pme with earth rod

[steptoe]

Can i just say something, and this is just random thinking from me, and Misssweden might like to comment too...

There is a lot of fuss about exporting TNCS and i know perfectly well why this is. BUT, hate to tell you, practically EVERYONE is on TNCS like it or not.... [Or so it seems to me...]

Ok, say Mr Scroggins down the road has a new service cable put in. The DNO will link the cable sheath and neutral in the service joint, and, if they can be arsed, [they ignore it sometimes]  the DNO will put a long copper earth rod thing connected to the now combined neutral and earth in the joint hole and fill all the earth back in.

Again, say there is an LV fault and they insert a nice piece of cable. It WILL be CNE, so you are all now on TNCS, just instead of it being linked in your service head, it is linked in next doors one, or out in the street..

Tin helmet on!!!!!

john....

Tut tut tut John,

You should know better than that,

Exporting TNCS is perfectly acceptable,

Exporting PME is at whole different game,

NO, they are not the same thing, regardless of the DNO deciding to come round and PME your install be putting a PME sticker on the cut-out :slap

I have alleged PME, I have put my own rod at the incoming MET, (front of property), I also have a rod at the rear where my shed supply exits the EZ, and I have a rod at the shed, about 4m (max) from rod 2,

Am I concerned about overlapping zones,?

What time is last orders,?   Guinness

 

[Manator]

It is strange but that little coil of copper is their earth rod!

 

[misssweden]

AFAIK, speaking to the WPD guys what App87 suggests is their standard practice, regardless of the joint kit.

I've been told that they "must" do it, if they can't put the rod in vertically they lay it horizontally.

Putting an earth electrode in vertically could be a challenge, they are quite soft

 

[apprentice87]

What makes me laugh, is that if you dig your own trench the DNO have all kinds of requirements. The trench bottom must not have any stones and nasty things, it must have a few inches of stone dust, [not ordinary sand, not allowed [salt in it???] cable must then be covered in stone dust.

Now, their own hole, they will install the joint, could not care a crap about bending radius of the cable, and then a lorry comes along and dumps in all kinds of concrete and stony crap right on the cable!!!!

My house, i had a load of spare stone dust, so i packed it all round the joint to support it and the cable and bedded the earth rod in the clay after straightening it out. When they came back the next day, they must have thought the stone dust fairy had been out in the night!!!

john..

Western power last November

Love the photo!!! Must have been a test joint for the apprentice!! They have even swept the bottom of the hole!!!

 

[ProDave]

Here's my joint hole.  Dug by me and filled in by me, that saved me almost £1000 off the cost of my connection!!!!

It meant they didn't have to send a digger and driver, just two guys in a van to make the connection.

I didn't see any sign of any form of earth, not even a pig tail left in the hole.

 

[Tony S]

It is strange but that little coil of copper is their earth rod!

There were two more joint holes around the corner from the one in the picture, both had the same little pigtail as the “rod”.

The outgoing is 95 3c wavecon to a block of flats, main is 185.

 

[Tony S]

Love the photo!!! Must have been a test joint for the apprentice!! They have even swept the bottom of the hole!!!

They didn't clear my fag ends out.

 

[Marvo]

If the OP is using an SWA to supply his shed and is knocking in an earth rod with a <200ohm Ra at said shed, why wouldn't you terminate the armour of the SWA at both ends to effectively connect the new rod the the MET if the supply is already PME? (Assuming the shed is supplied direct from the CU and not lollipopped off some spurious final circuit)

 

[apprentice87]

What is the difference between "exporting" pme to your garden shed, and the DNO "exporting" it to your house...

john...

 

[Essex1]

If the OP is using an SWA to supply his shed and is knocking in an earth rod with a <200ohm Ra at said shed, why wouldn't you terminate the armour of the SWA at both ends to effectively connect the new rod the the MET if the supply is already PME? (Assuming the shed is supplied direct from the CU and not lollipopped off some spurious final circuit)

So that the TT would effectively become a 'back up'?

What is the difference between "exporting" pme to your garden shed, and the DNO "exporting" it to your house...

john...

???

 

[steptoe]

What is the difference between "exporting" pme to your garden shed, and the DNO "exporting" it to your house...

john...

The DNO do NOT make or design an EZ

So that the TT would effectively become a 'back up'?

???

NO

think localised PME,

 

[apprentice87]

Ok. so what is the difference between the DNO "exporting" "PME" to your house, and you "exporting" it to your shed?? Errrr, none. none whatsoever....

First off; Why the requirement [by law] for the DNO to provide PME and not just TNCS, [Do not believe me?? then try reading the ESQCR] This is because of the dangers of a lost neutral, the more earth rods that are installed [thus making it PME] the lower the voltage rise on the neutral, and henceforth the DNO supplied earthing terminal, in the event of a fault, [obviously a good thing]

So what is the problem with TNCS?? Well, the problem [leaving aside "lost neutrals"] is this. As your earth is connected to the neutral, you will find that a PD exists between the earth terminal, and "true" earth, that is equal to the volt drop across the neutral conductor that appears between "your" bit, and the transformer that supplies the thing.

Ok, in your house, this is assumed not to matter, as all extraneous parts are bonded to the MET, so no PD, but what happens the second you step outside, [onto the lawn say] whilst holding your class one appliance?? You could very well find there is a PD then!!

[and no, the rod the DNO installed at "their" end of the service cable will not make any difference at all, as, in general, it will be too far away to bring the mass of earth outside your back door up to the same voltage as that which MAY [in theory] appear between the DNO earth and "true" earth.]

So you "export" the TNCS supply to your shed. So what??? You step outside the shed holding your class one tool, and it is no different at all, to stepping outside the back door of the house. Where is the problem??

