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Old February 8th 19, 05:00 PM posted to uk.d-i-y
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Default Question about sub-main sizing

On 08/02/2019 12:26, Andrew Gabriel wrote:
In article ,
John Rumm writes:
On 07/02/2019 18:38, wrote:
I haven't been able to find the rules for sub-main protection. Is it
permissible to run it from a 50A DP MCB in the garage CU, or does have
to be from a separate DP fused/switch from a Henley block?


No you can run from a MCB without any problem. Its commonly done.


One problem with using an MCB is you may get little if any
discrimination between that, and the downstream consumer unit
MCBs in the event of a fault. So if something shorts out on
a final circuit, you are quite likely to find the upstream
40/50A MCB trips, and you lose power on all the final circuits
on that submain. A BS1361 HRC cartridge fuse used to be a better
bet, but they have vanished (spare fuses still available, but
new fuse holders are not).


Yup, I have noticed that CU MCB based fuse carries are much harder to
find these days.

If you can size your sub main MCB a couple of ratings higher than the
highest downstream one, or, fit a type C at the sub main head end, you
might be able to mitigate the discrimination problems.

Having said that there are no guarantees, ISTR Adam mentioning a case
where an appliance fault took out the main incomer fuse, leaving its
plug fuse un-blown and the circuit MCB un-tripped!


--
Cheers,

John.

/================================================== ===============\
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http://www.internode.co.uk |
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| John Rumm - john(at)internode(dot)co(dot)uk |
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Old February 10th 19, 09:11 PM posted to uk.d-i-y
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Default Question about sub-main sizing

On 08/02/2019 17:00, John Rumm wrote:

Having said that there are no guarantees, ISTR Adam mentioning a case
where an appliance fault took out the main incomer fuse, leaving its
plug fuse un-blown and the circuit MCB un-tripped!



You are close John.

Two cases where this sort of thing has happened.

1. 1970s bungalow. Tried to slacken off the cooker switch from the back
box and the live cable fell out the terminal and shorted to earth. This
took out the house fuse but left the 30A fuse wire intact. I'll put that
down to an old fuse.

2. Flats with a 16mm T&E submain supply to each flat. A fault[1] on the
lighting circuit took out the 6A lighting MCB and the 63A head end MCB.
In this case there was no 10mm extra earth cable from the head end to
the flat as no supplementary bonding was needed (no gas and a plastic
water supply) and it would be very similar to the OPs set up. The 80A
BS1361 fuse in the suppliers cut out was OK.

[1] It was LE short in a new light fitting the tenant had fitted.

--
Adam
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Old February 10th 19, 09:26 PM posted to uk.d-i-y
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Default Question about sub-main sizing

On 07/02/2019 19:17, wrote:
On 07/02/2019 18:50, Cynic wrote:
I've not got tables to hand but check your earth loop impedance
against disconnection times for the 40A mcb

The sub-main earth loop impedance (20m of 10mm2 and 20m of 16mm2) should
be about 60mOhm (37mOhm + 23mOhm) so about 0.4 ohms total (TN-C-S) -
about 600A fault current. A type C MCB needs 10x rated current, so 500A
for a 50A MCB. Unless I've boobed it should be OK.



I think I must be missing something here... where does the 20m of 10mm^2
come into this?

Looking at just the submain as it currently stands:

16mm^2 T&E has a round trip resistance live + CPC of 4.23 mOhms/meter
[1], so that is a contribution of 85 mOhms.

If we are going with the standard estimate of 0.35 Ohms for the supply
impedance (in the absence of a measurement), that gives a total Zs of
0.435 Ohms. So just inside the limit for a 50A Type C using the new cMin
adjusted max ELI factors[2].

So a prospective fault current of 230 / 0.435 = 525A, adequate to trip
in the instant part of the curve.

Now if you increase the CPC size to 16mm^2 with the addition of a 10mm^2
single, then the situation improves further?




