|[AMRadio] Shorting sticks, another 2 cents worth|
jtml at losalamos.com
Sat Jun 3 00:53:14 EDT 2006
At work at the particle factory, we have some pretty hefty plate power supplies. 230 uF at 30 kVDC for one type and about the same uF at -88 kV for klystrons. We have some commercial (made by Ross Engineering) sticks and some homemade. The homemade sticks are phenolic rods about 3/4 diameter, 5 feet long, with a curved brass or steel hook on the end. Connected with a heavy crimped lug is a heavy cable like 'Monster cable' speaker wire, welding cable guage, that is vinyl covered so the internal conductor can be visually inspected. These are hanging on big switches so that they must be back in place before powering up. In addition, there are some visible bleeder dump switches and the door to the capacitor banks won't open unless the big switch is manually opened, which disconnects the rectifier output connections and puts a dead short across the bank input as well. All this is protected by a Kirk Key locking system, in which the door cannot be opened unless the switch is thrown first,
which cannot be done unless a small pilot circuit breaker is opened first, down by the control panels.
We don't use the resistor style sticks, except when there is a visibly damaged circuit like an open cap sitting there (about 3 kJ in each one). Then we use it to discharge first before using the solid hard ground sticks. The chances of having a loose or open resistor are high enough that it is safer to just run the risk of kaboom than depend on that resistor to always work.
For home I use smaller stick, about a foot long, in BC rigs.
In 1982, i was designing an FM broadcast TX in Quincy, IL, and I went to short the 20 uF oil cap on the lab model. The bleeder had burned open and I didn't know it. I neglected to look up at the voltmeter on the front panel. Boom. Scared the crap out of me, but it saved me from worse fate, that 'Jesus' stick. I have also heard them called Chicken sticks, although I don't think it is chicken to put them to use regularly. At work we would be terminated if we were not following the written procedure which gets annual training.
Here is an interesting fact about large oil filled caps (over 10-15 kV). They are usually made with series packs inside, each one is a much higher uF, but at 10 -12 kV or so. That is the normal build size. So what happens when a large oil filled cap, say a GE unit that is 2 feet tall, has a shorted pack inside? The stored energy goes up, as well as the capacitance. Reason is that the voltage per pack goes up, as one pack shorts out, and the total cap goes up since they are series connected. 1/2CV^2 remember? So when we inspect the caps every year or so, we measure the terminal value. If it starts climbing in incremental steps, then you know it is going to explode and fail soon. We try to remove them from service before they rupture during use (after 10-20 years of storing energy with pulsed discharge waveform).
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