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Radio Broadcast Technical Consulting and Sales
 
10032 Island Drive, Brainerd, MN  56401

 


Technical Tips from Mark W. Persons
Failed 3-Phase Electrical Power Panel


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.  

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.

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.


The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   Ham WMH

Questions?  Email Mark Persons:  teki@mwpersons.com        

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page last edited 02/22/2016