Another power line incedent video



Hello, and I think it's more important in this scenario to have the neutral properly earthed at the service entrance. Any floating conductor hot with respect to earth ground (including metallic water pipes, etc) presents a shock hazard that exists both internal and external to the building. If a 7200 MV wire should contact a LV service entrance wire one hopes that fault current is either conducted to ground (accompanied by tripping of the MV fuse) via the grounded neutral (at the pole and/or service entrance) or a building internal LV distribution circuit breaker/fuse is blown (MV wire touches LV "hot" wire rather than neutral.)
The purpose for MV multi-grounded neutral distribution systems is not safety (other than lightning protection); it is to provide for rapid operation of over-current protection devices under "hot" conductor-to-earth fault conditions and facilitate timely fault location and repair. This helps stabilize system voltages by not having leaky current faults (conductors touching tree branches, etc) that can accumulate over a period of time. In a few locales (e.g. desert areas with few trees in western U.S.) where this is not an issue MV distribution is done sans neutral and the cost of stringing an additional conductor is avoided. In those systems MV-to-LV transformers are connected line-to-line rather than the usual line-to-neutral.
Of course intentionally earthing one side of a voltage source makes the other side "hot" and introduces an electrical hazard that must be dealt with. Sincerely,
John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337
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snipped-for-privacy@ipal.net writes:

This discussion of MV neutrals is interesting, but I'll mention that in this specific case, there is no MV neutral. This is an old system, all the transformer primaries are connected phase to phase (delta), the poles have crossarms with two conductors on top (3 conductors further upstream where 3 phase is available) with no neutral. The secondary neutrals of different distribution transformers are all isolated from each other. I have no idea how (or even if) the primary circuitry is referenced to earth ground.
I have a "souvineer" from this incident, a rectangular fuse holder/switch marked 7200V from the primary wiring.
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I found this one from a few days ago:
http://www.click2houston.com/news/13339896/detail.html?rss=hou&psp=news
Not as much to see.
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