnected and a TT installed. I just want to know what is best as oppose to just contact the supplier especially when there are no tags on it. Just glad the meter one is on.



What I have been told and can see on the cert is the Zs is very low on my circuits, ie ring upstairs 0.56 lights 0.51 and that is repeated all over the other circuits. The Z1+Z2 continuity/resistance are all within the limits and were tested and not calculated.


The RCD test times are all very low at 1x ranges from 26ms to 64ms, and at 5x 1ms to 27ms. However I'm not sure if these are put in on the cert the right way around because the tester was saying he could not get the 30ma 40ms some made it and some did not the range was from 26ms to 60ms. Also they were tested live with nothing disconnected from sockets. Surely that would also skew results due to things like amps and items with capacitors in still holding a charge? I remember him saying the 1x were great but the 5x are going over.


What I'm getting at is, if the pme was just put in at anytime and the Ze is a tad too high then would this effect the RCD trip times at 5x current. Should an earth rod be added ie to the MET as a precaution. Or the PME removed and a TT system used. Plus is the cert a fail if your RCD times are over 40ms. I just want to know the score from someone without a vested interest in my job ie spark or supplier - just some knowledge to match what I'll be told next week.


As I said I've had another registered spark look at it and he said its all fine, its better than a TT. The cert seems ok and that I would not get rcd tripping as fast at 5x on a TT anyway. However he spotted 3 Zs that were filled in wrong ie they were 80 odd, when everything else was 0.3 on average. So getting that retested, as Z1+Z2 were 0.6 and the zs wrote in as 80.



I guess the bottom line is, the wiring all 'looks' great, and it has been done with all great gear. Its all terminated well, the board is fine and all the bonding is fine. But this is niggling me because I wanted better results and I'm don't have confidence in what I'm being told as I think the Ze should be lower, I think the RCD trip times for 5x should be lower, I keep getting sparks to look and none of them discusses or think about the issues you guys discus here. I got my certs yes, but I really do not care about them - I rather have no cert and everything is safe as possible than a piece of paper saying its ok I know he's dead but everything on paper looks ok.


I don't know if I am over reacting after all they are the professionals, and I've had a second and third opinion. But the issues are bothering me mainly because of the info I've read and in OSG, perhaps for the age of the building and the wiring entering the property its the best it can be - I don't know, any advice would be greatly appreciated that is more than contacted supplier and get another spark in again. Bottom line the job looks the part, but is it the best it can be or are these serious issues? Any help or views would be great guys ;-)

 

[telectrix]

first contact DNO and check if you have PME legitimately. i.e. supplied by them. then your Ze should be <0.35ohms. your Zs readings seem low considering a Ze of 0.45, maybe the Ze was tested on no trip? RCDs should ideally be tested at the RCD itself, with no loads connected. testing at sockets can sometimes give false readings, although the H&S thought police say it's safer. ( because you're not , perish the thought, putting insulated probes onto live terminals ).

 

[T0ny]

Thx Telectrix, I thought I would have to contact DNO, was concerned with no tags on the pme section. If I'd noticed it when I got the house would have called them then, I don't know how much grief it is having them missing, I just don't want it to be my grief lol as for the system as it currently stands what would you say the condition of it is? I'll have a word with the spark first before contacting DNO but I think it has to be done as how he be sure it really is PME if its not under 0.35ohms. Thanks for the advice, just trying to ascertain where my system is to date and how much of an issue I have. Bottom line is I want to work with the spark to get it sorted, nothing is ever dead simple and all jobs have some hassle - just glad I deal with wood

 

[Deleted member 26818]

