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Electrical maintenance on a mountain top

From the expert

May 2013

By Chuck Newcombe

I first became aware of the relationship between voltage breakdown and altitude as a teenager working in my father's TV repair shop in the late 1950s. That was long before digital solid state television sets appeared, using LCD flat panel technology operating at low voltages.

For you younger folks, that was also when all electronic devices involved the use of something called the vacuum tube instead of a transistor. And the display was created in a really large vacuum tube, or picture tube, also called a cathode ray tube. Most of these circuits required voltages of 250 V or higher in normal operation.

A caged fly-back transformer in a TV set created the highest voltage, about 30,000 volts, needed to accelerate electrons to the screen of the picture tube. The problem was that TV sets were designed at sea level, and they worked perfectly at low altitudes, but we were operating them in a valley on the eastern slopes of the Sierra Nevada Mountains, at an altitude of about 4,500 ft. I quickly learned that the high voltage on the coils of the fly-back could arc to the shielding of the cage because of the reduced air pressure. If allowed to continue for long, the arcing would eventually cause the high voltage transformers to fail.

The solution to the problem was something called "corona dope", a thick red goo that we could paint on the transformer windings to add insulation sufficient to stop the arcing. In extreme cases, we even removed the shielding completely to increase the air space around the transformer, stopping the arcing, but allowing some electrical noise radiation into the room.

I learned about something called "corona wind" back then. It's the actual movement of charged air away from an electrode with high voltage dc present. If you carefully place your hand near such an electrode, you can feel the cooling effect of that breeze.

It wasn't too long before my inventive young mind hatched the idea for a corona wind loudspeaker. The idea was to build a speaker with no moving parts - the air moving away from a panel of high voltage electrodes could be modulated with high voltage audio to create sound. I built it from parts taken from several defunct TV sets, and it worked - kinda, but as I quickly found out, you didn't want to put your ear too close to the contraption.

This experience was brought to mind many years later, during my career at Fluke, when military specifications, and later international safety standards, required a listing of maximum operating altitudes for our meters. While voltages in the circuitry seldom exceeded 1,000 volts, the close spacing between traces on the circuit boards had to be great enough to ensure that breakdown or arcing would not occur during normal operation, or under transient voltage conditions.

A recent example, the Fluke 87V DMM, lists a maximum operating altitude of 2,000 meters, which is more than 6,500 ft. That's certainly good enough for the valley of my childhood and for such cities as Denver, CO, the mile high city.

But what can you do in Colorado's San Luis Valley south of Denver? The floor of the valley is at about 7,500 feet, and these days there are agricultural electrical systems in use there, operating at 480 V. Or, how about the electrically operated ski lifts at Vail, where elevations range up to 11,000 ft?

The Fluke 87V is rated at 1000 V CAT III, 600V CAT IV for the listed altitude maximum, but what happens if we can accept a lower rating, say 600 V CAT III?

It turns out that if you are operating in conditions that meet the lower rating, safe operation up to 5,000 meters (over 16,000 ft) is possible. This means that you could safely use the meter on a 480 V panel inside the telescope facilities on the top of Hawaii's Mauna Kea, at more than 13,000 ft. The caution here is to understand the differences between CAT III and CAT IV environments. Simply put, CAT III conditions exist inside a facility, downstream from the service panel, while CAT IV applies to utility lines outside or underground on the upstream side of the service panel.

There are several other instances in which a meter can be pushed to operate at extremes of environmental conditions. Among the best examples are those used by clever troops and contractors in the desert heat of Iraq. It seems that clear zip lock bags, trays of ice and other tricks have been used to combat the extremes of temperature which occur there under direct sunlight.

My early TV experience was the first indication I had that altitude could play a part in proper electrical performance. I'm sure that many of you have had similar experiences relating to extreme temperature, as in Iraq or Antarctica.

It might be worth it to review the general environmental specs of your meter to ensure the best available accuracy and safety for the measurements you make.