"newshound" wrote in message
o.uk...
On 3/18/2017 11:18 AM, wrote wrote:
Despite extensive googling, there seems to be nothing that tells me how
much
current flows along wires on a national grid pylon. They only list
voltages.
Anybody know?

A 400 kV National Grid circuit may carry 1 kA in each of its three phases,
thus transmitting a power of 700 MW.
A 132 kV distribution circuit may carry 300 A in each of its three phases,
thus transmitting a power of 70 MW.
An 11 kV distribution circuit may carry 150 A in each of its three phases,
thus transmitting a power of 3 MW.
A 400 V final distribution circuit may carry 200 A in each of its three
phases, thus transmitting a power of 150 kW.

200A per phase at 400V (240V phase-to-neutral) doesn't sound very high. We
have a 60 A "company fuse" and I presume our neighbours do too. With an
electric fire (3 KW), an electric shower (maybe 8 kW) and an electric oven
and hob (maybe 6 KW), you'd be getting towards that limit but still
remaining legal. Now imagine lots of people roundabout doing that. It
doesn't take many houses to run up 200 A - or a total of 600 A across all
three phases. How many houses are typically fed from a single feed from the
substation or 11 kV-to-400V pole-mounted transformer? What is the average
current that is assumed per house when sizing up the number of houses that
can be fed from one substation circuit? I presume it not the full 60A of the
company fuse rating.

And Supergrid pylons (400 and 275 kV) are normally double circuit, aren't
they? Three wires on each side of the "tree".

How much of the route from the power station to the consumer is redundant
multi-circuit? At one point, typically, does it change over to a given house
only being fed by one set of wires, and if that line develops a fault there
is no backup circuit?

Is there a backup route as far as the final substation that transforms to 11
kV or 400V, or is it higher up the chain?

I presume for maximum redundancy they try to use feeds from different places
rather than two sets of wires carried on the same pylons, in case an
accident takes out *all* the wires (both circuits).

I'm intrigued at the way house gets its electricity supply. There is
overhead mains on wooden poles (originally four separate wires, now a single
fat cable with four wires) and our house is the middle house of two adjacent
blocks of three houses. There is a single feed from the wooden poles to the
end of one block, and then four wires running along the back of one block,
overhead across the gap to the next block and along there, with each house
taking its feed from neutral and one of the three phases - I think no two
adjacent houses are on the same phase. I suppose this is less unsightly than
every one of the six houses having its own single-phase feed from the street
poles.