Got sorted last night. Fault was only reported at about 21.00.

Interestingly, the call centre bloke said a low volts report wasn't an
emergency in the same way as no power at all. Even with the volts being
low enough to stop near everything but lights working. I'm sure they make
these things up as they go along...

--
*My dog can lick anyone

Dave Plowman London SW
To e-mail, change noise into sound.

On Fri, 12 Mar 2010 15:44:53 +0000 (GMT), Dave Plowman (News) wrote:

Got sorted last night. Fault was only reported at about 21.00.

Interestingly, the call centre bloke said a low volts report wasn't an
emergency in the same way as no power at all.

Yebbut. Trained monkeys an' all that.....



--
The Wanderer

I have become more optimistic.
I now believe that things can get worse.

"Dave Plowman (News)" wrote in message
...
Got sorted last night. Fault was only reported at about 21.00.

Interestingly, the call centre bloke said a low volts report wasn't
an
emergency in the same way as no power at all. Even with the volts
being
low enough to stop near everything but lights working. I'm sure they
make
these things up as they go along...

--
*My dog can lick anyone

Dave Plowman London SW
To e-mail, change noise into sound.


The danger issue with low voltage is that fuses will not blow within
the specified times - with low volts and another fault fire is a
distinct possiblity.

It sounds like it is a TNCS supply (based on 25 yr age) in which case if the
N is about to go open circuit this is dangerous. Might be worth testing if
anything more than 10v N to earth with load on; if so I would isolate supply
until fixed. Danger is that if N is failing/broken on DNO side you just have
a L and dependant on how good your PME is you might end up with live metal
surfaces.

Alternatively you might just have a drugs farm next door!

S

Andrew Mawson wrote:

The danger issue with low voltage is that fuses will not blow within
the specified times
[snip]

That doesn't sound right. How can the fuse know anything about the
supply voltage?

Cheers,

Colin.

"Colin Stamp" wrote in message
...
Andrew Mawson wrote:

The danger issue with low voltage is that fuses will not blow
within
the specified times
[snip]

That doesn't sound right. How can the fuse know anything about the
supply voltage?

Cheers,

Colin.

A fault generates less fault current at the lower voltage resulting in
a fuse taking longer to blow.

AWEM

Andrew Mawson wrote:
"Colin Stamp" wrote in message
...
Andrew Mawson wrote:

The danger issue with low voltage is that fuses will not blow
within
the specified times
[snip]

That doesn't sound right. How can the fuse know anything about the
supply voltage?

Cheers,

Colin.

A fault generates less fault current at the lower voltage resulting in
a fuse taking longer to blow.

AWEM


Ah, so I guess you're talking specifically about earth faults in systems
where fuses are the only protection.

Actually, I wonder how low voltage would affect the operation of an RCD.
Would a 30mA RCD become a 60mA one at 120V?

Cheers,

Colin.

Andrew Mawson wrote:
"Colin Stamp" wrote in message
...
Andrew Mawson wrote:

The danger issue with low voltage is that fuses will not blow
within
the specified times
[snip]

That doesn't sound right. How can the fuse know anything about the
supply voltage?

Cheers,

Colin.

A fault generates less fault current at the lower voltage resulting in
a fuse taking longer to blow.


as the wiring its designed tp protect takes longer to catch fire..

More stoopthink from the half knowledgeable..
AWEM

Colin Stamp wrote:
Andrew Mawson wrote:
"Colin Stamp" wrote in message
...
Andrew Mawson wrote:

The danger issue with low voltage is that fuses will not blow
within
the specified times
[snip]

That doesn't sound right. How can the fuse know anything about the
supply voltage?

Cheers,

Colin.

A fault generates less fault current at the lower voltage resulting in
a fuse taking longer to blow.

AWEM


Ah, so I guess you're talking specifically about earth faults in systems
where fuses are the only protection.

Actually, I wonder how low voltage would affect the operation of an RCD.
Would a 30mA RCD become a 60mA one at 120V?

No.

the average RCD works at under a volt: It's a current sensor only.

He's talking ********.

Cheers,

Colin.

The Natural Philosopher
wibbled on Friday 12 March 2010 20:35

Andrew Mawson wrote:
"Colin Stamp" wrote in message
...
Andrew Mawson wrote:

The danger issue with low voltage is that fuses will not blow
within
the specified times
[snip]

That doesn't sound right. How can the fuse know anything about the
supply voltage?

Cheers,

Colin.

A fault generates less fault current at the lower voltage resulting in
a fuse taking longer to blow.


as the wiring its designed tp protect takes longer to catch fire..