TT would have an earth electrode, and a conductor from the MET (Main Earth Terminal) to the Electrode.
If There isn't an electrode, then either the system's not TT or it's un-earthed.
TN-S would have a conductor from the MET to the sheath of the incoming supply cable.
If there isn't one, then it's either not TN-S or again it's un-earthed.
TN-C-S (PME) would have a conductor from the MET to the 'cut out' the box on the end of the supply cable, that contains the main fuse.
There should be a part on the right of the main fuse with one or more holes for the conductor to enter.
Again if there is no conductor, it's not TN-C-S (PME), and if it's not any of the others, the system is un-earthed and the earthing would have to be through the main bonding.
Very unlikely, as during testing for Ze, the bonding conductors are disconnected (partly to prove that the system is earthed) and the Ze reading taken using the supply live conductors and the the main earth conductor.
As far as being able to tell whether the PME has been made by the supply company or by someone else, how would anyone be able to tell?
To make a system PME, an earth conductor is taken from the MET to the part on the right of the main fuse and terminated inside to a connector block that has the supply neutral connected to it.
Some heads have a link that needs to be conneted between two connector blocks, some just have one long block.
Sometimes the original TT or TN-S earth conductor are also connected to this connectror block.
Again there is no way, to tell who conducted the work, other than by speaking to the DNO, and hoping that their records are correct.
Check with the DNO as to whether they have it on record as PME, if they do then report the high Ze.

 

[T0ny]

Its defo not tt or tn-s. Some houses in the street have an earth rod, I do not. I have 10mm earth coming from the entry where the fuse is on the meter and that goes to my MET in the CU, next to those are the bonding to gas and water then circuits. The 10mm earth was disconnected and the Ze test carried out. Will check with DNO after chatting to sparky just to keep him in the loop. Don't get me wrong I'm more than happy with what is here now compared to what it was. I just want to try and get an understanding of how good/bad the system is until it gets resolved, and if it needs resolving ;-)

 

[Andrew Youll]

first contact DNO and check if you have PME legitimately. i.e. supplied by them. then your Ze should be <0.35ohms. your Zs readings seem low considering a Ze of 0.45, maybe the Ze was tested on no trip? RCDs should ideally be tested at the RCD itself, with no loads connected. testing at sockets can sometimes give false readings, although the H&S thought police say it's safer. ( because you're not , perish the thought, putting insulated probes onto live terminals ).

Where I am Northern Power Grid and Western Power, all go... uhm we don't know what is PME and what isn't... Western Power will allow for a higher Ze if it is a PNB install from a pole mounted transformer.

otherwise I'd get a 80A service fuse installed due to high Ze as per 5s disconnection in regs for HBC.

 

[Engineer54]

Very simple to check if overhead lines are a PME system. Just walk along the line of poles, every third pole (or thereabouts) should have an earth conductor running down the pole to an earth electrode. Usually that pole will also have a small warning plate for the down earth conductor, stating it's a PME connection.

Very rare for an overhead PME system to be over 0.35 ohms, due to the amount of multiple earth connections being made to the system....

Don't whatever you do, opt for a TT system earth, you'll be very unlikely to get anywhere near that 0.45 ohm. Nothing to stop you incorporating a good 2.4m rod into the system, in fact it's advisable. I don't like the CU's earthing bar being used as the installations MET!! Far better to have an external MET earth block. makes life much easier for testing and fault finding purposes!!

 

[T0ny]

Thanks Guys, all great advice. Yip Engineer54 the earths do look crowded in there. Will have a nose at the pole but there is tonnes of snow at the moment. I can see conduit/cable running down every post to the deck, will have a close nose when I brave the snow Will put a 2.4m rod in as well within the next week or so. Again thanks for all the info - its all very interesting ;-)

 

[telectrix]

if you install a rod, make sure it's at least 5ft. away from the building walls. try not to spike any incoming services or drains. use a proper earth pit .

 

[Andrew Youll]

if you install a rod, make sure it's at least 5ft. away from the building walls. try not to spike any incoming services or drains. use a proper earth pit .

If you add an electrode to a PME do you then need a 100mA Type S RCD? as if you lose the neutral back to the transformer you are then on a TT? Also if you lost the neutral somewhere along your supply would you become the PEN conductor for the street?