[1]
http://wiki.diyfaq.org.uk/index.php/...sistance_table

[2]
http://wiki.diyfaq.org.uk/index.php/...2C_Cmin_factor


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
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Old February 10th 19, 10:01 PM posted to uk.d-i-y
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Default Question about sub-main sizing

On 10/02/2019 21:26, John Rumm wrote:
On 07/02/2019 19:17, wrote:
On 07/02/2019 18:50, Cynic wrote:
I've not got tables to hand but check your earth loop impedance
against disconnection times for the 40A mcb

The sub-main earth loop impedance (20m of 10mm2 and 20m of 16mm2)
should be about 60mOhm (37mOhm + 23mOhm) so about 0.4 ohms total
(TN-C-S) - about 600A fault current. A type C MCB needs 10x rated
current, so 500A for a 50A MCB. Unless I've boobed it should be OK.



I think I must be missing something here... where does the 20m of 10mm^2
come into this?

Looking at just the submain as it currently stands:

16mm^2 T&E has a round trip resistance live + CPC of 4.23 mOhms/meter
[1], so that is a contribution of 85 mOhms.

If we are going with the standard estimate of 0.35 Ohms for the supply
impedance (in the absence of a measurement), that gives a total Zs of
0.435 Ohms. So just inside the limit for a 50A Type C using the new cMin
adjusted max ELI factors[2].

So a prospective fault current of 230 / 0.435 = 525A, adequate to trip
in the instant part of the curve.

Now if you increase the CPC size to 16mm^2 with the addition of a 10mm^2
single, then the situation improves further?





Of course the second boobie makes things better:-)

And I got the resistance of 16mm plus 16mm wrong in my calcs.

--
Adam


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Old February 11th 19, 11:15 AM posted to uk.d-i-y
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Default Question about sub-main sizing

On 10/02/2019 20:30, ARW wrote:
On 07/02/2019 19:17, wrote:
On 07/02/2019 18:50, Cynic wrote:
I've not got tables to hand but check your earth loop impedance
against disconnection times for the 40A mcb

The sub-main earth loop impedance (20m of 10mm2 and 20m of 16mm2)
should be about 60mOhm (37mOhm + 23mOhm) so about 0.4 ohms total
(TN-C-S) - about 600A fault current. A type C MCB needs 10x rated
current, so 500A for a 50A MCB. Unless I've boobed it should be OK.


You might have boobed it¬* - twice, if I am not mistaken!


1. You are using 240V not 230V for your fault current. (Max Ze for a 50A
C type is 0.37ohm and remember that is when the cables are at 40 deg)


2. Your resistance calculations should be 16mm earth and 16mm live
(unless you wish to chop out the 6mm cpc of the 16mm T&E and not bother
using it)

Either way you are over the 0.37 ohms allowed.

It might be better to swap the head end with some thing like this

https://www.tlc-direct.co.uk/Products/CGMSF100.html


Yes, it's always better when boobs appear as a pair ;-)

I'd done the quick check at 230 but old habits (my fingers initially
typed old hobbits) die hard so 240 reappeared when I did the proper check.
When I decided to add the 10mm2 cable the brain forgot about the
existing CPC. I can't think of an excuse for that :-(

As you spotted, there was a third boob in your calcs ... brings back
memories of a character from HHG2TTG ;-)
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Old February 11th 19, 11:25 AM posted to uk.d-i-y
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Default Question about sub-main sizing

On 10/02/2019 21:26, John Rumm wrote:
On 07/02/2019 19:17, wrote:
On 07/02/2019 18:50, Cynic wrote:
I've not got tables to hand but check your earth loop impedance
against disconnection times for the 40A mcb

The sub-main earth loop impedance (20m of 10mm2 and 20m of 16mm2)
should be about 60mOhm (37mOhm + 23mOhm) so about 0.4 ohms total
(TN-C-S) - about 600A fault current. A type C MCB needs 10x rated
current, so 500A for a 50A MCB. Unless I've boobed it should be OK.



