PME Trip Times etc advice

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Hi all, great site I'm not a big DIY'er but looking to pick some brains - I'm after some advice as I don't have total confidence in the 4 sparkies I've currently used. 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 wired around about the time the colours changed from what the sparks told me.

Everything has been rewired 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.

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, 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 (have lurked for a while). 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 and see if they are connected to PME. I can't confirm yet but will get nosey :) but I all looks like TT to me.

So my first question is this, is this safe. The sparks view is that the reading is higher than the so called 0.35 (I think) 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 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 when testing he said could not get the 30ma under 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?

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.

As I said I've had another spark look at it and he said its all fine, its better than a TT. The cert seems ok some errors 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.

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 it 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, 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?
 
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I wouldn't be overly worried about it as TNS supplies have a limit of 0.8 Ω so you are still way below that.
 
Woah! rather long. A few facts.

PME is Protective Multiple Earth and is a type of supply from the Distribution Network Operator.

A PME Ze of 0.45Ω is a bit high but may be acceptable.

The fact that it is lower than TT is true but totally irrelevant and meaningless. TT would be many ohms.

What is the Prospective Fault Current (PFC) on your certificate?


The Zs measurements are unfeasibly low for such a Ze - especially the lighting circuit.

You cannot be told that your circuit Zs is very low if that is not expected.
The R1+R2 (not Z here) is a straight value of ohms per meter of cable.
So this must be a tiny property as the lighting circuit can only two metres long.


RCD times - would not be affected by any of your other (Zs) results.

@ 1 x IΔn (30mA) - maximum 300ms.
@ 5 x IΔn (150mA) - maximum 40ms.

These tests should be done with no load on the RCD as obviously any fault in the circuit will affect the results.
 
Thanks EFLImpudence soz for the length :)

Ok, for the prospective fault current is 0.506

lighting 1 is 1.30 zs and is about 26m in total
lighting 2 is 0.42 zs and is 17m
lighting 3 is 0.53 zs and is 16m

All rcd times met 1 x IΔn (30mA) - maximum 300ms
but only 3 met 5 x IΔn (150mA) - maximum 40ms. the rest were mid 55ms.

Don't get me wrong I'm happy that what I got here now was a damn sight better than what was here at the beginning - I just want to make some sense of it. The Ze is not massively above the recommended for tncs, I guess I'd be a lot happier if my trip times were under 40ms then at least I know the supplementary protection is fast enough. Hope that helps, but any advice would be great ;)
 
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lighting 1 is 1.30 zs and is about 26m in total
lighting 2 is 0.42 zs and is 17m
lighting 3 is 0.53 zs and is 16m
R1+R2 for 1.5/1mm² twin & earth is 30.2mΩ/m

Lighting 1 = 0.79Ω leaving 0.51Ω for Ze
Lighting 2 = 0.51Ω leaving -0.09Ω for Ze
Lighting 3 = 0.48Ω leaving 0.05Ω for Ze
 
Ok, for the prospective fault current is 0.506
That would seem to be about right with a Ze of 0.45Ω.

lighting 1 is 1.30 zs and is about 26m in total
That could be right.

lighting 2 is 0.42 zs and is 17m
lighting 3 is 0.53 zs and is 16m
As Zs = R1+R2 + Ze you will see this is highly unlikely.
Zs can be lower than expected because of parallel paths but not that much.
As you have RCD protection on all circuits and so presumably no supplementary bonding it is difficult to see where these paths may be on a lighting circuit.

17m. of 1.5/1.0 T+E has an R1+R2 of 0.51Ω (1.0/1.0 higher) so ...
 
A PME Ze of 0.45Ω is a bit high but may be acceptable.

Yep it will still allow a 100A cut-out fuse to operate within 5 sec for a fault prior to the customer's protective devices
 
Thanks again EFLImpudence at least the Ze with the PFC is computing, so I guess I have nothing to worry about there - to a degree?

1 of the lighting circuits with the low ZS does have a bathroom so I think the rad in there has an earth to the lighting circuit - will check. Only bonding I know is gas is in 16mm and back to met, water 16mm and back to met and all pipework in bathroom and kitchen is cross bonded. Plus all heating is cross bonded.

With regards to the Ze is it worth installing a earth electrode to the met - ie rod outside to met in CU? Some say yes that's a welcome bonus to TNCS and some say no. I'm guessing there is pro's and cons probably based on if the neutral goes live from the DNO?

