it would just be good if it was easily described, looking back at the installation certs over the years, its been called TN-S TNC-S and TT but mostly TNC-S


its mostly a matter of interest for me, than any great importance. it just amuses me how much confusion it causes


and, I have had, of course, a couple of people exclaiming that 'its illegal!'

Johno12345:

and, I have had, of course, a couple of people exclaiming that 'its illegal!'

LoL - as the saying goes "If you can't change the people, then change the people"


Regards


OMS

oh yes, thats the biggest aspect of choosing contractors here, I show people around and can usually tell if they are going to fit in. Plenty look petrified and I just know that they will be in the office every 30 seconds wanting a box moving, or complaining about something. 


I tend to show electricians one of our old red spot or 100a BS3036 boards to see the reaction. It's not even an age thing, its just attitude 


the other thing I like is when people have a positive reaction to permits 


I have some great contractors that I can show the job, tell them what I want the end result to be, and leave them to it, next thing, its all done. That's what I want in an industrial environment. I don't want to babysit

Agreed on the TT aspect, the logic from that came about from the earth rods...


no alternative supply here, just the 11kV ring main



I do rather wish I had some decent qualification in this area, would be a good thing to have in my job I think. Not to mention that I find it all very interesting

Thanks Alan, at least ill have a definitive answer when I can get a look inside the spreader box then!


might be a good while though, so I think ill do the Yorkshire thing, and 'not let it werry me'

If it is pre 1980 Print a card in red ink on white background, or later than that black ink on yellow background like the service cut-out stickers you almost never see.




for the door to the equipment, that says any one of these ,

and watch all contractors copy it faithfully onto the test sheets from now on. The reports will become consistent at least.

If there is a separate  earth coming from the transformer, and that earth is connected to the star point of the transformer with the neutral conductor then tat is TN-S. There should be an earth electrode connected to the same star point which is not in the earth fault path.


If only 3 phases and neutral come from the transformer to the main panel and there is a link between the neutral in the panel and the installation earth then that is TN-C-S (PNB). PNB is one of the 2 types of TN-C-S.


If the supply to the building comes from the transformer and that supply may serve other users and the cable has earth electrodes along the cable route (the "M" in PME) then that is TN-C-S (PME), the other type of TN-C-S.


If the supply has no earth from the supply transformer and only the 3 phases (or a single phase) and neutral provided there should be an earth electrode connected to the installation MET to make the installation TT. In this case the earth fault path is through the consumers earth electrode through the general mass of earth and back to the star point of the transformer via the transformer electrode.


Some installation supplied from a cable coming from the street may look like a TN-S say with a connection from the lead sheath of a PILC cable to the MET but they are not because the DNO has jointed the cable and combined the earth and neutral in that joint. So the method of earthing cannot be verified by visual inspection. My local DNO (UKPN) have declared that where they have provided an earth from a cable in the street there whole network is PME. That PME cannot be undone by spitting the earth and neutral in a BNO network (which should be the case anyway). In this case you will see a separate neutral and earth at the supply head but the method of earthing is still PME.


I feel sure that my brief explanation will be disagreed with but my reference sources are BS 7430 and IET Guidance Note 8. GN8 has very clear drawings of the different earthing systems.


If you do not have a copy of Guidance Note 8 then I would highly recommend you get one. If Kirsty asked me if I could only take one GN to my desert island which one would it be I would say GN8. My luxury would be Kirsty herself, just of course for the convivial conversation to while away the time!