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#61
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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 |
#62
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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 |
#63
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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 |
#64
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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 |
#65
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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 |
#66
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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) |
#67
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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 ![]() |
#68
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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 ;-) |
#69
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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!!! |
#70
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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 ![]() |
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