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Radio World Article

September 8, 2010

Radio World Magazine


Overcome the Tyranny of Power

by Mark Persons


It seems that every time we turn around there’s another power problem to solve. We may be radio broadcast engineers but we need power to keep a station on the air.

Fig. 1 is from the inside of a broadcast transmitter where 240 VAC power comes in.  A loose connection on the power connector block caused wires to burn off. Wires have been pulled back from the top center terminal of the block to show the debris left from the incident. Insulation burned off the overheated wires for about two inches from the block. The connector will need to be replaced after a temporary repair is made.
Left: Fig. 1, Right: Fig. 2

The photo shows a Harris HT-5 FM but this kind of problem can and does happen to all transmitters where high-current conductors are terminated. Best to check tightness of these kinds of connections whenever practical.

Fig. 2 shows a similar incident in a Continental 816R 20 kW FM transmitter. A heavy 208 VAC wire on the primary of the high-voltage power transformer is burned off.

I have seen this twice over the years. The original crimped terminal gets warm and then hot as resistance increases. The wire often needs to be cut back five inches or more to find clean copper. A field replacement can be a screw terminal as shown in Fig. 3.

Fig. 4 shows a 120 VAC plug that was providing power to three equipment racks in the engineering area of a radio station. The cable-to-plug wiring developed higher-than-normal resistance resulting in overheating. The insulation on one conductor turned brown from the heating. As you can see, the plastic on the plug suffered as well.

Left: Fig. 3, Right: Fig. 4

The problem was caught on a routine inspection when it was noticed that the plug was running hot. This plug would have gone into total failure after not too much longer. The wall outlet didn’t look very good either.

Fig. 5 is the top/input end of a three-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 also were 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.
Fig. 5

The panel was a Square D model D324N. It had been in service for just nine years. 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 percent of the capacity of the panel. In this case, 120 amperes is just 60 percent of 200 amperes. That should be plenty of safety margin. I suspect that Square D had a problem with this panel design. The replacement panel was made by Siemens.

Fig. 6 is the left input lug of that same panel after it was gently lifted out. The 3-0 wire connected to it had overheated and its insulation was largely fried off. Not only did the fused disconnect need to be replaced, but the wire leading to it needed to be replaced.

There are inexpensive infrared thermometers that can spot overheating problems before they become big. Fig. 7 shows a Sperry IRT100 temperature sensor, with laser pointer. It was less than $50 from the local Home Depot store. The moral of the story is that preventive maintenance can save you problems later.

Left: Fig. 6, Right: Fig. 7

Mark Persons, WØMH, is certified by the Society of Broadcast Engineers as a Professional Broadcast Engineer and has more than 30 years experience. Visit www.mwpersons.com

From the Radio World September 8, 2010 issue:  http://www.radioworld.com/article/106572

See you down the road. I'll leave the soldering iron on for you.

Questions?  Email Mark Persons:  teki@mwpersons.com     

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