I think I must be missing something here... where does the 20m of 10mm^2
come into this?

Sadly, it was me that was missing something - I'd forgotten about the
extg CPC being in parallel with the new single. I must be getting old.

Looking at just the submain as it currently stands:

16mm^2 T&E has a round trip resistance live + CPC of 4.23 mOhms/meter
[1], so that is a contribution of 85 mOhms.

If we are going with the standard estimate of 0.35 Ohms for the supply
impedance (in the absence of a measurement), that gives a total Zs of
0.435 Ohms. So just inside the limit for a 50A Type C using the new cMin
adjusted max ELI factors[2].

So a prospective fault current of 230 / 0.435 = 525A, adequate to trip
in the instant part of the curve.

Now if you increase the CPC size to 16mm^2 with the addition of a 10mm^2
single, then the situation improves further?




[1]
http://wiki.diyfaq.org.uk/index.php/...sistance_table


[2]
http://wiki.diyfaq.org.uk/index.php/...2C_Cmin_factor




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Old February 11th 19, 07:12 PM posted to uk.d-i-y
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Default Question about sub-main sizing

On 11/02/2019 11:15, wrote:
On 10/02/2019 20:30, ARW wrote:


You might have boobed it¬* - twice, if I am not mistaken!


1. You are using 240V not 230V for your fault current. (Max Ze for a
50A C type is 0.37ohm and remember that is when the cables are at 40 deg)


2. Your resistance calculations should be 16mm earth and 16mm live
(unless you wish to chop out the 6mm cpc of the 16mm T&E and not
bother using it)

Either way you are over the 0.37 ohms allowed.

It might be better to swap the head end with some thing like this

https://www.tlc-direct.co.uk/Products/CGMSF100.html


Yes, it's always better when boobs appear as a pair ;-)

I'd done the quick check at 230 but old habits (my fingers initially
typed old hobbits) die hard so 240 reappeared when I did the proper check.
When I decided to add the 10mm2 cable the brain forgot about the
existing CPC. I can't think of an excuse for that :-(

As you spotted, there was a third boob in your calcs ... brings back
memories of a character from HHG2TTG ;-)


I am glad someone like the bit of humour I try to put into some of my posts.

The actual fused isolator I was looking for (should you go that route)
is the Scolmore DB750. But I am sure the one I linked to is the same thing.

Scolmore one here

https://www.rselectricalsupplies.co....ched-db750_813

There is a shroud showing on the first link that allows T&E to enter
properly.
--
Adam
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Old February 11th 19, 07:18 PM posted to uk.d-i-y
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Default Question about sub-main sizing

On 08/02/2019 16:25, wrote:
On 08/02/2019 12:26, Andrew Gabriel wrote:
In article ,
¬*¬*¬*¬*John Rumm writes:
On 07/02/2019 18:38,
wrote:
I haven't been able to find the rules for sub-main protection. Is it
permissible to run it from a 50A DP MCB in the garage CU, or does have
to be from a separate DP fused/switch from a Henley block?

No you can run from a MCB without any problem. Its commonly done.


One problem with using an MCB is you may get little if any
discrimination between that, and the downstream consumer unit
MCBs in the event of a fault. So if something shorts out on
a final circuit, you are quite likely to find the upstream
40/50A MCB trips, and you lose power on all the final circuits
on that submain. A BS1361 HRC cartridge fuse used to be a better
bet, but they have vanished (spare fuses still available, but
new fuse holders are not).


I was planning to use a 50A type C MCB to protect the sub-main and 16A
type B RCBOs in the new CU - this should give discrimination. The reason
for using 16A, rather than 32A, is that I have several radials and one
ring with spurs. All the high current loads are fed from the other CU,
in the garage.


That was the second point I meant to ask you about (I was tired
yesterday as I had had a weekend off work).

That's more about diversity than discrimination.

http://wiki.diyfaq.org.uk/index.php/Diversity

is worth a read if you have not already seen it.

--
Adam
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