My main angle is the safety ie ar the rct times at x5 a major problem or not and how could they be improved if they are. Also surely if someone at some point just connected up the pme and it was not done by the dno then surely the Ze would read a lot higher and there would be no stickers saying PME on my meter. Again thanks for the help, the more I read the easier it gets, but may have to call over the pub later to cure the headache it gives ;)
 
at least the Ze with the PFC is computing, so I guess I have nothing to worry about there - to a degree?
Seems ok.

1 of the lighting circuits with the low ZS does have a bathroom so I think the rad in there has an earth to the lighting circuit - will check. Only bonding I know is gas is in 16mm and back to met, water 16mm and back to met and all pipework in bathroom and kitchen is cross bonded. Plus all heating is cross bonded.
Could be the explanation.
Check the R1+R2 or R2(double the single R2 value to estimate R1+R2) values on the cert.
When added to Ze they should make more sense

With regards to the Ze is it worth installing a earth electrode to the met - ie rod outside to met in CU? Some say yes that's a welcome bonus to TNCS and some say no. I'm guessing there is pro's and cons probably based on if the neutral goes live from the DNO?
It's one of those things which possibly could happen but the vast majority of installations manage without.

My main angle is the safety ie ar the rct times at x5 a major problem or not and how could they be improved if they are.
Only by replacing the device.

I was unsure of this -
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 when testing he said could not get the 30ma under 40ms some made it and some did not the range was from 26ms to 60ms.
If 5x values are over 40mS you can only replace the RCD. Nothing else can make the time longer unless his meter was faulty.

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?
If all the MCBs were turned off that is ok.
If not, any fault would make the times shorter.

Also surely if someone at some point just connected up the pme and it was not done by the dno then surely the Ze would read a lot higher
Would certainly be higher without PME.
It should not be done without authorisation.
You could check with neighbours. If you are the only PME then have a word with DNO.

and there would be no stickers saying PME on my meter.
Presumably.
 
Top man EFLImpudence all makes sense to me and is more or less what the sparks said. My board is a 63a 30ma rcd and 63a 30ma rcd they did advise to maybe change one to an 80rcd and keep the other as 63rcd. I don't understand what that is about will have a read up.

I do have the option to have the rcd's replaced by the supplier. So that may be worth considering. Will check the R1+R2 and just the R2 as you say its normally around half.

Yep some of the rcd trip readings at 5x were in the 55ms range but a few were around 30 ish. But it was tested live and none of the breakers were off and had everything plugged in pc's, amps, tvs etc. Tested from the furthest point.

Will try and have a nose at neighbours set up's see if any have the PME connected. Again thanks for all the info, helps me make sense of it all. Thank god I do a different trade. Its very interesting but fair play you sparks are expected to know a great deal of physics, christ my gas was done and tested in an hour and was a doddle to understand :) I like physics and have read about Faraday cages etc and magnetic fields its all very interesting ;) fair play to you guys :)
 
RCDs need to be tested in isolation i.e. without any circuits/loads connected.
Re the 63A/80A thing, what circuits do you have?
 
Hi Spark thx, will get him to test them in isolation on final visit. They definitely were not tested that way.

circuits are

lights, central heating, fridge, hob, shower, oven, ring main downstairs, ring upstairs, kitchen ring, outside lights.

Oven, hob, ring, lights / shower ring, ch, fridge lights.
 
WRT the trip times, I suspect the geezer has gone round testing from socket outlets. As you said, this was done with stuff plugged in, so may affect readings.

The RCD tests are best done in isolation. I normally disconnect the tail to the N bar and switch off all outgoing breakers, thus the test is only on the RCD itself.

I picked up a second visit from a colleague where he had tested the RCD and it had failed.

However, when I tested it in isolation, it passed with flying colours.

Turned out the fridge/ freezer was cocking up the test result.
 
WRT the trip times, I suspect the geezer has gone round testing from socket outlets. As you said, this was done with stuff plugged in, so may affect readings. The RCD tests are best done in isolation. I normally disconnect the tail to the N bar and switch off all outgoing breakers, thus the test is only on the RCD itself.
That's certainly normal/good practice, but I find it a little hard to see how having loads connected to the circuit could increase measured RCD trip times. If the loads had any effect, it would presumably be to increase the current imbalance during trip time testing - which I would have thought would result in at least as rapid tripping as with an unloaded circuit.

Kind Regards, John
 
I thought and I'm only guessing here that the 'time' can only be measured when there is no power in the circuit hence a switch going from on to off. If everything is connected and the trip fires, but if there is still some current from items plugged in leaking into the circuit, then that would read as switch not off and add time? IE increase the ms.
 

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