Electrical Engineering - Three-phase circuit breakers? [UK power distribution]

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Three-phase circuit breakers? [UK power distribution]

Adam Funk

2007-02-21, 9:25 am

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?

Andrew Gabriel

2007-02-21, 1:25 pm

In article <nk0ua4-ull.ln1@news.ducksburg.com>,
Adam Funk <a24061@yahoo.com> writes:
> 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?

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
[email address is not usable -- followup in the newsgroup]

Adam Funk

2007-02-21, 5:25 pm

On 2007-02-21, Andrew Gabriel wrote:

> It's not a problem because the load also dropped to 2/3rds of
> normal.

Right. 0 VA was coming into mine!

> 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.

OK, thanks. I thought it was important to balance the output of the
transformer.

--
Leila: "What if he's innocent?"
Agent Rogersz: "No one is innocent."

John Rye

2007-02-22, 5:25 pm

Hello Adam

In article <nk0ua4-ull.ln1@news.ducksburg.com>,
Adam Funk <a24061@yahoo.com> wrote:
> 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?

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

--
John Rye
Hadleigh IPSWICH England
<http://web.ukonline.co.uk/jrye/index.html>
---< On Line using an Acorn StrongArm RiscPC >---

Adam Funk

2007-02-23, 9:25 am

On 2007-02-22, John Rye wrote:

> 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 :-

OK, that's (obviously) different from what I thought.
Thanks.

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