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
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?
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.
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.