FLAT Capping for meter tails chased into a breeze block wall.

[Stephen H]

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?

[jgharston]

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

[Stephen H]

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.

[Nick]

"Stephen H" <ilov...@pleasespamme.com> wrote in message
news:jttTs.163699$T25....@fx13.fr7...

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.

[John Rumm]

On 15/02/2013 16:48, jgharston wrote:

"Clipped direct" and embedded in masonry both count as Reference Method
C (i.e. have the same current rating)



--
Cheers,

John.

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[Bob H]

If the op hasn't , then I would advise getting some galvanised steel
rather than just mild steel

--
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[Stephen H]

Thats what I was going to get as the salts in the plaster and water will
attack mild steel.

[Andrew Gabriel]

In article <0wwTs.77218$lh.4...@fx14.fr7>,

Stainless steel might be even better (but more expensive).

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

[John Rumm]

Three layers of regular shaped galvanised capping might do the job
easily and cheaply.

[Andy Wade]

On 15/02/2013 22:31, John Rumm wrote:
>
> Three layers of regular shaped galvanised capping might do the job
> easily and cheaply.

Not according to the ESC:
http://www.esc.org.uk/industry/industry-guidance/industry-guidance-on-the-wiring-regulations/1-new-or-rewired-domestic-and-similar-installations/
[Q1.47]

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.

No corrosion probs after four years.

--
Andy

[ARW]

http://www.metals4u.co.uk/

any use?

--
Adam

[Stephen H]

wow... fabulous... just what I was looking for.

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?

Regards,

Stephen

[ARW]

Yes

--
Adam

[Stephen H]

So as I am using 25mm meter tails and 16mm earth between meter cupboard
and new CU,

considering worst fault condition would be 100A (switch-fuse isolator is
100A and so is the cutout) I'm looking at running a 2nd 16mm earth wire
in parallel from the metal plate back to the meter cupboard?


What would you use to attach the 16mm copper wire to the steel plate
which is maintenance free?

Regards,

Stephen.

[ARW]

Trust me. A fault current will be many hundreds of amps.

> What would you use to attach the 16mm copper wire to the steel plate
> which is maintenance free?

I would use a 10mm earth. Maintainance free would need welding.
--
Adam

[Stephen H]

wouldn't the 10mm2 earth wire vapourise before the main 100A cutout
opened? otherwise why use a 16mm earth between meter cupboard and the CU?

[ARW]

There is no guarantee that the 25mm tails would not be vapourised.

--
Adam

[John Rumm]

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:

http://wiki.diyfaq.org.uk/images/0/06/Curve-BS1361.png

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:

s = (2300^2 x 0.1 ) / 115 = 6.32mm

Hence in that case a 10mm earth is fine.

[Andy Champ]

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.

Andy

[Tim Watts]

On Tuesday 19 February 2013 00:44 Stephen H wrote in uk.d-i-y:

>
> wouldn't the 10mm2 earth wire vapourise before the main 100A cutout
> opened? otherwise why use a 16mm earth between meter cupboard and the CU?

What makes the cable vapourise?

Hint - it's the *energy* dumped into the cable, not through it.

If the cable resistance/linear metre is R, then the energy dumped into that
length in time t is

I2Rt

So as long as t is very small, I can be large, eg 1000A.

Without being bothered to look up the actual numbers:

Say I=1000A and t is 0.1 seconds (will likely be less)

10mm2 cable has a linear resistance of 0.0017 Ohms

So energy dumped is

1000*1000*0.0017*0.1 = 170J

The mass of copper in 1m length is about 0.09kg

So the temperature rise will be:

S=specific heat capacity = 390J/kgK
m=mass
T=temp rise

T=E/mS

T=170/(390*0.09)


T=4.8K or less than 5 celcius temperature rise.

Feel free to double check :)


--
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[John Rumm]

Its also a reflection that the fault protection needs to leave the
circuit undamaged, whereas it does not matter if the fusewire is glowing
red hot for a brief moment (it has no insulation to burn off!).

So that means limiting the terminal temperature of the conductors so
something that won't destroy the insulation - so for "normal" sized PVC
insulated copper cables that is 160 deg C.

> 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.

That level of protection is in effect part of the k factor used in the
adiabatic check. So if the sum says you have enough copper CSA, then its
telling you that the worst case final temperature after the fault is
cleared should still leave the insulation undamaged.