Just a thought I've had reading this, or am I complicating this?

 

[Engineer54]

If you add an electrode to a PME do you then need a 100mA Type S RCD? as if you lose the neutral back to the transformer you are then on a TT? Also if you lost the neutral somewhere along your supply would you become the PEN conductor for the street?

Just a thought I've had reading this, or am I complicating this?

No it doesn't become a TT system!! The earth rod is just another earth rod on the multiple earthed neutral. The advantage is, that it is local. And it's no good that rod being 200 ohm's either, it needs to have a ''minimum'' of lower double figure Ra value, Think of values around 10 to 20 ohms!!

 

[Andrew Youll]

No it doesn't become a TT system!! The earth rod is just another earth rod on the multiple earthed neutral. The advantage is, that it is local. And it's no good that rod being 200 ohm's either, it needs to have a ''minimum'' of lower double figure Ra value, Think of values around 10 to 20 ohms!!

Thanks for clearing that up.

 

[Deleted member 9648]

TOny for a non-sparky you have a surprising amount of electrical knowledge!

 

[Octopus]

I think you should either ask the sparky to come back and redo the Ze and some Zs tests, or pay a 2nd one to do this for you.

Doing this could cost you £40 - £50 but could save you loads more.

PS can you upload some pickies?

 

[T0ny]

Thanks Wirepuller and Murdoch. My electrical knowledge is only from trying to understand most of the stuff I've read and seen onsite, plus I like physics and my son does physics so I end up watching a lot of stuff with him. If I was younger I think I'd do a few courses and the 17th as I quite like it, but as for being on my knees and crawling around tight spots I'd rather not ;-) they've had a hammering over the years. I think most people can become fascinated with electricity when you ask the question "what is electricity, what actually is it" and to nearly everyone its probably a mystery (me included) which is nuts considering we all have been through school and the very thing that shapes our world, in general the average joe knows nothing about it - in fact it's magic juice to most us.

I am getting tests done again in around 3 weeks, will have the Ze re-checked and do the RCD 1x 5x again along with Zs. I think I have rights now to upload pics. Also when my mate has finished working away, he's in Egypt at the mo, and will be back in about 7 weeks, he said he'll give it a test as well.

 

[Octopus]

Thanks Wirepuller and Murdoch. My electrical knowledge is only from trying to understand most of the stuff I've read and seen onsite, plus I like physics and my son does physics so I end up watching a lot of stuff with him. If I was younger I think I'd do a few courses and the 17th as I quite like it, but as for being on my knees and crawling around tight spots I'd rather not ;-) I think most people can become fascinated with electricity when you ask the question "what is electricity, what actually is it" and to nearly everyone its probably a mystery (me included) which is nuts considering we all have been through school and the very thing that shapes our world, in general the average joe knows nothing about it - in fact it's magic juice to most us.

I am getting tests done again in around 3 weeks, will have the Ze re-checked and do the RCD 1x 5x again along with Zs. I think I have rights now to upload pics. Also when my mate has finished working away, he's in Egypt at the mo, and will be back in about 7 weeks, he said he'll give it a test as well.

When this is done, keep quiet and see what all the "results" are before saying anything. I have to wonder if the tests done before were reliable given what you've outlined.

Please "report back" with the findings.

 

[T0ny]

Will do Murdoch, I was there for the first tests. But now I know a lot more and I've read up on how to test, and read a fair bit in the forum so its not so much of a mystery to me now. I'm confident with the Ze because I was next to him and seen it. I think a few Zs have been jotted down wrong and the rcd trip times were rushed. That's my gut feeling with it all. It will be the same guy in 3 weeks, but he's not bothered his attitude is you can only record what you measure and if I ask him can we test this way or try this to see what it is, he will. So at the moment, I'll be getting a retest, and then in a month or so my mate will pop over and have the day testing. In reality out there, its pretty hard and hit and miss to get sparks who are as thorough and knowledgeable as you guys on here. Any of the lads who are brilliant are usually flat out with work or burned out on large jobs - not all, but most people in other trades like myself all find it hard to get good sparks with some free time.

