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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| Internode Ltd - http://www.internode.co.uk |
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ARW wrote:

On 22/11/2016 20:23, Roger Hayter wrote:
Andy Burns wrote:

Andy Burns wrote:

Jack Lawson wrote:

MCB swapped for Type C 32 Amps

32A? Properly you should use 4mm^2 with that.

Or make it a very short ring with 2.5mm^2

We've established upthread that the 2.5^2 is short enough.


Not short enough for a C type 32A MCB on a TN-S supply that is at the
upper limits of it's maximum allowed Zs.

Sorry I missed we'd changed from type B to type C. That makes a
dfference I suppose? With my limited knowledge I think I just know
that ordinary circuits will be all right with 20A type B and if I want
anything more unusual I'll ask.

--

Roger Hayter

On Tue, 22 Nov 2016 17:13:00 +0000, Andy Burns wrote:

Andy Burns wrote:

Jack Lawson wrote:

MCB swapped for Type C 32 Amps

32A? Properly you should use 4mm^2 with that.

Or make it a very short ring with 2.5mm^2


Please see earlier posts - it is not a ring - it is a very short spur direct in
to a dedicated MCB.

On 22/11/2016 23:25, Roger Hayter wrote:
ARW wrote:

On 22/11/2016 20:23, Roger Hayter wrote:
Andy Burns wrote:

Andy Burns wrote:

Jack Lawson wrote:

MCB swapped for Type C 32 Amps

32A? Properly you should use 4mm^2 with that.

Or make it a very short ring with 2.5mm^2

We've established upthread that the 2.5^2 is short enough.


Not short enough for a C type 32A MCB on a TN-S supply that is at the
upper limits of it's maximum allowed Zs.

Sorry I missed we'd changed from type B to type C. That makes a
dfference I suppose?

The C type has the same overload protection (i.e the thermal part of its
response is the the same) as the type B, but its magnetic response is
less sensitive. A type B will open "instantly" on fault currents of 5
times the nominal current, so 160A for a B32. The C type devices require
10 times the nominal current to open instantly, or 320A for a C32

The effect is to make them far more tolerant of high inrush currents,
but at the expense of possibly lowering the protection for fault
currents, so in some cases one will need to check (by measurement or
calculation) if a C type is safe to use on a particular circuit.

With my limited knowledge I think I just know
that ordinary circuits will be all right with 20A type B and if I want
anything more unusual I'll ask.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/