We had a power cut the other day, which in itself is not that
interesting. However after the initial loss of power we then had a
period of about 40 mins of sustained undervolts where we were receiving
a stable supply at about 80V. The EDF energy emergency line said it was
a fault on an underground cable on the HV network affecting about 650
homes in to local area.

What sort of fault with a underground cable typically results in
symptoms like that?


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

We had a power cut the other day, which in itself is not that
interesting. However after the initial loss of power we then had a
period of about 40 mins of sustained undervolts where we were receiving
a stable supply at about 80V. The EDF energy emergency line said it was
a fault on an underground cable on the HV network affecting about 650
homes in to local area.
What sort of fault with a underground cable typically results in
symptoms like that?

At a guess, one where there is crap interconnection with surrounding
network !

You might have been towards the end of a "radial" feed, where the
cable fault would normally support the network from the other end,
i.e.

sub-------------lots-of-load-----------------you---fault---------sub

....where everything between you can the first sub was basically
overloading the network causing volt drop at your location. Without
the fault, the backfeed would keep the volts up, and act like a big
ring-main :-}

Colin Wilson wrote:

At a guess, one where there is crap interconnection with surrounding
network !

You might have been towards the end of a "radial" feed, where the
cable fault would normally support the network from the other end,
i.e.

sub-------------lots-of-load-----------------you---fault---------sub

...where everything between you can the first sub was basically
overloading the network causing volt drop at your location. Without
the fault, the backfeed would keep the volts up, and act like a big
ring-main :-}

That did occur as a possibility, but I was surprised how stable the
supply was at our end. I would have expected "lots of load" to be quite
variable in nature, and hence the supply voltage to fluctuate somewhat.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Sun, 23 Mar 2008 23:44:07 +0000, John Rumm wrote:

We had a power cut the other day, which in itself is not that
interesting. However after the initial loss of power we then had a
period of about 40 mins of sustained undervolts where we were receiving
a stable supply at about 80V. The EDF energy emergency line said it was
a fault on an underground cable on the HV network affecting about 650
homes in to local area.

Remember that the 'help' line is staffed by non-technical people, who will
come up with all sorts of rubbish on the grounds that the general public
won't have much of clue about what's said.

What sort of fault with a underground cable typically results in
symptoms like that?

Hmm, faults in underground cables on the hv system tend to be catastrophic
in their nature - the faulty circuit is unusable until it's repaired.

Open circuit on an overhead system is a distinct possibility and would lead
to three phase transformers single phasing, that could well give rise to
the sustained and steady low voltage. A few minutes sketching out a vector
diagram will confirm that, but you need to know that distribution
transformers are connected Delta/Star hv/lv.

--
the dot wanderer at tesco dot net

Hmm, faults in underground cables on the hv system tend to be catastrophic
in their nature - the faulty circuit is unusable until it's repaired.

How the hell did I miss that it was a HV fault :-}

On Mon, 24 Mar 2008 08:53:01 -0000, Colin Wilson wrote:

Hmm, faults in underground cables on the hv system tend to be catastrophic
in their nature - the faulty circuit is unusable until it's repaired.

How the hell did I miss that it was a HV fault :-}

By no means certain anyway. The quality of info from call centre staff is
highly suspect.

--
the dot wanderer at tesco dot net

In article , The Wanderer
scribeth thus
On Mon, 24 Mar 2008 08:53:01 -0000, Colin Wilson wrote:

Hmm, faults in underground cables on the hv system tend to be catastrophic
in their nature - the faulty circuit is unusable until it's repaired.

How the hell did I miss that it was a HV fault :-}

By no means certain anyway. The quality of info from call centre staff is
highly suspect.


Loss of one phase on the HV side and never believe what anyone tells you
from a call centre;!....
--
Tony Sayer

tony sayer wrote:
In article , The Wanderer
scribeth thus
On Mon, 24 Mar 2008 08:53:01 -0000, Colin Wilson wrote:

Hmm, faults in underground cables on the hv system tend to be catastrophic
in their nature - the faulty circuit is unusable until it's repaired.
How the hell did I miss that it was a HV fault :-}
By no means certain anyway. The quality of info from call centre staff is
highly suspect.


Loss of one phase on the HV side and never believe what anyone tells you
from a call centre;!....

Hence the question really, since I was taking what they said with a
pinch of salt! ;-)

However I knew it was unlikely to be a LV fault since I can see the sub
station (which seems rather too glorified a term for what is just a
tranny on a pole) from here and it only feeds the local group of about 8
houses, and judging by the lack of street lights in the distance from
some directions it was far more widespread than just our road.

The first call to the call centre said HV fault, but did not specify any
more detail (I assumed it was an overhead line at that point), only
later did they add the bit about it being an underground cable, which
seemed less likely given the typical HV infrastructure in these parts.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

The Wanderer wrote:

Open circuit on an overhead system is a distinct possibility and would lead
to three phase transformers single phasing, that could well give rise to
the sustained and steady low voltage. A few minutes sketching out a vector
diagram will confirm that, but you need to know that distribution
transformers are connected Delta/Star hv/lv.

Ah, yup that does sound more plausible. It would also tie in with the
timescale. The "help" centre said that an engineer would be attending as
soon as he finished the current job - i.e. there is no one there yet so
they were just guessing! About 40 mins later the power went off
altogether. Which would tie in with "ok, I am here now, oh look, half
the wires are down, we might as well switch off that bit!"

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:
We had a power cut the other day, which in itself is not that
interesting. However after the initial loss of power we then had a
period of about 40 mins of sustained undervolts where we were
receiving a stable supply at about 80V. The EDF energy emergency line
said it was a fault on an underground cable on the HV network
affecting about 650 homes in to local area.

