External earth fault loop tt system

[brightspark2010]

Evening all could anyone please tell me the the maximum ze for tt systems the jobs entail Rec cut out fuse bs88 16a to feed 1x 30 ma rcd fuse spur for feeding socket outlet what should the max ze be please

 

[Engineer54]

Good point, I hadn't thought that far ahead. Was just having a rant. I don't think we did distribution transformers on my 5 week course, although that may have been in the two weeks I was off having it large in Ibiza. Can't have missed that much.

On a serious note, the DNO is responible for getting current to and from the installation (Line and Neutral) so why shouldn't they also be responsible for the fault current path as well?

They are, where they provide an earthing point!! Sometimes the system just cant take any more end user PME connections. It's not always as simple as it first seems, and why you should always ask the DNO, if a PME connection is available. And more importantly, why you should never make your own PME connection, even if you know your next door neighbours on either side has been provided with a PME connection!!

 

[R G]

Having worked for one of the RECs and got involved with supply and distribution the pme system is the preffered system used at all times by them , you will probably notice more and more overhead TT systems being converted with the new aerial buched conductor(ABC) network , you will also probably have noticed that every 4th pole as required by Distribution Supply regs that the neutral is bonded down to earth with a stake , this is for safety reasons in case the neutral breaks down , There are drawbacks of course firstly the need to test for polarity on the incomming supply and also the loss of the neutral on the incoming supply , and the consequences. But there is no difference whether there are 10 or 100 properties being supplied , as long as the Ze is below the 0.35 . The fault current that returns back to the transformer is no longer fault current from the cut out back to the Tx , only within the customers premises , Yes it is correct that the REC would need to be contacted if you want a PME conversion , you can not do it yourself , but as time goes by more and more of the old TT systems will be converted , Indeed many olde

 

[Engineer54]

Having worked for one of the RECs and got involved with supply and distribution the pme system is the preffered system used at all times by them , you will probably notice more and more overhead TT systems being converted with the new aerial buched conductor(ABC) network , you will also probably have noticed that every 4th pole as required by Distribution Supply regs that the neutral is bonded down to earth with a stake , this is for safety reasons in case the neutral breaks down , There are drawbacks of course firstly the need to test for polarity on the incomming supply and also the loss of the neutral on the incoming supply , and the consequences. But there is no difference whether there are 10 or 100 properties being supplied , as long as the Ze is below the 0.35 . The fault current that returns back to the transformer is no longer fault current from the cut out back to the Tx , only within the customers premises , Yes it is correct that the REC would need to be contacted if you want a PME conversion , you can not do it yourself , but as time goes by more and more of the old TT systems will be converted , Indeed many olde

Go and ask your Engineer if that is always correct or not!! You may be very surprised by the sound of it!! lol!! Think you must have been working on relatively all new PME installations, and not on existing converted cables that are in excess 60+ years old!! Most of the older TN-S cables that are now beginning to breakdown haven't been completely converted either, many being over the 0.35 ohm, and also carrying TN-S earthing on sections of the same cable!!

I can think of a few valid reasons for the DNO to refuse connection to a PME converted supply, as well as too converted overhead systems as well!!

 

[R G]

TN-S systems mostly remain TN-S systems , what we are talking about here is a TT system!!
The older TT systems that are converted are allways assesed and Ze readings taken into account before conversion . Their are exceptions where even with a PME supply you would have to install your own earthing system of course!!

 

[D Skelton]

Got to agree with Eng that TT is by far the most reliable method of earthing. Disregarding the Ra value and the reliance on RCDs argument. The only thing you are relying on is the earthing arrangement at the sup tranny. No supply cable breakdown to worry about at all!

 

[R G]

Yes your right of course you do have more control of your earth arrangements . It is not unknown for neutral faults to come about on PME systems , sometimes actually melting the bonding cables and taking out appliances in some instances , I have seen it a few times

 

[Deleted member 9648]

Nope, i doubt that very much!!

Wind up merchant, nothing more, nothing less!! lol!!

Are you accusing me of lying?...:toetap05:

 

[Engineer54]

TN-S systems mostly remain TN-S systems , what we are talking about here is a TT system!!
The older TT systems that are converted are allways assesed and Ze readings taken into account before conversion . Their are exceptions where even with a PME supply you would have to install your own earthing system of course!!

I think you should really go and talk to your Engineer if you are really being serious about older DNO TN-S supply distribution cables remaining TN-S!! So what do you think the DNO's do when the lead sheathing of these old cables starts breaking down, ....dig it all up and replace them?? I would confidently say there are more partially converted TN-S cables throughout the UK than fully converted!!


Yes the thread were on, is about TT systems. But you made a post statement that isn't correct, (as you have done again in this post too!!) so i've replied!! Actually, an overhead PME system either converted or purpose installed, will probably all be full blown PME supplies, as they are much easier and cheaper to PME!!

 

[Engineer54]

Are you accusing me of lying?...:toetap05:

Maybe i wouldn't go as far as that!! But a wind up merchant, you most definitly are!! lol!!

 

[SYKRAPS]

Just read this thread from start to finish and IMO I think that it will be very useful to many people as an informative and well constructed debate........well done to 'most' involved

 

[Deleted member 9648]

Maybe i wouldn't go as far as that!! But a wind up merchant, you most definitly are!! lol!!

Can assure you I take the subject very seriously. And once again,after all the debate,nobody has given me a valid reason why relying on RCD's for earth fault protection with an Ra <200 ohms is dangerous....but relying on the same RCD(s) with an Ra of 10 ohms is safe....end of debate for me.(again)

 

[Engineer54]

Of course not, you've freely admitted that you've never even tried to achieve better, and that was the basis of this thread!! All too easy to fall in to compromise mode, and rely on a single RCD ....Well while it's working correctly that is!! ..lol!!

 

[Knobhead]

Of course not, you've freely admitted that you've never even tried to achieve better, and that was the basis of this thread!! All too easy to fall in to compromise mode, and rely on a single RCD ....Well while it's working correctly that is!! ..lol!!

How many times do we read on here of failed RCD’s? I wouldn’t like to think I was relying on something that is known to be fallible.

 

[High Tower]

Lads a personal thank you from me for keeping this on topic, professional and civil, it's nice to see proper debate without dummies being spat out!!

I see the points on both sides of the argument but if it was me I think I'd be going for the lowest RA possible, I think in the last 18 months I have changed 8-10 faulty RCD's which would worry me if I had to rely on one solely to disconnect a TT system

 

[SPARTYKUS]

Go and ask your Engineer if that is always correct or not!! You may be very surprised by the sound of it!! lol!! Think you must have been working on relatively all new PME installations, and not on existing converted cables that are in excess 60+ years old!! Most of the older TN-S cables that are now beginning to breakdown haven't been completely converted either, many being over the 0.35 ohm, and also carrying TN-S earthing on sections of the same cable!!

I can think of a few valid reasons for the DNO to refuse connection to a PME converted supply, as well as too converted overhead systems as well!!

like the assumed changes in the laws of physics