Crossing London tube tracks
 

On Tue, 08 Apr 2008 09:52:47 +0100, tony sayer wrote:


yes I remember that too, presumably 4 parallel sets of 3 lamps in series
so the -240/+410 gives each lamp about 250V?

I thought an 'eff off size shorting bar was the best insurance;!...

You don't drop a shorting bar across a live supply. The effect would be
some what spectacular and will probably throw molten metal around,
possibly causing nasty injuries.
People have been injured in the power industry by closing earthing
switches onto a live supply and then being splattered with molten metal
when the earth switch vaporised. (As they are not designed to switch live
and take the full fault current across their closing contacts)

You don't short the circuit till you have tested it's actually dead.
Then short it so that if some one accidentally turns it back on while you
are working on it, the short circuit causes it to trip off again. And you
make short your short is securely attached so that in the event is really
does get energised, the sudden large current flow doesn't cause the
shorting wire/bar to fly off and disconnect itself, thus allowing the
circuit to be fully energized and then fry you.


--
Matthew Geier

Crossing London tube tracks
 

On Tue, 08 Apr 2008 20:08:55 +0100, Charles Ellson wrote:

but the usual rule about treating anything as
live unless it is obviously earthed seems to be the main defence.

As a friend who worked in the power industry once said - 'The only thing
I trust is a metre air gap'.
Any one working on a high power system will want the assurance some
idiot can't turn it back on and kill them, hence the highly visible short
to earth bars/wires. If some one tries to turn it back on, the short to
earth should be a sufficiently good 'fault' to trip it off again saving
your own skin from such stupidity.

An people do try to turn things back on again. I've seen at least one
report were a circuit was isolated for work, and a remote control centre
forgot about the work, saw the isolated circuit as a fault and tried to
remotely turn the power on again...
The earthing cables saved the guys out on site.

--
Matthew Geier

Crossing London tube tracks
 

In article , Matthew
Geier scribeth thus
On Tue, 08 Apr 2008 09:52:47 +0100, tony sayer wrote:


yes I remember that too, presumably 4 parallel sets of 3 lamps in series
so the -240/+410 gives each lamp about 250V?

I thought an 'eff off size shorting bar was the best insurance;!...

You don't drop a shorting bar across a live supply. The effect would be
some what spectacular and will probably throw molten metal around,
possibly causing nasty injuries.
People have been injured in the power industry by closing earthing
switches onto a live supply and then being splattered with molten metal
when the earth switch vaporised. (As they are not designed to switch live
and take the full fault current across their closing contacts)

You don't short the circuit till you have tested it's actually dead.
Then short it so that if some one accidentally turns it back on while you
are working on it, the short circuit causes it to trip off again. And you
make short your short is securely attached so that in the event is really
does get energised, the sudden large current flow doesn't cause the
shorting wire/bar to fly off and disconnect itself, thus allowing the
circuit to be fully energized and then fry you.



Ever heard of SIDE?.

Switch off
Isolate
Dump
Earth

One wouldn't recommend for a moment chucking an earthing bar or cables
across a live line!.

A shorting/earthing system should be suitable for the currents involved.

There will be a difference for linesmen working on an 11 kV overhead to
someone on an LU line!...



--
Tony Sayer

Crossing London tube tracks
 

On 08 Apr 2008 21:04:35 GMT someone who may be Matthew Geier
wrote this:-

You don't drop a shorting bar across a live supply. The effect would be
some what spectacular and will probably throw molten metal around,
possibly causing nasty injuries.

An occasional poster to uk.railway has written of the time he spent
testing the various designs of such bars on live railway supplies
(as they have been used since the earliest days). IIRC one of the
parts of these tests was to see how the designs minimised the
chances of injuries.

While ideally they should be used just to ensure that supplies
cannot be re-energised I think that in an emergency they are still
used first. When radio communications become rapid, as they are in
some places, this may be changed/has been changed relatively
recently as it will then be at least as rapid to make a radio call
compared to getting hold of the bar, clambering out of the cab and
applying the bar.

People have been injured in the power industry by closing earthing
switches onto a live supply and then being splattered with molten metal
when the earth switch vaporised. (As they are not designed to switch live
and take the full fault current across their closing contacts)

Short-circuiting bars are not the same as earthing switches.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

Crossing London tube tracks
 

On 08 Apr 2008 21:13:49 GMT someone who may be Matthew Geier
wrote this:-

Any one working on a high power system will want the assurance some
idiot can't turn it back on and kill them, hence the highly visible short
to earth bars/wires. If some one tries to turn it back on, the short to
earth should be a sufficiently good 'fault' to trip it off again saving
your own skin from such stupidity.

That rather depends on the system. For example it is difficult to
apply an earthing wire to a cable one is about to open up to joint,
but one is a sensible precaution on a bare conductor.

Where the system covers a relatively small area then the obvious way
to manage things is to use locks. If people are doing the same sort
of work over a small area then they can have their own lock which
they keep the key to. An example would be a workshop where several
people work on the electrical parts of a train.

If there is a larger system with someone responsible for the whole
system then the way to manage it is to use locks, the keys for which
are locked into a box. This box has a key for those doing the work
and a key for the person responsible for the system.

