On Tue, 22 Nov 2011 03:22:16 -0000, John Rumm wrote:

On 22/11/2011 02:32, Lieutenant Scott wrote:
On Mon, 21 Nov 2011 09:17:19 -0000, Dave Liquorice
wrote:

On Mon, 21 Nov 2011 03:21:51 -0000, Lieutenant Scott wrote:

BTW I take it the smoke alarms are ok on batteries, because all the
mains can fail due to a fire in the wire before your meter. Happened to
my neighbour due to loose connection. Electric board too stupid to fuse
this wire at substation.

The feed would have been protected against a phase to phase or phase
to earth faults but a "loose coonection" is neither of those it just
gets hot due to the volt drop across it. 500A, 0.01 ohm = 2.5kW,
doesn't take much of a "loose connection" to produce significant
quantities of heat...

Firstly it's ONE hundred amps into a house, not FIVE. It should be fused
at 100 amps AT THE SUBSTATION to protect from faults on the external
side of your meter, but it isn't. There is no protection whatsoever
against shorting a house feed wire.

Houses are not usually individually wired to the substation. Typically
the three phases will leave the station and then be daisy chained to
many properties in rotation down the road. Hence you can't fuse them at
100A at the substation.

(However diversity does allow them to be fused far lower than one might
expect - 600A per phase is not uncommon IIUC even when there are dozens
of properties sharing a phase)

The wire feeding 1 property is not capable of 600 amps, therefore needs a more sensitive fuse.

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