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.

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.