TNCS or TT supply to external metal pressurised plantroom

Question

Hi, 
 
 I have been tasked to run a 3phase supply to an external metal container plantroom and the pipework is all in copper feeding back into the building. 
 
 The main supply in the switchroom is a TNCS supply. 
 
 I am thinking of making the supply to the container a TT system. 
 
 I am concerned as the main water pipe is in the building and will be earthed to the MET at the TNCS cablehead. - along with the sprinkler system. 
 
 I am wondering if I should just make the supply to the container a TNCS supply with a separate C.P.C as it only about 4 meters away from the main building. 
 
 The container will be a fixed to a concrete plinth and will be a permanent structure. 
 
 Any advice would be much appreciated. 
 
 Regards 
 
 Alan

AJJewsbury · Accepted Answer

These things tend not to have a simple yes/no answer ... more a case of least-worst option, which varies with the details of the particular situation. 
 It sounds like simply TT'ing the container alone would not help - as the copper pipework (which presumably would have to be main bonded in both "buildings" as extraneous-conductive-parts) would connect the container to the TN-C-S (PME?) system anyway, so defeating the purpose of TTing it. 
 Maybe you could arrange for an insulating section to be inserted into the pipework between the two buildings (the "PME outside tap" approach) - if the insulating section was short you'd also have to arrange that pipework on both sides of the break weren't within reach of each other (2.5m+) - perhaps enclosing the pipework on one or both sides with something insulating. 
 Another possible (if not particularly likely) option is to TT the whole installation - main building as well as container (tends to be inconvenient and expensive, but an option). 
 Or, as you say, use TN-C-S (PME) across both buildings. There's nothing inherently wrong with that approach - there are plenty of steel framed/clad buildings on PME earthing. The main worry with TN-C-S being used "outdoors" is the risk of earthed metalwork (the container in this case) being at a significantly different voltage to the ground it's stood on and consequently of someone outside (on the ground) touching the container receiving a shock - both under "normal" conditions and open-PEN events. The distance from the main building isn't that relevant - it's the distance from the nearest PME electrode that's more significant (unless the main building makes a good electrode of itself - e.g. steel framed with the steels extending into the foundations) - and even then the benefit is limited as most electrodes will only "pull up" the voltage at the surface of the ground to something close to PME earth voltage for perhaps a metre or two, maybe three. 
 Usually the justification for using PME outdoors is more of a general risk assessment - e.g. to compare with a few other situations... 
 Caravan - very likely touched by someone on wet grass/soil, possibly barefoot - PME totally prohibited. 
 EV car charging - very likely to be touched, but more likely to be on solid ground (concrete/paving and especially tarmac tend to be a lot less conductive than bare soil) and less likely to be barefoot - PME permitted with some precautions (e.g. open-PEN detection). 
 Class I outside light - not likely to be touched day-to-day - PME permitted and very frequently used. 
 Steel street lighting column - less likely to be touched - often surrounded by tarmac - PME permitted (but additional electrode recommended, which in some open-PEN situations can hold the voltage down to safe levels, given the limited load). 
 Steel supply transformer outdoors - rarely touched, and ground typically covered with large gravel pieces, which provide a high resistance even when wet - PME perfectly normal. 
 There are other mitigations that can sometimes be thrown at the problem too - depending on where you are in the project and what else is in the surroundings - e.g. a bonded buried grid electrode around the outbuilding can hold up the ground to similar voltage to PME earth - although "extending the equipotential zone" like that can itself cause knock-on problems (especially if there's something like steel fencing radiating out from that location). 
 So it's really a matter of looking at the detail of your situation and seeing how close to other ones it seems - is it the sort of thing people will be going into every day, or just once a year for a checkup/service? Are barefoot (esp. children) a possibility? Is the ground surrounding it likely to be wet soil, or something less conductive? Are "perceived" electric shocks (i.e. a sensation of shock from small, but not of themselves dangerous, voltage differences) likely to be a problem? 
 - Andy.

TNCS or TT supply to external metal pressurised plantroom

Community Rules & Guidelines