The school supply is a 300Amp incomer on a PME which has a MCCB Panelboard in place. 3no Temporary Porta cabin Classrooms are to be delivered to site which will remain in use for around 3to6 months then removed from site. They are to be wired into 3no x 3phase spare ways located in the MCCB Panelboard. I have suggested they must be independently made into TT supplies to comply with clause 717. The contractor suggests it is OTT because all other services entering the Porta cabin Classrooms will be piped in plastic ie water connection etc. The cabins are cladded so are not your typical metal exteriors. My fear is the loss of Neutral from the PME. All circuits in the classrooms will be 30mA. Would love to know peoples thoughts. Am I over thinking it with it being temporary ?
For temporary buildings, eg temporary classrooms, a PME earthing terminal may be supplied if the installation is constructed so that a person in contact with the general mass of earth cannot touch any metalwork of the temporary installation and the installation complies with the bonding and earthing requirements of BS 7671. In such cases, a temporary building may be treated in the same manner as a permanent building. A PME terminal shall not be offered for a temporary building which is not constructed as above (e.g. metalclad buildings) .
Thanks for clarity. Iam concerned that 1 of the 3 porta cabins may well end up being of metal construction to be used for storage but this is tbc. If this is the case then TT would apply and also time delay RCD Main switch in said Porta cabin.
If it was all new and miles from the main building, then I'd be inclined to agree, TT it, children can be like livestock, and find ways to damage things you were not expecting. However, this is a retrofit to an existing installation and there may well already be things installed that expose the PME earth of the main building anyway - lighted bollards , perimeter fences with lights or cameras, electric door lock mechanisms are just a few examples that may be, as well as outside taps.
If so you need good clearance between the PME and any TT zone, to avoid the risk of someone straddling the offset ground voltage, at least 2.5m, ideally quite a bit more. Similarly, unless the cabins are well spaced, then the earths should be in common - which may be as simple as as bonding the electrodes. If y9ou are not far from the main building, then the PME may make more sense.
The risk either way is not great, there are plenty of PME supplied lamp posts and bus stops, and not many accidents
