On 22/11/2016 20:12, ARW wrote:
On 22/11/2016 16:02, Jack Lawson wrote:
On Fri, 18 Nov 2016 12:02:49 +0000, Jack Lawson wrote:



Earlier this week I bought from Homebase a mitre saw:
http://www.homebase.co.uk/en/homebas...s-2125u-389395


I had read reviews and seemed to fit my needs perfectly.
Got it home - and tested it. I found that when I started the saw, it
tripped
my RCD on the supply. Tested it further - sometimes it tripped -
sometimes
not. This was on initial start-up with no load.
Took it back - got a replacement.
The new one is just the same. About 1 in 10 of start-ups the RCD
will trip.
The saw seems perfect for my needs - very accurate mitres and 90
degree cuts
are spot on - I think I will live with what for me is a minor problem
- but
thought I'd just share.

MCB swapped for Type C 32 Amps - job's a good 'un


Thanks to all for advice.



No-one suggested that you should fit a C type 32A MCB.

Indeed that may or may not be safe depending on the external earth
impedance at the origin, since that MCB will need 320A to open on the
instant part of the trip curve.

If you had the notional book value of 0.8 Ohms of external earth
impedance on a TN-S earthing system, and added to that the 42 mOhms
contribution of the cable, you would only be able to muster about 273A
of fault current. (which would take something like 7 seconds to open
that MCB).

An adiabatic check would also show the conductor size required:

s = sqrt( 273^2 x 7 ) / 115 = 6.28mm^2

so well in excess of the 1.5mm^2 area of earth conductor.

If we work it backwards from the 320A required, the total ELI can't be
more than 230 / 320 = ~0.7 ohms. So fine without measurements on a
TN-C-S system, but will need checking on TN-S




--
Cheers,

John.

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