PME Earth Impedance and RCT times - advice ;-)

[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.