Such systems become unmanageable over large areas with remotely
operated switchgear.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

Crossing London tube tracks
 

In message , David Hansen
writes

An occasional poster to uk.railway has written of the time he spent
testing the various designs of such bars on live railway supplies
(as they have been used since the earliest days). IIRC one of the
parts of these tests was to see how the designs minimised the
chances of injuries.

While ideally they should be used just to ensure that supplies
cannot be re-energised I think that in an emergency they are still
used first. When radio communications become rapid, as they are in
some places, this may be changed/has been changed relatively
recently as it will then be at least as rapid to make a radio call
compared to getting hold of the bar, clambering out of the cab and
applying the bar.

On LU we are trained to lay SCDs (Short Circuit Devices) live as an
absolute last resort although if we have to that's what we do. We are
also trained to look away though ;)

The ones on LU are designed to clamp themselves to the rails and carry
the fault current required to trip out the breakers in the switchrooms.
--
Steve Fitzgerald has now left the building.
You will find him in London's Docklands, E16, UK
(please use the reply to address for email)

Crossing London tube tracks
 

Matthew Geier wrote:
On Sun, 06 Apr 2008 03:28:47 +0100, Charles Ellson wrote:

On Sun, 06 Apr 2008 02:33:53 +0100 (BST), "Dave Liquorice"
wrote:

On Sat, 05 Apr 2008 23:59:50 +0100, Bruce Fletcher (Stronsay, Orkney)
wrote:

Test if a wire is live by touching it to the BACK of the hand. The
muscle action is then to throw your hand away
Better still, test with a meter!
A meter can be misleading if it has a high impedance, as many modern
meters do. It'll measure voltages induced into the wire that are not
actually there if you apply even a small "load".

In that case it will still show _at least_ the possible voltage that
contact might be made with even if the available current is minimal.

This is why the serious 'sparkies' carry analogue meters and 'test
lamps'.
Digital Meters can read apparent high voltages that were induced by
induction from near by cables. Analogue meters tend not to do that - they
pull more power from the load being tested. (A tradesmen is unlikely to
be carrying around a super expensive high-impedance laboratory analogue
meter, were as high impedance digital meters are common).
I have a recollection that at least one test manufacture makes DVMs that
have a lower than normal impedance (for a DVM) for exactly this sort of
use, ensuring circuits are well and truly dead before beginning work on
them.

Of course test lamps are a 'real load' and tell the story. But the test
lamps used by an electrician when working on 'domestic' mains won't last
very long if you tried to use them to see if the 3rd (or 4th) rail was
live. A bit too much juice!
Some one who regularly works on railway power systems may actually have
in their kit a 'test lamp' for 750v supplies.


Four regular 40w lamps wired in parallel should do the trick. The problem comes
when one of them blows :)

Crossing London tube tracks
 

On 11/04/2008 12:23, funkmish wrote:

Four regular 40w lamps wired in parallel should do the trick. The
problem comes when one of them blows :)

Four 240V lamps wired in parallel across a 750V supply would blow rather
quickly I'd expect. You should try to explain that you meant to say "in
series" at this point ;-)

Crossing London tube tracks
 

Andy Burns wrote:
On 11/04/2008 12:23, funkmish wrote:

Four regular 40w lamps wired in parallel should do the trick. The
problem comes when one of them blows :)

Four 240V lamps wired in parallel across a 750V supply would blow rather
quickly I'd expect. You should try to explain that you meant to say "in
series" at this point ;-)

Yes, I meant in series, hence my comment about when one of them blows! No idea
why I typed parallel!!!

Crossing London tube tracks
 

Matthew Geier wrote:
On Sun, 06 Apr 2008 03:28:47 +0100, Charles Ellson wrote:

On Sun, 06 Apr 2008 02:33:53 +0100 (BST), "Dave Liquorice"
wrote:

On Sat, 05 Apr 2008 23:59:50 +0100, Bruce Fletcher (Stronsay, Orkney)
wrote:

Test if a wire is live by touching it to the BACK of the hand. The
muscle action is then to throw your hand away
Better still, test with a meter!
A meter can be misleading if it has a high impedance, as many modern
meters do. It'll measure voltages induced into the wire that are not
actually there if you apply even a small "load".

In that case it will still show _at least_ the possible voltage that
contact might be made with even if the available current is minimal.

This is why the serious 'sparkies' carry analogue meters and 'test
lamps'.
Digital Meters can read apparent high voltages that were induced by
induction from near by cables. Analogue meters tend not to do that - they
pull more power from the load being tested. (A tradesmen is unlikely to
be carrying around a super expensive high-impedance laboratory analogue
meter, were as high impedance digital meters are common).
I have a recollection that at least one test manufacture makes DVMs that
have a lower than normal impedance (for a DVM) for exactly this sort of
use, ensuring circuits are well and truly dead before beginning work on
them.

Of course test lamps are a 'real load' and tell the story. But the test
lamps used by an electrician when working on 'domestic' mains won't last
very long if you tried to use them to see if the 3rd (or 4th) rail was
live. A bit too much juice!
Some one who regularly works on railway power systems may actually have
in their kit a 'test lamp' for 750v supplies.

Four regular 40w lamps wired in *series* should do the trick. The problem comes
when one of them blows :)