Transportable unit earthing system

The problem, as with a boat or a caravan, is what happens if the PME 'earth' goes for a wander voltage wise relative to the terra-firma earth that someone may  be standing on while in good contact with the body of the container.

This is less of a problem if to grab the door handles you are on a paving slab, and more worrying if your feet are in 6 inches of muddy water.

Also who uses it and how often - a lot of difference if the users are lots of barefoot children, or animals, or at the other extreme one  workman in boots 3 times a year.


Street lights considered OK on TNCS , because  even if the PEN is removed and the full load of 100W or whatever load  is at the top of the pole runs to earth the current is small, half an amp or less, and the 'rod resistance ' of the street lamp  is likely to be less than 100 ohms, so the voltage accessible during fault is likely to be less than 50V.

So, how good an electrode is your container - if it is sitting in mud and going rusty, probably low tens of ohms, and how much load is in use. If its a couple of light fittigns, it may well be fine even if the PEN were lost. IF it is several kW of heating, then not so likely.

Then you must consider how likely is the PEN to be lost  - is the PME coming in on singles overhead through trees that never get pruned  - perhaps a  high risk of one wire fault.

bundled overheads are less risky, as failure tends to be all or nothing.

Buried supply depends on the age and type of cable.


If you are not happy, the safest is to TT island the containers  and pretend they are caravans - the supply cable can be armoured and the armour connected to the PME earth, but  rod only for the CPC at the load end.  Equally, depending on how many of the risk markers above are on the low side you may decide that   to leave it PME is just fine.

If you are not happy, the safest is to TT island the containers  and pretend they are caravans - the supply cable can be armoured and the armour connected to the PME earth, but  rod only for the CPC at the load end.  Equally, depending on how many of the risk markers above are on the low side you may decide that   to leave it PME is just fine

Agreed ... but in forming the "TT island", you should consider the fact that separation is required from any buried metalwork connected to the PME earthing terminal. Conversely, this has the adverse effect that if you can touch any exposed-conductive-parts connected to the TT island, at the same time you are standing on ground affected by the rise of earth potential due to the broken PME CNE conductor, you haven't achieved anything, and you will simply have returned the touch voltage due to the PME broken neutral. BS 7430 provides suitable information to determine the required separation distances, and the effect of buried metalwork connected to the PME system on the surface potential. All of this is, of course, site-dependent.

True, and be careful, a mixed system with simultaneous  access can become the worst of both worlds.

The area of influence around an electrode, in which most of the voltage drop occurs, be the electrode a rod or a patch, like the base of a container, is related to its longest dimension, and will be a few of them.


If you have many containers over a wide area, it is not wise to link the TT earths together, as although it does lower the overall Zs, this matters not much when there is an RCD, and it can be better during fault if each container is only locally earthed, so it can 'ride the waves ' of any local bounce in ground potential, much like a ship bobbing in the sea.

In the end, you want the door handle of the cabin, and the ground around it,  where the person holding that handle will have their feet, to be within a few volts of each other. What that voltage  is relative to some other far-away earth potential is less relevant.