What sort of fault with a underground cable typically results in
symptoms like that?


Interesting. We also had a power cut (5.5 hours) followed by sustained
variable low voltage (192 - 225v) for 6 evenings, varying from 4 - 11 hours.
On Sunday it was low from about 1230 pm to 5 am next morning. May not seem a
great problem, but the central heating boiler would not operate below 220v
(confirmed by Worcester Bosch).

EDF gave various reasons on the reasons for the low voltage (one being an
apparent problem with a temp generator). It seems to me that the periods of
low voltage corresponded fairly well with maximum load as one would expect.
I wondered if the "fault" was perhaps deliberate to avoid straining a supply
which had suffered in some way from the previous power cut, with low voltage
leading to a lower current throughout the system.

The fault took 6 days to fix.

--
Jeff
(cut "thetape" to reply)

"John Rumm" wrote in message
...
The Wanderer wrote:

Open circuit on an overhead system is a distinct possibility and would
lead
to three phase transformers single phasing, that could well give rise to
the sustained and steady low voltage. A few minutes sketching out a
vector
diagram will confirm that, but you need to know that distribution
transformers are connected Delta/Star hv/lv.

Ah, yup that does sound more plausible. It would also tie in with the
timescale. The "help" centre said that an engineer would be attending as
soon as he finished the current job - i.e. there is no one there yet so
they were just guessing! About 40 mins later the power went off
altogether. Which would tie in with "ok, I am here now, oh look, half the
wires are down, we might as well switch off that bit!"

--
Cheers,

John.

How long was your power off, I reckpn It may have been a broken HV jumper,
as the power did not trip completley.

Oh the Happy memories of fixing them in the crappy weather.

Regards

Steve

On Mon, 24 Mar 2008 15:37:21 -0000, Jeff Layman wrote:


Interesting. We also had a power cut (5.5 hours) followed by sustained
variable low voltage (192 - 225v) for 6 evenings, varying from 4 - 11 hours.
On Sunday it was low from about 1230 pm to 5 am next morning. May not seem a
great problem, but the central heating boiler would not operate below 220v
(confirmed by Worcester Bosch).

That's probably because you were switched to another hv circuit. Generally
hv circuits are run as interconnected radial circuits, with numerous
isolation points along the circuit. You may be 4-5 km along on your normal
feeder, but 7-8km on a temporary hv backfeed, IYSWIM.

EDF gave various reasons on the reasons for the low voltage (one being an
apparent problem with a temp generator). It seems to me that the periods of
low voltage corresponded fairly well with maximum load as one would expect.
I wondered if the "fault" was perhaps deliberate to avoid straining a supply
which had suffered in some way from the previous power cut, with low voltage
leading to a lower current throughout the system.

Highly unlikely. The hv system gets it's supply through system transformers
that have automatic voltage control. As the load increases the system
transformers push out more volts, could be upto 11.3 - 11.4kv. System
losses and load affect the voltage along the circuit, so at the normal open
point the notional voltage may be only 10.8kv. Distribution transformers
can be set for a range of input voltages 0, ±2.5%, ±5% of nominal 11kv.

When you're on a long alternative backfeed, the volts start to run out
somewhere past the normal open point, so you may be only getting 10.5kv.

Alright it doesn't help you with your c/h boiler, but at least you have
some volts whilst the fault is repaired.

--
the dot wanderer at tesco dot net

Stephen Dawson wrote:

How long was your power off, I reckpn It may have been a broken HV jumper,
as the power did not trip completley.

About seven hours.

Oh the Happy memories of fixing them in the crappy weather.

It was wet and windy, but not as bad as it has been in the last couple
of weeks.

Still I probably got the better end of the bargain being sat in front of
the log fire playing board games by candlelight! ;-)

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

The Wanderer wrote:
On Mon, 24 Mar 2008 15:37:21 -0000, Jeff Layman wrote:


Interesting. We also had a power cut (5.5 hours) followed by
sustained variable low voltage (192 - 225v) for 6 evenings, varying
from 4 - 11 hours. On Sunday it was low from about 1230 pm to 5 am
next morning. May not seem a great problem, but the central heating
boiler would not operate below 220v (confirmed by Worcester Bosch).

That's probably because you were switched to another hv circuit.
Generally hv circuits are run as interconnected radial circuits, with
numerous isolation points along the circuit. You may be 4-5 km along
on your normal feeder, but 7-8km on a temporary hv backfeed, IYSWIM.

EDF gave various reasons on the reasons for the low voltage (one
being an apparent problem with a temp generator). It seems to me
that the periods of low voltage corresponded fairly well with
maximum load as one would expect. I wondered if the "fault" was
perhaps deliberate to avoid straining a supply which had suffered in
some way from the previous power cut, with low voltage leading to a
lower current throughout the system.

Highly unlikely. The hv system gets it's supply through system
transformers that have automatic voltage control. As the load
increases the system transformers push out more volts, could be upto
11.3 - 11.4kv. System losses and load affect the voltage along the
circuit, so at the normal open point the notional voltage may be only
10.8kv. Distribution transformers can be set for a range of input
voltages 0, ±2.5%, ±5% of nominal 11kv.

When you're on a long alternative backfeed, the volts start to run out
somewhere past the normal open point, so you may be only getting
10.5kv.

Alright it doesn't help you with your c/h boiler, but at least you
have some volts whilst the fault is repaired.

Thanks for the info - explains it all very well.

Main grouch was that we have gas central heating (gas also from EDF), but
had to resort to fan heaters when the boiler failed. Cost of latter is 4x
former per KWh.

--
Jeff
(cut "thetape" to reply)