Unexpected reading between Earth and Neutral

[turnitonagain]

That's all well and good if the neutral supply conductor has been earthed at several locations along its length, eg such as found in PME neutrals. It's not such a great idea to willy nilly earth the neutral as and where you come across a TT system.

No it's not allowed in the UK. The only allowable connection to the supplies service neutral is by the DNO, and then generally only when the neutral conductor meets the all the requirements as PEN conductor... Nominally 0.35 ohm but can be higher as stipulated/approved by the local DNO...

fair point, so how do reduce the 30.4 ohms as it is far too high in any circumstances,

 

[turnitonagain]

fair point, so how do reduce the 30.4 ohms as it is far too high in any circumstances,

thanks for your reply, no system is perfect, it would make sense to allow to use the neutral conductor also as a path for fault current, as the mass of the earth might not be good enough conductor on its own hence 30.4 ohms. in ireland that is what we do in a tt system, in fact we would regard the customer earth electrode as secondary to the neutralizing conductor. also i am not taking away the absolute importance of having an earth electrode in place, if for example the supply neutral if overhead and single ever became open circut. this is only an observation of mine not a rewrite of the rules.
.

 

[Engineer54]

fair point, so how do reduce the 30.4 ohms as it is far too high in any circumstances,

Ah well, there lay's another problem with BS7671!! lol!! They don't give a maximum value, only suggest that a Ra value over 200 ohms is unlikely to be stable!! Far too many actually think that is the maximum level, but they'd be very wrong...
So most totally rely on a 30mA RCD rather than try and reduce the Ra value while the more enlightened, will also install an upfront 100mA S type RCD to cover the possibility of an inoperative 30mA RCD...

There's plenty of threads on this subject in the archives here!! lol!!

 

[turnitonagain]

good point, it is not a good idea to rely on rcd or similar devices on there own, because as you are aware, nobody ever or seldom tests these units, and if they were expected to trip after many years of inactivity, as i have come across many times doing a rcd trip test, which here for a b type standard is 400 ms max.

 

[marc8]

I understand that creating your own PME is not great, but the majority of the network these days is PME so what's not clever to do it, call in the board & get it changed it's free & as above the network is mostly PME anyway.
i am not saying anything about the fault at this time just about making the house PME

 

[turnitonagain]

do not create your own pme if not allowed to do so, by your dso. if the dso will do it for free, good. you will now have as good as possible Ze

 

[turnitonagain]

remember earthing is all about creating the lowest possible [impedance] path for fault current to reach the supply traffo as quick as possible , so the protective device will open as quickly as possible. [ i know this obvious but no harm in reminding ourselves].

 

[Risteard]

We are expected in Ireland to neutralize an installation if we come across a tt system

Although a TT system (aka Direct earthing) is extremely rare in the south of Ireland, such a system is permitted by the ETCI Wiring Rules.

 

[polo1]

Ah well, there lay's another problem with BS7671!! lol!! They don't give a maximum value, only suggest that a Ra value over 200 ohms is unlikely to be stable!! Far too many actually think that is the maximum level, but they'd be very wrong...
So most totally rely on a 30mA RCD rather than try and reduce the Ra value while the more enlightened, will also install an upfront 100mA S type RCD to cover the possibility of an inoperative 30mA RCD...

There's plenty of threads on this subject in the archives here!! lol!!

Eng, you feelin ok? You didn't mention "twigs" :49:

 

[Engineer54]

I understand that creating your own PME is not great, but the majority of the network these days is PME so what's not clever to do it, call in the board & get it changed it's free & as above the network is mostly PME anyway.
i am not saying anything about the fault at this time just about making the house PME

First off the majority of UK's LV networks are Not PME at all, the overhead supplies that have been PME'd can be considered as true PME but the underground supplies are far more likely to be TNC-S that are partly or in the process of becoming PME in the true sense of the word and can have a Ze value, say anything from less than 0.35 ohm to over 0.80 ohms or more....

There are several reasons why you shouldn't be making your own N-E connection at the DNO's service head, the available Ze may not be suitable, is one....now you have a think about it and come up with at least one or two more??

Oh and it used to be free, but the privatised DNO's have seen it as another source of income that they can squeeze out of customers. Some still are free but not many, and i'd guess not for too much longer!!
.

 

[marc8]

#25 read the post I said call them in it is free & if you were to make your own PME what do you think you would check for exactly the same as the board would do (uk power or whoever sorry not the board anymore) you check the ZE across L/N, if I am doing a CCU change then that would be the first thing I check & if it's a TT I advice them they could get it changed for free. Have a guy who works for uk power as cable jointer / fault finder ect on their network & as he has said many times it is nearly all PME now.

 

[turnitonagain]

never make your own pme unless you are allowed, its important that the dso has a low enough ze, and if tt ze is too high, call the dso and they may neturalise, i would never recommend using a mechanical device eg rcd, solely to protect from earth faults, its better to have a low ze in the first place.