Power to a portakabin stake or not to stake?

[darren]

Hi Guys.

Got a job coming up where the customer wants me to install power to two portakabins. I have a few questions and would really appreciate it if someone could help out.

They are located on a waste recycling site and will be used as the sites offices the previous cabins that they are replacing have been there for 6+ years.

The cabins are going to be placed one on top of the other strapped together with metal bracing bolted to the ground and cabins with metal stairs going to the top cabin.

If i was to assume I'm treating it as permanent fixture and a tn-s supply can anybody see a problem with running out a 16mm 3 core sub main and using the spare 16mm as the earth or should i make it TT? (main supply is less then 5M away from cabins)

Not 100% sure on what the supply is so say its tn-c-s could i again continue the earth or would i have to TT it?

Both units will have a separate feeds and will be fed from a 3 phase dis board, if possible i would like to avoid the TT approach seeing as the area is concrete heaven strong enough to withstand the hammering of 25+ ton trucks 24*7 with soft ground nowhere to be seen!


Thanks in advanced




Darren


if I'm talking bs let me know, i have recently qualified and every time i think i have learned something it turns out that there is always more to it

 

[Deleted member 9648]

No issues with a TNS supply but you have to tread more carefully with a TNCS.....I guess from your description the site would be classed as a construction site for the purposes of the regulations.....it is only permitted to use a TNCS for fixed building of a construction site.(704.411.3.1)...it is prohibitted for a caravan or similar consruction.
I think your portacabins can be classed as fixed buildings therefore you can use the TNCS but if there are any services which require main bonding you will have to run it back to the MET....structural steel would only require main bonding if it extends inside the units.

 

[darren]

Thank you wirepuller, glad i don't have to be trying to drill through that concrete

I Have one more question about the size of the cpc, when i do the calcs it shows i need about 23mm cpc for some 16.0mm however 543.1.4 table 54.7 states i only need 16.0mm is there something I'm missing here?

 

[malcolmsanford]

Table 54.7 shows minimum sizes for Protective Conductors. What it is saying for your 16mm is that you can use as a minimum a protective conductor the same size as your line condutor so for you that is 16mm providing it is of the same material.

As we know though that by doing the Adibiatic equation in 543.1.3 you can work out that for that 16mm line, a 10mm or even a 6mm earth can sometimes do.

So what it's saying is 543.1.4 if you can't really be chewed to work it out then use a cpc the same size as your line and you can't go wrong.

 

[darren]

and that is why I'm even more confused! i performed the following equation s=√(1200².5)/115 which gives a result of 23.33 mm, although the pscc might be a little wrong (from memory doing a minor works there about a year ago)

have i got my calcs wrong here?

 

[Adam W]

Try OSG table 4.1 - easier than all that maths.
Sorry to sound pedantic but could the old 'what's covered under PAT' argument be applied here, ie "it's called 'portable' appliance testing so fixed equipment isn't covered", meaning:
It's called a 'PORTA cabin' as in 'portable cabin', and therefore not a permanent structure?

... Which brings about my next question:
Is it bolted to the ground, and if not would doing so make it a permanent structure?

 

[Deleted member 9648]

A portable unit would usually be connected via a plug and inlet socket arrangement...although I agree it is a grey area.....IMO 2 units stacked and bolted together with a steel stairway and hard wired can reasonably described as fairly permanant...

 

[darren]

I'm pretty sure I'm going to have to call this one a permanent structure seeing as i know that it is going to be there for anything between 5 - 10 years and requires a hell of a lot of dismantling to move. I think if i am to use the spare core in combination with the swa there is not going to be a problem, i hope :/

Really appreciate all the feed back guys


Darren

 

[Guest123]

I'd be inclined to go with 704.1.1 (vii) and take it that these offices/toilets/canteens will be in the "administrative" area of the construction site so the use of a TN-C-S would be allowable.

Also sizing the CPC the same as the supply neutral will be fine.

 

[Jurassic Sparks]

Portable Appliance will be excluded, they have a definition by weight I think its over 20 kilos or items not able to be moved
we had machinery at my last place and it all weighed around 600 kilos although free standing, so not covered under PAT