More stoopthink from the half knowledgeable..
AWEM


There's also the factor of disconnect time to prevent an excessive touch
voltage remaining for more than the specified time - to do with the
potential for administering a lethal shock.

But and RCD if present will sort that out.

--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.

On Sun, 14 Mar 2010 19:22:42 +0000, John Rumm wrote:

So not only do we have a dangerous touch voltage present for an extended
period, the chances are the CPC in the cable will fail before it gets
cleared. Resulting in electrocution risk, cable damage, and fire risk.

In addition (a separate issue) don't certain kinds of motors overheat if
run on low voltage?

--
Use the BIG mirror service in the UK:
http://www.mirrorservice.org

On Sun, 14 Mar 2010 19:22:42 +0000, John Rumm wrote:

But who is guilty of the stoopthink here I wonder? There is more to be
concerned about than just catching fire.

As an example, take a class one appliance plugged into a spur from a
ring circuit with an earth loop impedance of 1.1 ohms. Circuit
protection is a 32A type B MCB.

Prospective fault current at normal voltage is 230/1.1 = 209A, or well
over the 160A required to open the MCB in 0.1 secs.

Let's say we have a serious undervolt with the supply at 100V. Our
prospective fault current now falls to only 90A. According to Fig 3.3B
in the 17th edtn of BS7671, that gives a fault clearance time of about
45 seconds. That alone should give cause for concern, since the fault
clearance time is now well out of either the 0.4 or 5 sec limits
allowed.

However, it gets worse....

With normal voltage, in the even of an earth fault, we can calculate the
minimum CSA of CPC required to clear the fault without failing:

min CSA CPC = sqrt( 209^2 x 0.1 ) / 115 = 0.6mm^2

Which is well under the 1.5mm^2 available on a spur connected by a
single run of 2.5mm^2 T&E with 1.5mm^2 CPC.

However if we repeat the above calculation for the undervolt situation
we now get:

min CSA CPC = sqrt( 90^2 x 45 ) / 115 = 5.25mm^2

Well in excess of even the combined 3mm^2 available on the ring, never
mind the spur.

So not only do we have a dangerous touch voltage present for an extended
period, the chances are the CPC in the cable will fail before it gets
cleared. Resulting in electrocution risk, cable damage, and fire risk.

I knew sustained low volts was "not to be recommended" but that
excellent post really shows how nasty the consequences can be. One
for the FAQ?


--
Cheers
Dave.

Dave Liquorice wrote:
On Sun, 14 Mar 2010 19:22:42 +0000, John Rumm wrote:

But who is guilty of the stoopthink here I wonder? There is more to be
concerned about than just catching fire.

As an example, take a class one appliance plugged into a spur from a
ring circuit with an earth loop impedance of 1.1 ohms. Circuit
protection is a 32A type B MCB.

Prospective fault current at normal voltage is 230/1.1 = 209A, or well
over the 160A required to open the MCB in 0.1 secs.

Let's say we have a serious undervolt with the supply at 100V. Our
prospective fault current now falls to only 90A. According to Fig 3.3B
in the 17th edtn of BS7671, that gives a fault clearance time of about
45 seconds. That alone should give cause for concern, since the fault
clearance time is now well out of either the 0.4 or 5 sec limits
allowed.

However, it gets worse....

With normal voltage, in the even of an earth fault, we can calculate the
minimum CSA of CPC required to clear the fault without failing:

min CSA CPC = sqrt( 209^2 x 0.1 ) / 115 = 0.6mm^2

Which is well under the 1.5mm^2 available on a spur connected by a
single run of 2.5mm^2 T&E with 1.5mm^2 CPC.

However if we repeat the above calculation for the undervolt situation
we now get:

min CSA CPC = sqrt( 90^2 x 45 ) / 115 = 5.25mm^2

Well in excess of even the combined 3mm^2 available on the ring, never
mind the spur.

So not only do we have a dangerous touch voltage present for an extended
period, the chances are the CPC in the cable will fail before it gets
cleared. Resulting in electrocution risk, cable damage, and fire risk.

I knew sustained low volts was "not to be recommended" but that
excellent post really shows how nasty the consequences can be. One
for the FAQ?


Its still ********.

If the cable can blow before the fuse trips on *any* installation on ANY
voltage, its ipso facto dangerous.


Playing around with the numbers to try and prove its different in some
carefully chosen case doesn't alter that.

Fuses blow on over current. Wires blow on over current. If the voltage
isn't enough to blow a trip its not enough to blow a cable, or the cable
is ill matched to the trip. Period.

A little knowledge...