 

[Octopus]

Rushed maybe?

Odd results shouldn't just be noted down!

Can't you upload a copy of the test results (blanking out personal details)?

 

[moose]

I am missing the point here i always do, but dont RCDs have a 1666ohm anyway LMAO, the only issue i see is X5 tripping above 40ms get a new RCD. im in for it now HEhehe.

 

[rattlehead85]

I would tend to think that the Ze test giving you 0.45 was probably done with a multi-function tester with the attenuator function activated. This would give you an overblown figure due to a higher resistance placed into the loop path by the MF which is used to prevent tripping of RCD's. This test will need to be done again without this function operated. Chances are you may well end up with a value below 0.2 ohms. Your Zs values as already pointed out do appear to be on the low side considering you have a high Ze.
This also gives credibility to the M.F setting theory.

 

[Engineer54]

I am missing the point here i always do, but dont RCDs have a 1666ohm anyway LMAO, the only issue i see is X5 tripping above 40ms get a new RCD. im in for it now HEhehe.

Another one that thinks an RCD is a replacement for a functional ADS system!! Luckily in this case Zs isn't a real problem...

 

[T0ny]

Just a tad more info, been out today to look at the street set up. Spotted a few earth rods which are all at the front door, but many more that you can't see ie rod but no earth on it and no rod. The houses have one overhead supply coming from a pole and that feeds both semi detached homes via an external junction box on one house. There are 5 poles per street. The first and last have supply running up from the ground to the top. Then there is one pole in the center of the run that has a cable running down to the deck from a grey box. I can not see anything external saying PME, you can see there has been some changes over the years as the rubbish is just left there.

I'm guessing the 1st and last pole (could) have PME, earth electrode at base, the center pole has pme and the others do not, the problem is on the deck they are in hedges etc so I can't see if there is an earthing pit etc plus the snow don't help. The street over has the same configuration. I am about 25m from that center pole with a cable going to the deck.

What reading on Ze would you expect if you were the only one on pme and 20 other houses are on TT. Would there still be an earthing effect via the neutral to everyone's tt earth? So my earth going to external neutral, then that neutral going to other houses and in some appliances the neutral acting as earth, which ties to their earthing system? Would I be right in thinking Ze would be a lot higher than 0.45 due to resistance? I just thought I would have seen more earthing points on the poles outside for the area. I remember helping out doing an earthing mat once for a HV LV transformer and we must have put in 25 rods all linked up in pits about 2m apart.

PS I take it ADS is not this - but please can I have one! mwhahahahaaa http://en.wikipedia.org/wiki/Active_Denial_System

 

[moose]

ADS - Automatic Disconnection of Supply page 403 BGB has a list of items to help on ADS, you need all the above to help in this matter but test sheets you say you got a reading of 80ohms nah matter your last line of defence is the RCD with 1666ohms Im sure Every little helps. Love this I feel like im wanted, Im learning loads, oh im not an engineer im average commertial spark wanting to learn, thats why im on this forum, but realy dont mind the abuse as you dont learn sitting in the background . Thanx for Help LMAO..

 

[Engineer54]

It's very likely that your street and the next street and maybe other streets are all on the same PME distribution line so it's not just the earth rods on the poles on your street, it's the combination of all of them.... You won't see any earth pit's on pole PME earthing electrodes, they are cad welded affairs below ground.

If the other house have TT then it shouldn't affect them, though may well do via any bonding of services. Much more likely is that the earth rods you have seen, are from a time before this overhead supply was PME'd!! Many being still connected to the MET of those houses... No it will not increase Ze values, the supply taken to your house is directly from the overhead supply as a branch or link, as will the other houses, so your Ze is directly related to the overhead supply, not to the other houses connected to the same supply. It would, if the supply looped in and out of each house, which it obviously doesn't ...lol!!

