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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: Bad neutral connections, lightning current paths posted by Jacques on July 8, 2006 at 10:45:35:
In one instance, my sister and in another neighbors have both experienced "ball lightening." Interestingly, the blue ionized cloud can pass right through walls and glass windows as though they weren't there. In the lab building where this particular telephone switch was installed, a direct straight shot to earth was not possible because two floors below there was an auditorium. So this lab did not meet Bell standards in that regard. Had the building sustained a direct lightening hit, it would have been one of countless pieces of lab gear vulnerable to damage. This was part of a network not associated with the normal utility network, only a stand alone company network with experimental testing of software as its purpose.Most of the transformers I install are internal to buildings where the disconnecting means to the secondary are within feet of the transformer itself. In fact, I ordinarily will provide both primary and secondary disconnect switches at the transformer to facillitate both ease of service and testing and a convenient disconnecting means in the event of an accident. The same is also true of substation transformers. (OSHA rules require at least two electricians to be present when working on live exposed 480 volt and above circuits.)
The high frequency nature of current flow in upset conditions is interesting. We used to run the wires gounding the neutral to building steel in EMT or rigid metal conduit to protect them from physical damage. But it became evident that the ferrous nature of these the alloys in these materials created a high impedence ground at high frequencies due to series inductance. To reduce the amount of rework required to correct existing installations, it became common practice to install a bonding jumper between the wire and the conduit at one end making the conduit a secure part of the ground circiut as well. New installations are physically protected usually by installing them in PVC pipe.
BTW, there are many folk superstitions about lightening in the US. Among my favorites when I lived in Maryland was the common belief that crab soup prepared during a lightening storm would turn sour.
"Is it a point of disagreement, or a misunderstanding somewhere. I note the following:"Article 250 of the NEC (National Electric Code) sets forth the general grounding requirements for electrical wiring in a structure."
Do you know if this was the result of a relatively recent change in the code? BTW, this question came up as a serious issue of contention in a large UPS installation I did in 1993 (1.5 MVA.) The equipment (someone else selected) had a 3 phase Y 480-277 input/ and a 3 phase Y 480-277 output. There were two modules each served by a 1000 amp bus duct and a maintenance bypass 2000 amp bus duct. The contractor wanted to ground the secondary of the UPS and said if he didn't his license would be at risk. The manufacturer said if he did, the UPS would trip out on retransfer to commercial power from battery backup. The consulting engineers said...follow the code and wouldn't commit to anything. Analyzing the circiut, I said it appeared to me that the code would not permit grounding of the neutral at the UPS because of the single ground provision (it would still be illegal even as you cited the code since the UPS secondary neutral was available to the user only after the disconnecting means.) The disagreement was settled by a conference call to the NEC chairman of the committee which had written article 250 who by conincidence was retired but still on the company payroll. His analysis was that it should not be grounded but his explanation was different saying that the UPS acted in effect as an autotransformer. Live and learn.
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- Re: Bad neutral connections, lightning current paths - Soundmind 13:00:15 07/08/06 (0)
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