I am chasing a channel in a breeze block wall to take 25mm meter tails and 16mm earth and then capping this over with 150mm wide 3mm thick 2m length of steel for mechanical protection.
The whole lot will then be plastered over.
I have been round the electrical wholesalers to get 3mm thick flat steel capping.
Then can only offer me steel box trunking or bridge profiled thin sheet steel capping.
Can you actually get 150mm wide, 3mm thick 2m lengths of steel capping from electrical wholesalers or do I need to go to a sheet metal supplier instead?
On 15/02/2013 16:48, jgharston wrote:> Stephen H wrote: >> I am chasing a channel in a breeze block wall to take 25mm meter tails > ... >> The whole lot will then be plastered over. > > I don't like plastering in tails as strictly speaking they're rated > for surface mounting. > > JGH >
The meter tails have two layers of insulation. The earth has only one outer layer.
In T&E, the neutral and live have two layers of insulation, where earth has one layer of insulation.
The only difference really is that the outer sheath is not shared in meter tails where it is shared in T&E by all three conductors. Yet we bury T&E in capping and then plaster over.
I had this discussion with Adam Wadsworth and I believe John Rumm too on this very NG a couple of months ago, who are electrical engineers conversant with 17th Ed regs.
It appears its within regs to do what I am doing as the steel provides mechanical protection (hence 3mm thick, not sheet metal) and there will be a switch fuse at the start of the meter tails to protect against short circuits.
I think you would be better going to a steel stockholder rather than an electrical wholesaler. The stockholder can advise on standard sizes and possibly shear to suit. Probably cheaper as well. No doubt you've considered the rusting effects. Nick.
On one job I did I just used a piece of 3 mm black mild steel strip, prepared as follows: de-burr cut ends, drill & csk some fixing holes, degrease, prime with 2 coats of Galvafroid.
I'm considering stainless steel 100mm wide x 3mm thick x 2000mm long to protect 25mm tails and 16mm earth from metals4u. I am thinking of nails being driven in at oblique angles so I I need quite a wide plate to protect against this as well as perpendicular nails.
Mild steel would corrode with the water and salts in the plaster. So i have to decide between 304 or 316 grade stainless steel.
Before I go ahead and order from metals4u, will this satisfy the mechanical protection requirements for 25mm meter tails and 16mm earth or do i need to go wider than 100mm or thicker than 3mm?
Would you bother earthing the stainless steel plate back to the MET block?
The answer to the question is to carry out some measurements and do the sum so that you know for sure...
First job, is you need to know your earth loop impedance. There are three ways you can tackle this. The best is to measure it with an appropriate meter. The next would be to ask the supplier, and lastly you could assume a nominal "worst case" value for the type of earthing (0.8 ohms for TN-S supplies, and 0.35 for TN-C-S (PME)).
Let's say you have a BS1361 100A incomer, and a TN-C-S supply. You don't know the ELI (aka Ze), so assume the 00.35 ohms.
That means your prospective fault current is 230 / 0.35 = 657 A
So look at the characteristics of the fuse:
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From the table you can see that with only 657A of fault current it will take getting on for 5 secs to open the fuse.
Now all we need to know is what CSA of copper is required to withstand that fault current for 5 secs. This we can calculate with the Adiabatic check:
s = sqrt( 657^2 x 5 ) / 115
(where 115 is the k factor for PVC insulated cable)
s = 12.7mm
So in that case, 10mm is not enough.
However, if you now go and measure your ELI and find it is actually say
0.1 ohms, you get a different result.
The PFC goes up to 230 / 0.1 = 2300A
That opens the fuse in 0.1 secs, and the earth conductor size required becomes:
This is interesting, and a bit surprising. The fuse is after all a bit of wire which is designed to melt when it carries too much power. This implies that the temperature curve for the fuse in an overcurrent situation is rather different to that for a bit of copper wire in the same situation. I suppose that is because the fuse is going to be dumping more heat to its environment when it's suffering a mild overload.
Though there is one thought - after you've had a fault that has tripped your main fuse it would be a good idea to inspect the cables involved in that fault. Possibly also after a fault that has taken out a ring/cooker/shower circuit too.
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