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.

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