RE: [OT] Reconfiguring Ringmain to garage.

["Nigel Giddings" <nigel@xxxxxxxxxxxxxx>]

Wayne,



It doesn't sound as if there is any thing electrically unsafe about what
you suggest. Effectively hanging a small dis board off of a ring main.



I think the issue is what will you gain?



Depending on the load applied to the rest of the 'Ring Main' at the time
of a fault you may trip the main ring main MCB and any protection you
have included down-line may not trip...



If the Main MCB is rated at 32A and your sub ring is 16A if the
MAIN-MAIN is already pulling 17A the fault current will trip the 32A and
may not trip the 16A MCBs. This is a big problem with sub circuits feed
from main ccts protected by MCBs. A typical house has a 60, 80 or 100A
fuse which is much slower to blow than an MCB so tripping 'up' the line
is not normally an issue.



Adding a 30mA RCD or RCBO to your sub ring may have advantages if the
main RCD is 300mA and you are not close to tripping the main RCD due to
earth leakage. Many installations leak power to earth especially
Switched Mode Power supplies and UPSs...



The ideal solution is a separate cct on you main dis board feed with 4mm
(cable type depends on route) and then run a dedicated 2.5mm ring around
the garage/workshop. Keeping domestic appliances separate from the tools
is a good idea. I would also include a cut off switch so that as you
leave the workshop all the non-essential power is isolated...



HTH



Nigel



-----Original Message-----
From: Wayne [mailto:Wayne@xxxxxxx]
Sent: 06 September 2006 00:20
To: ukha_d@xxxxxxx
Subject: [ukha_d] [OT] Reconfiguring Ringmain to garage.



Hiya all,
would like to run this past 'the collective' to see if this is
acceptable/correct/ not breaking any rules etc...

Our garage is attached to our house via a porch and currently has a
breaker in our main distribution board feeding a ring. Theres 7 sockets
on this ring in the garage at the mo, being used for things like washing

machine / freezer and a slowly growing collection of power tools (drill
press/ table saw/ various hand-held tools).

Currently the ring is the standard 2.5mm twin and earth going from the
main board - picking up each socket and then back to the board - nothing

out of the normal there.

What I would like to do is have a 'sub board' (?) in the garage that
could be expanded on as and when the tool collection grows. I'm not
looking at running 3 phase stuff or anything industrial in there, all of

this is going to have the old standard 13amp plug on the end.

As I understand things, 2.5mm is rated at 20amps - so having a ring
basicly says that the cable could take 40amps. (the breaker in the house

is a 32amp standard jobbie and I'm not looking at changing that).

The Question: (eventually)
Is it possible to take both the ring feeds from the main board and use
this to feed this 'sub board' without having to run any more cable back
to the main board? Maybe with an RCD split to allow me to keep things
like the freezer always on but other sockets to trip if there's a
problem (at the min I've got an 'inline' rcd plug that I move with the
tools I'm using.).

It would also mean that as the equipment list grows I could just add
another breaker in this 'sub board' and feed directly to it.

Thoughts?

Hopefully this makes sense! Thanks.
Wayne.




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6/9/2006



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