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High voltage capacitor safety and general safety by cloa
Posted: 24 Mar 11

As for going into the HV section- I will focus just on the high voltage, while the HV power might be secured, there still may be 220 VAC and or 24VDC control voltage present.

There are typically 2 items that heed to be secured in an HV section, the source of the HV, and any HV storage. There's 3 simple rules I follow when I secure the HV section so I can safely work on it.

1. Make sure the HV source is disabled. This is usually a transformer fed by 480 volts 3 phase.

2. Make sure the HV storage is discharged. This will typically be a large HV capacitor.

3. Make sure the HV storage capacitor stays discharged.

Very simple. Many people get 1 and 2 right but miss number 3.

What can go wrong? Lets look at number 1- You have to be absolutely sure that primary voltage WILL NOT make it to the HV transformer. I properly identify and shut off the breaker that feeds the HV transformer. You could lockout the machine but sometimes there is a need to go into the HV section with the control voltage active to do tests.

As for number 2- It's all about the grounding rod. Make sure it's mechanically and electrically sound. Look it over, give the wire a good yank to make sure it's not loose, then it hit with an ohmmeter to make sure it actually conducts to ground. The grounding rod is long for a reason, you hold the end that keeps your body parts as far away as possible from the HV danger. There is typically more than one grounding rod on a machine, ONLY the one that can reach the HV source and storage can make things safe, the other rods are good as a double check only. A common rule of thumb is to stay 10 mm for every 1,000 volts away from any live HV.

And number 3- Too many miss this one- Most of the capacitors are electrolytic. Just like the battery in your car, they WILL recover a charge after they've been run down. Typically there is a bank of resistors that will discharge this capacitor once the HV source is deactivated. If you hit that capacitor with the grounding rod and you get a discharge, those resistors are not working. The best thing to do here is to securely strap the output of the capacitor to ground. When these capacitors are shipped from the manufacturer, they have a wire connecting the output to ground for this very reason. Always assume a capacitor is charged.

I like to stay 4 or 5 steps away from getting hit by the high voltage. Sometimes when I'm rushing around I'll find myself a 1 or 2 steps from getting hit. When this happens I'll stop what I'm doing and reflect on how much I like living and make more of an effort to stay safe.

Now that I've shared what I feel is important, I'll share some of the other things I've come across over the years- Don't drink the electrolyte in the capacitors. Under certain conditions the ballast tubes can give off X-rays. Don't wrap HV cable around your neck. HV makes Ozone. Broken glass is sharp. Transformers are really heavy. Electrical parts can contain hazardous materials....... and so on and so on.....
If you decide to get out of bed in the morning, I in no way take any responsibility. I take none if you decide to stay in bed.......
- Paraphrased from Krugtech

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