Three-phase circuit breakers? [UK power distribution]

Recently there was a partial power failure on my street (including my
house), but our adjacent neighbours' electricity was still on. When I
phoned to report it, the power company said there was already someone
on the way to our substation to fix it.
AIUI, urban residential power supplies in the UK are provided from
three-phase underground cables. The cable running down our street has
four conductors: earth/neutral and three lives (red, yellow and blue)
each at 240 volts but 120 degrees apart. Each house is connected to a
phase in turn and to the earth/neutral, so for example houses 1, 7, 13
and so on are on the red phase; 3, 9, 15, ... on the yellow; and 5,
11, 17, ... on the blue. This provides load-balancing between the
phases.
I was surprised that one phase could trip while the other two on the
same transformer stayed on --- I would have expected the output of the
last transformer to go through a breaker that would disconnect all
three phases in the event of a fault. If only one phase trips,
doesn't that leave the load on the transformer unbalanced? And why
isn't that considered a problem?
Reply to
Adam Funk
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It's not a problem because the load also dropped to 2/3rds of normal. If the houses without power could have reconnected themselves to another working phase, then it might have been a problem because the original load would then have been distributed across just 2 phases, which could have overloaded those two transformer windings if they were near their load limits. The street distribution cable will have a full size neutral conductor, so the local distribution network will not be assuming a balanced load, and will cope fine with one or two of the 3 phases tripping out.
Reply to
Andrew Gabriel
Right. 0 VA was coming into mine!
OK, thanks. I thought it was important to balance the output of the transformer.
Reply to
Adam Funk
Hello Adam
Andrew Gabriel has already given you most of the answer, but it is a bit misleading to talk of circuit-breakers tripping. The 240 V cables supplying houses are protected almost invariably by three independant fuses. When these blow the problem is normally a cable fault, and frequently only one or two phases will fail. After the fuse has blown there are two possibilities :-
(a) The heat of the arc will have dried out the fault, and inserting a new fuse will restore the supply for an indeterminate time between a few minutes and several months.
(b) The fault will be permanent, and the supply company will set about the odten difficult task of locating and repairing the fault.
John
Reply to
John Rye
OK, that's (obviously) different from what I thought. Thanks.
Reply to
Adam Funk

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