NOTES ON HIGH VOLTAGE SAFETY

CHTM holds safety to be a top priority, and is committed to operating in a manner that ensures the safety of our members and our community.

What is high voltage?
Any voltage greater than 40 volts is generally considered potentially (!) dangerous. The exact value that becomes dangerous depends upon many factors; it is, as most of us know, the current flow – and path taken through the body - which kills. The danger is compounded by the fact that our body resistance actually becomes less as the applied voltage increases. Therefore as voltages become more inherently hazardous as they increase, a simultaneous physiological phenomenon magnifies the danger further several times. Unfortunately, the commonly used AC power frequencies of 50/60 Hz. are also the most dangerous possible. Europe, which uses 220V AC domestic power, has a greater problem with electrocution than we do in the U.S. with 120 V mains (although we have a corresponding increase in fire hazard from I^2 *R heating).
In surgery, where electrodes are sometimes placed inside the body, as little a 500 uA. can kill and even a few volts are dangerous. We are mostly water and therefore fairly conductive internally.

High voltage safety is an attitude!
The highest fatality rate amongst motorcyclists is not immediately after issuing a new license. It is approximately two years after licensing, when the motorcyclist begins to feel confident in his skills and gets careless. It is very important when using high voltage devices to make a conscious effort to assume nothing regarding the safety of the device – adequate insulation, is power on/ off, etc. The problem is complicated by the fact that many devices such as power supplies and CRTs, which use D.C., can store voltage internally in capacitors, so that even with the unit unplugged from the wall a lethal hazard can still exist. Always short the equipment side of the high voltage cable to ground (safely – use deadsticks) after power down but before removing the cable. It is essential to disconnect the high voltage cable from the power source before working on circuitry, bearing in mind that high voltage can be stored in the high voltage cable itself if it is coaxial. The inner conductor, surrounding dielectric and (usually grounded) coaxial shield make an excellent capacitor and are in fact sometimes used for this purpose.
Never assume that insulation is intact, devices are actually grounded, etc.
I have personally observed the following:
1). Dielectric breakdown. An apparently insulated cable had pinhole punctures in the dielectric, which resulted in high voltage “jumping” to nearby grounded objects.
2). Stored coaxial cable which was actually charged to many K.V.
3). Supposedly grounded equipment that was actually “floating “.
4). Power supplies believed “off” but actually “on” due to power switch failure (mechanical switches are one of the most likely components to fail) and/or pilot lamp burnout.
5). Continuous high voltage arc from a 100KV (@ 1 amp!) power supply - there was a sharp point on the wiring - to a grounded object over a foot away!

Some general advice when working with high voltage:
1). Remove metal objects such as rings, jewelry, watches before working with high voltage – they make excellent electrodes! Keep one hand in a pocket or closed behind your back. Be sure the floor is dry (remember much equipment uses cooling water) and wear preferably rubber soled shoes. Always be aware of where your body is and avoid accidentally contacting grounded objects – a nearby refrigerator you might brush against for example.
2). Prove to yourself that all exposed metal surfaces are actually grounded. Verify the outlet ground using a simple neon tester device you can get at hardware stores. Verify continuity from the power cord to the equipment cabinet. (You may need to scrape some paint). Remember that proper grounding insures that metal connected to it can not be at high voltage and is therefore safe in this regard, but it can provide an excellent current path to ground through you if you touch it and are for some reason at a high potential. (Static shock is an example we are all familiar with.) Do not forget that your body has capacitance to ground and therefore a D.C. current can flow instantaneously to charge this capacitance even if you are well insulated; R.F. current (as can be found in plasma etchers) can flow continuously through your capacitance to ground.
3). Prove to yourself that there is no voltage present anywhere in the device you are working on. Never rely on just one switch to power down a high voltage supply. Turn the power switch off and disconnect the cord from the wall outlet, or trip the main circuit breaker to the equipment as well if it is hard wired. Be sure no one will inadvertently reconnect the power while you are working on the device.- follow lockout / tagout protocol. After disconnecting power, use “deadsticks” to short out high voltage leads and capacitors.
There are two styles of “deadsticks”. One type has a single electrode on an insulated wand with a connecting cable – the cable is to be connected to a known good ground. Water pipes can not be relied upon. When using this type of deadstick to discharge capacitors, be sure to discharge both sides of the capacitor to ground.
The other variety consists of two insulating wands with an electrode at the end of each. The electrodes are jumpered together so that a short can be applied between any two points. Be sure to touch each side of any capacitors to ground after you have discharged them by shorting their terminals together.
When working with power supplies of several hundred volts or higher, be especially aware of dielectric absorption. This is a phenomenon whereby an apparently fully discharged capacitor, previously shorted by deadsticks, can 'self-charge' upon removal of the short. The appropriate procedure is to apply a short to the storage capacitor while working on the system.
4). Never assume insulation integrity. As a rule of thumb, allow 1 mil of insulation for every 100 V to be isolated. This is just an approximation. Remember that it is important that there are no sharp points in the wiring. For a given voltage the E-field intensity increases with decreasing radius of curvature, so a given thickness of insulation which was previously adequate may no longer be safe if a repair is made and a sharp point inadvertently introduced from a cut wire or solder spike.
5). Never work with high voltage alone, and never leave a high voltage experiment unattended. If the length of the experiment requires it to be left unattended for some time, place a high voltage warning sign with a warning lamp near the experiment. Only place the warning sign and light out when the experiment is actually powered or people will come to disregard it.
6). Do not work on high voltage apparatus when you are tired and not alert even if it means a delay. The experiment will be delayed quite a bit longer if there is an accident.
7). Probably everyone here has had CPR training. Most people are surprised to learn that the current success rate with CPR is only about 5%! The implications are obvious – do everything possible to avoid needing it in the first place. Remember that you are the person who can best protect yourself when working with high voltage, or any dangerous equipment and materials.

Safety - your own and your colleagues' - is YOUR responsibility.