You normally see Ze values over the stated max 0.35 ohms for PME, on older converted TN-S systems, where the DNO distribution line hasn't been fully PME'd. I'd say that condition is fairly common too. Overhead systems are normally fully PME'd because of the ease of converting them....

 

[Engineer54]

ADS - Automatic Disconnection of Supply page 403 BGB has a list of items to help on ADS, you need all the above to help in this matter but test sheets you say you got a reading of 80ohms nah matter your last line of defence is the RCD with 1666ohms Im sure Every little helps. Love this I feel like im wanted, Im learning loads, oh im not an engineer im average commertial spark wanting to learn, thats why im on this forum, but realy dont mind the abuse as you dont learn sitting in the background . Thanx for Help LMAO..

So long as you understand that RCD devices are defined as ''Additional'' protection on TN systems and not a get out of jail device for a non functional ADS on new and additional installations, that's fine!!

On TT systems, especially when looking at around 200 ohms Ra values, they are the Only earth fault protection afforded to the installation, which brings it's own problems... lol!!

 

[moose]

To be honest with you I looked at the problem, as a spark walking in of the street and just seen results as ok as all ADS was implace in his first post so the readings where not an issue, I would personaly have told him to phone his supplier to look in to the high value to check there lines assuming the spark did not use the NO Tripp on his meter as this is eratic thats maybe my first assumption to walk in to this house.

on your last Post On TT i have seen suppliers common practice just stick a Rod in place for his issue, near CU.

 

[T0ny]

Thx again guys, I agree moose its better to ask questions than sit in the dark. I've seen plenty of lads on site that just follow the book verbatim and don't really understand why. But I guess that's the whole point of the book, you don't have to know everything you just need to follow it - obviously better to know why - but that causes headaches from too much thinking

What is the Perspective Fault Current, mine is 0.506ka - just curious to what it means

PS I'm going to write a schedule so I can keep it safe with the cert as I know all the runs at the moment, where as I'll not remember anything in 6 months let alone 6yrs. Will upload it when done.

 

[Octopus]

Thx again guys, I agree moose its better to ask questions than sit in the dark. I've seen plenty of lads on site that just follow the book verbatim and don't really understand why. But I guess that's the whole point of the book, you don't have to know everything you just need to follow it - obviously better to know why - but that causes headaches from too much thinking

What is the Perspective Fault Current, mine is 0.506ka - just curious to what it means

PS I'm going to write a schedule so I can keep it safe with the cert as I know all the runs at the moment, where as I'll not remember anything in 6 months let alone 6yrs. Will upload it when done.

I think you should sit tight until the sparky comes back and does some retests - or as I previously suggested pay another one to do some basic tests for you - this could be the best £50.00 you spend this year.

Floundering around on forums isn't going to give you definitive answers

 

[Engineer54]

To be honest with you I looked at the problem, as a spark walking in of the street and just seen results as ok as all ADS was implace in his first post so the readings where not an issue, I would personaly have told him to phone his supplier to look in to the high value to check there lines assuming the spark did not use the NO Tripp on his meter as this is eratic thats maybe my first assumption to walk in to this house.

on your last Post On TT i have seen suppliers common practice just stick a Rod in place for his issue, near CU.

Never known any supplier to install a Rod at a domestic installation!!

 

[T0ny]

Will do Murdoch. Thx

 

[moose]

If you have the BGB on PART 2 DEFINITIONS it explains the meaning of Prospective Fault Current (Ipf), for those that dont here is what it says,

PFC the value of overcurrent at a given point in a circuit resulting from a fault of negligible impedence between live conductors having a difference of potential under normal operation conditions, or between a live conductor and an exposed-conductive-part.