pme with earth rod

[Duane B]

Hi Guys

Ok so DNO law says no no no on export of the pme earth. so mr customer wants a supply to garden buildings etc . so swa is used earth connected at supply end to protect the cable, insulated at other end so 100% guarantee no earth present, just a Line & Neutral feed.

ok so install earth rod time,  how do we test to ensure we are not putting the rod into another earthed radiated zone i.e overlapped from the pme system.  always here people going on about bang a rod in test Ra etc but am I missing something here or overthinking it

because lets face it how many times have we all seen pme systems exported down the patio area

Cheers guys

 

[Andy™]

just stick in rod, test and youre done

 

[Rob.]

Exporting or extending?

 

[Duane B]

simply bang it in carry out Ze "Ra" make sure under 200 ohms done.  no need to worry about being in another overlapped earth zone ?????????

Exporting or extending?

well surely they could mean the same, unless an extension on the house with 2nd db is extending and up the garden is exporting

 

[Manator]

Exporting or extending?

Bang on there Rob

There are many guides published on exporting and extending, and the difference is very clear. PME earthing means just that, P= Points of, M=Multiple, and E= Earthing.

A properly installed PME will already have an earth rod immediately before connection to the property, although this is very often done away with.

Overlapping earth zones you refer to is the step voltage calculations, and this is more critical when installing sub stations, which is why a grid mesh is often used to distribute the zone equally.

It is acceptable to extend a PME, but not always to export.

 

[sprocketflup]

There are occasions when a TNCS/PME earth can be exported though, are there not?

 

[Manator]

There are occasions when a TNCS/PME earth can be exported though, are there not?

This is back to what Rob said, extend or export.

Say you have a garage next to your house,and you decide that you want a light and a socket in there. Do you extend, or export the PME?

Have a look at this

http://electrical.theiet.org/wiring-matters/16/elect-inst-outdoors.cfm?type=pdf

 

[Duane B]

but for a property on a pme for outbuildings supply summer house etc etc we wouldn't need step voltage calcs would we. or be concerned about overlap when the rod was installed. 

 

[apprentice87]

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....

 

[Sidewinder]

but for a property on a pme for outbuildings supply summer house etc etc we wouldn't need step voltage calcs would we. or be concerned about overlap when the rod was installed. 

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.

 

[Manator]

Every electrical installation is governed by an equipotential zone, so everything within that zone has a next to zero resistance to earth. This is very important.

Now lets say that you have now installed an equipotential zone in your garden shed, everything within the shed is equalized out should a fault be present. Then you walk out the door of that shed, onto your neatly mown damp lawn, whilst switching the light off. Within the zone, all voltages are equal, but now you have walked upon a moist relatively low resistant earth. We all know which path electricity likes best.

I have edited this inline with what Sidewinder said as I was posting.

As said, the disconnection times should be such that any difference is registered and removal from supply is carried out in such a short length of time that no harm should come to the user.

 

[Adrian]

Should we be TT ing our lawn mowers then

 

[misssweden]

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.

Not all joint kits are designed to come with an earth electrode...

 

[Manator]

Should we be TT ing our lawn mowers then

Not at all, as long as a fully working RCD is present everything should be fine.

 

[Andy™]

and we all know they are tested quarterly and never fail

 

[apprentice87]

Hi Misssweden,

Thought all service joints were supposed to have the earth rod where the service joins the main, [pme and all that!!]

john..

 

[Sidewinder]

Not all joint kits are designed to come with an earth electrode...

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.

 

[Duane B]

then as we've gone to TT off the back of the pme we just give our self a 0.2 sec disconnection time to meet lol

 

[Manator]

then as we've gone to TT off the back of the pme we just give our self a 0.2 sec disconnection time to meet lol

Well I now use the Schwarz method in my calculations,as it is far more accurate. If you google it, you may be enlightened as to how and why these calculations are done.For everyday domestic installations you can always use the on-site guides and the best practice guides available.

For more complex installations you need to know where your earth is, what its resistance is to proper earth, and only then can you design a proper earthing arrangement.

Having said that, even the most humble of domestic installer can learn a great deal from the science behind it, and become a better person for at least having a basic knowledge of the science behind it.

I have only had cause to use it on two occasions in the last 30 years or so, but I know the science behind it, and acknowledge its use and purpose.

 

[Tony S]

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.

Western power last November