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#91
April 16th 08, 09:43 AM
posted to uk.railway,uk.rec.subterranea,uk.transport.london
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Crossing London tube tracks
Incorrect, filling a fuse body with sand provides additional protection
against arcing in an overcurrent situation. Glad we got there in the end. The fuse failed to break the current. The mechanism by which it failed to break the current was that an arc formed, which is the way fuses which fail to break the current fail. We don't know why the fuse operated and failed to break the current. It may just have expired through old age, or there may have been a large current flowing for some reason. However, this does not change the fact that the fuse failed to break the current. If it had broken the current then there would have been no arc. Circuit breakers can fail to break the current in the same way. They can also fail to break the current because the contacts don't move or don't move far enough. That is why 'serious' circuit breakers have compressed air blown onto the gap to suppress arcing. Jeff |
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#92
April 16th 08, 10:03 AM
posted to uk.railway,uk.rec.subterranea,uk.transport.london
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Crossing London tube tracks
"David Hansen" wrote in message ... On Wed, 16 Apr 2008 08:54:23 +0200 someone who may be "Clive" wrote this:- Incorrect, filling a fuse body with sand provides additional protection against arcing in an overcurrent situation. Glad we got there in the end. The fuse failed to break the current. The mechanism by which it failed to break the current was that an arc formed, which is the way fuses which fail to break the current fail. Finally ! |
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#93
April 17th 08, 04:28 PM
posted to uk.railway,uk.rec.subterranea,uk.transport.london
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Crossing London tube tracks
On Wed, 16 Apr 2008 10:43:30 +0100, "Jeff" wrote:
Incorrect, filling a fuse body with sand provides additional protection against arcing in an overcurrent situation. Glad we got there in the end. The fuse failed to break the current. The mechanism by which it failed to break the current was that an arc formed, which is the way fuses which fail to break the current fail. We don't know why the fuse operated and failed to break the current. It may just have expired through old age, or there may have been a large current flowing for some reason. However, this does not change the fact that the fuse failed to break the current. If it had broken the current then there would have been no arc. Circuit breakers can fail to break the current in the same way. They can also fail to break the current because the contacts don't move or don't move far enough. That is why 'serious' circuit breakers have compressed air blown onto the gap to suppress arcing. They used to, most modern "serious" circuit breakers of a significant size use Sulphur Hexafluoride, or as it is more often called, ozone killer. -- |
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#94
April 17th 08, 04:52 PM
posted to uk.railway,uk.rec.subterranea,uk.transport.london
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Crossing London tube tracks
On Wed, 16 Apr 2008 10:43:30 +0100, "Jeff" wrote:
That is why 'serious' circuit breakers have compressed air blown onto the gap to suppress arcing. That beats the old system of a chap fanning the arc away with a cane brush supplied for that purpose Bruce |
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