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Technical
Tips from Mark W. Persons |
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Here is
the top/input end of a 3-phase/200 Ampere fused disconnect switch used
at a transmitter site to provide 240 VAC power to a 25 KW FM
transmitter. The leftmost lug is missing because of overheating.
The left fuse was replaced temporarily because it failed from heat
fatigue. The center and right fuses are discolored and were also
discarded after the switch was replaced because it is good engineering
practice. The lesson here is that reliability is more important
than saving a buck. The panel was a Square D model D324N. It had been in service for just nine years. Just two years earlier, an identical panel was replaced on another transmitter at the same site. That is a bad track record considering less than 120 amperes of current were being drawn per leg. Electrical practices dictate that loads should be no more than 80% of the capacity of the panel. In this case, 120 amperes is just 60% of 200 Amperes. That should be plenty of margin. I suspect that Square D had a problem with this panel design. The replacement panel was made by Siemens. Click on the photo for a larger image. |
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Here is
the left input lug after it was gently lifted out of the fused
disconnect panel. The 3-0 wire connected to it has overheated and
its insulation has fried off. Not only did the fused disconnect
need to be replaced, but the wire leading to it needed to be replaced.
Click on the photo for a larger image. |
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There
are inexpensive infra-red thermometers that can spot overheating
problems before they become big. The photo
shows an inexpensive Sperry IRT100 temperature sensor, with laser
pointer. It was under $50, as I recall, from a Home Depot store. The
moral of the story is that preventive maintenance can save you problems
later.
Click on the photo for a larger image. |
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The stories go on and on.
Stop in again sometime. I'll leave the soldering iron on for you.
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Questions? Email Mark Persons: teki@mwpersons.com |
