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SAFETY: these tips from expericed members could save your life!

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LAN_403

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Mon Feb 02 2015, 08:58AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

A DETAIL CONCERNING SAFETY WHEN USING LARGE ENERGY/PULSE CAPACITORS (AND OTHERS):

I have had a lifetime of experience with these beasts and I am SAFE around them, using every precaution I know. I will never forget one close call I had with not only a capacitor that I 'thought' was discharged, but it read zero voltage across its terminals! This is an example of only one type of situation that really worries me: as rare as they are, they can pop up and kill--even if the operator does everything correctly!! So, I must share with you to make you aware of one such situation in particular that, only by luck only, I survived.

I was working with a bank of -relatively- small pulse capacitors (50uF @ 5kV each). I had discharged the bank and separated each cap. To store them, I touched the terminals with a heavy discharging rod, then wrapped a thin shorting wire across the terminals for storage. Now, when the time came to work with these again, I removed the shorting wires. I was connecting a bank of ten in parallel, while doing this, I keep the 'first one in line' shorted--most would call this safety overkill. (since I work with the huge caps so I guess the beasts just scare me). I grabbed one, removed the shorting wire and proceeded to push on a copper shorting bar--since this was the first cap in line. As I pressed it on there was a huge bang and the copper bar flew off! I just figured somehow it didn't get discharged, so I measured the voltage at the terminals and it read zero--I figured the bang discharged it. As I again started to press the bar across
the terminals--BANG--again. This time the copper shorting bar --remained-- across the terminals. I could see it was touching both terminals, but now I wasn't taking any chances--I measured the terminals and got zero volts. To verify that the voltmeter was working, I measured voltage on another component and it worked fine. I safely removed the bar from this capacitor, re-measured the voltage and continued to get 0 volt readings. I wrapped the shorting wire around the terminals (a new wire which was made of shiny copper and tested properly for continuity) and stored it.

The next day, still spooked by this capacitor, I checked the terminal voltage with the shorting wire in place, got zero v, so I measured the continuity of the shorted terminals--got a short circuit reading. I thought what could be wrong now? Again after removing the shorting wire and while pushing the copper bar across the terminals, a --violent-- short occurred! NONE of the other capacitors acted abnormally like this one. I eventually determined that this capacitor was at least partially shorting when pressure was applied to the terminal(s)--this usually indicated a bad or 'loose' connection. This was indeed the case. This capacitor had been used in a several high current pulse discharges, and I concluded that this violent discharging must have resulted in a internal 'hairline' break that was being 'reconnected' by simply applying a downward force to the terminals. Everything fit this scenario. By now the capacitor was actually fully
discharged. I could NOT recharge it by touching the terminals with the charger poles--I actually had to apply a downward force (connecting that internal break) on the terminals to charge (or discharge) it. I later determined that it was the 'negative' lead that was defective

Even though rare, I will NEVER assume this defect is not present, especially when dealing with huge energy storage capacitors. I learned (for educational purposes) how to charge and discharge this defective capacitor--and with no significant downward pressure applied to the defective terminal, the capacitor would read zero voltage whether actually charged to 5000V or zero V. It consistently read zero capacitance (microfarads) on the meter.

So how do you ensure against this? Well, after some thought, I came up with a very simple procedure: After discharging and verifying zero voltage across the capacitor, THEN test it for the proper capacitance with a capacitance meter! Touch the terminals very lightly for both the voltage and capacitance readings. That's it--CHECK FOR CAPACITANCE!

Now, before shrinking a coin, I always do a CAPACITANCE reading, after removing the shorting conductor, on the capacitor(s) I am about to use.

hen918

Mon Feb 02 2015, 09:10PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Thanks for sharing! I have two of those 50uf 5kv pulse caps in my coin shrinker/can crusher. (thank you Dr Slack!) lesson learnt.
Another tip: no capacitor will ever be fully discharged. You will always get some voltage left even after a dead short.

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