Now, if you want to eliminate this problem, you could of course bang in rods all over the place and this will create a zone where the "true" earth will tend to rise up to match the "DNO" earth, so now we can live happily ever after, and run about our lawn in sheer bliss whilst holding our ancient Black and decker drill!!!! JOB DONE!!!!

Yes!! job done!!, [assuming that is] you do not mind the fact that your earthing and bonding system could now very well end up being used as the neutral conductor for half the street.....

Ok, next idea!! We will TT the shed!!!! Yes, great, but now your earth fault protection depends ENTIRELY on the most unreliable piece of electrical apparatus there is, namely an RCCB... So yes, you could TT the shed, but i hope you have either; 1, good life insurance, or 2, a load of RCD's in series to provide redundant protection....

BUT, guess what?? You have TT'ed the shed, you have installed 200 RCD's in series to provide the ultimate in "redundant protection"..... So what could possibly go wrong now!!!!!       Errm, well, if your shed earth rod is within about 30 feet of anyone else's rod, or any metallic services, then your extraneous parts in your shed will just, once again, tend to rise to the same voltage above true earth, as the DNO TNCS supplied earth was in the first place....

You have gained precisely nothing....

john......

 

[IzzyS]

The over cautious nature of BS7671 takes care of this wrt <200 Ohms for the Ra, a 30mA RCD so that one does not have to be concerned about step voltages.

Calculate the potential step voltage on a rod with a max Ra of 200 Ohms and a max applied fault current of 30mA for 0.2s for a final circuit or 1s for a distribution circuit.

Then work out the potential typical step voltage and energy that could be imparted to a body over a typical step distance in these disconnection times.

I may have misunderstood your post, but it seems to me that this is by far the most common misconception on this and any other electrical forum - RCDs do not limit the magnitude of the current at all. If you have an EFLI of 200 ohms it will (only) be 240/200 = 1.2 amps.  

But that is 40x the "safe" amount and will kill you just as readily,  The only thing an RCD does (if it works) is disconnect in a short time.

Remember that a 30mA RCD on a circuit with a measured Zs of 1.2 ohms will still pass 200 amps.  Ouch.

 

[Sidewinder]

Izzy,

I see your point.

I can see why my post is confusing.

The fault current will be the fault current, however, it will not be instantaneous, it will have a rise time from zero.

The RCD will have an affect, in that it will detect the current at it's tripping threshold say 27mA, however, we can use 30 as that's the nominal.

Now as the fault current rises and hits 30mA, the RCD disconnection process will begin, therefore yes the fault current will continue to rise above 30mA up to a finite value that it will reach in the time that the RCD disconnects.

This may well be more than the rated current of the RCD, and it may be less than the steady state fault current that would be caused by the "nominal" fault resistance, for example teh 200 Ohms you give in your post.

The affect the RCD has on the scenario will depend on the rise time of the fault current into the fault, even a fault of negligible impedance does not occur instantansously, nothing really does, we live in an analogue world, even though we like to think it's digital, it aint.

So, yes you can work the step voltage out at the steady state fault current, however, we would hope that there is a chance that a functional RCD will disconnect in less than the required time, thus, hopefully before the fault current reaches it's peak.

Akin to the energy limiting affect of fusing, however, not quite the same.

I used the 30mA because (nominally) you will, see that level of current flow.

Depending on the rise time of the fault current verses the response time of the RCD, then, you may or may not see the full calculated fault current.

Plus the fault current is an unknown, as is its' rise time, where as the RCD response current & response time, are hopefully, as you say if the damned thing works at all, a "known" quantity.

 

[Davethsparky]

One of my favourite forum topics, it like a home from home here!

it always amazes me how many electricians manage to make such a simple subject complicated.

outbuildings which have no extraneous parts which require bonding don't require the equipotential zone to be extended and so you can connect to a PME supply and use the PME Earth.

outbuildings which do have extraneous parts do require bonding so you can either connect to the PME Earth and run a main bond of the correct size for the incoming supply to the outbuilding and bond all the extraneous parts to an earth marshalling terminal (EMT) there, or else set up a Seperate earthing system for the outbuilding (TT) with a properly installed earth electrode preferably made up of multiple deep driven rods and then bond all extraneous parts as required to the MET of that earthing system. 

 

[Phoenix]

Ok, say Mr Scroggins down the road has a new service cable put in. The DNO will link the cable sheath and neutral in the service joint, and, if they can be arsed, [they ignore it sometimes]  the DNO will put a long copper earth rod thing connected to the now combined neutral and earth in the joint hole and fill all the earth back in.

Again, say there is an LV fault and they insert a nice piece of cable. It WILL be CNE, so you are all now on TNCS, just instead of it being linked in your service head, it is linked in next doors one, or out in the street..

Tin helmet on!!!!!

This is an issue which acknowledged in some sectors

The Blue book (the accepted standard for petrol station services) introduces the concept of 'guaranteed TN-S' is introduced. Generally the only way you can have guaranteed TN-S is if there is a dedicated transformer for the site. The public supply network can not be relied upon to be TNS even if it looks like it at the intake, and even if it is, might not remain that way after jointing has been carried out.

Outside of specialist installations, I think we do get over anxious about using TNCS... a lot of the world used TNCS and doesn't worry unduly about exporting it (though most places do insist on a rod at the point the supplpy enters the building to reinforce the M in PME

 

[Duane B]

Nice saving digging own hole for the cable joint. 

 

[sprocketflup]

Sooooo....are there any statistics anywhere that show how many people are getting hurt as a result of incorrect earthing arrangements and set ups?

 

[Sharpend]

Sooooo....are there any statistics anywhere that show how many people are getting hurt as a result of incorrect earthing arrangements and set ups?

Call the LFB they seem to be in the know