# Why do high voltage transmission line workers need a Faraday cage suit?

In this video the high voltage transmission line workers are wearing a Faraday cage suit. Why is this needed?

Without the Faraday cage, the resistance of the human would be very high compared to the transmission line, so practically no current would flow through the worker (i.e. the reason why birds don't get zapped).

With the cage, some current would probably flow through it, but still nothing should go through the worker. The only purpose of the cage that I could think of is that when the man touches (or is near, rather) something (e.g. a tree, or the helicopter) at a different potential than him, the current would go through the cage instead of him. Is this a reason and is it the only one?

A clearer question: why a Faraday suit and not an insulating suit?

• Show me a high voltage technician who can fly and we can think about doing away with the protective gear. Until our friends have both feet on the "ground", they will need to put on something a little bit more "metal". As for high impedance: the human body is pretty low impedance, with the only high impedance piece being a sliver of dry skin. Once that's punctured (which doesn't take more than a couple dozen volts), we are all the electrical equivalent of a large bag of saline solution. :-) – CuriousOne Jun 26 '15 at 19:54
• @CuriousOne So you agree that the Faraday cage suit is only needed to get off and on the helicopter? – eimrek Jun 26 '15 at 20:43
• You can't compare the resistance of a human to the transmission line to draw the conclusion that practically no current will flow through a person. What matters are the potential difference in free space surrounding the wire in the space the person occupies. In principle, you can be killed by currents in your body simply by being near to a powerline, without any current flowing from the powerline to your body. – Count Iblis Jun 26 '15 at 21:24
• If you could do it without a safety harness and wire and neglecting the effects that Count Iblis mentioned? Of course. Let me know when you are ready to perform that circus act and I will show up for the show. – CuriousOne Jun 26 '15 at 23:30

My brother-in-law faced this EXACT problem, as he worked on high tension lines. There is a corona discharge from these lines due to the very high voltages involved. From experience, the linemen learned that this corona discharge is injurious to internal organs. To prevent injury, the linemen wear the Faraday cage suit, because such a suit keeps the electric charge on the outside of it (the charge on conductors resides on the outside surface), which prevents any corona discharge effects from entering their body.

High voltage lines come in triplets for a different reason than mentioned below. Electric generating companies generate 3-phase power, as this is supposed to be more efficient to generate, and I expect that such a power generation practice results in more acceptable mechanical loads on the generators. Each phase carries AC current, and each phase is 120 degrees out of phase with the other two lines in the triplet. This means that even though each line in the triplet may carry 100,000 volts rms current, it is easily possible to short one line against the other if two of these lines touch each other or are connected by a conductor.

• Coming in a bit late, but: the reason for using alternating current rather than DC is that it's easy to use relatively inexpensive, easy to make, no-moving-parts devices, namely transformers, to step up the voltage. If you increase the voltage while keeping energy flux the same, the current is lower. Lower current means low resistance losses. Now that solid-state DC-DC voltage converters exist, high-voltage DC lines are coming into common use. (3 phases rather than 2 or, say, 11 is a different question.) – CarlF Nov 21 at 12:08

The electric field around high voltage transmission lines (or "high tension" lines) is extremely high, and can be close to the breakdown threshold of air. That's why the highest voltage lines use multiple (often three) parallel conductors, to increase the effective conductor radius and reduce the peak electric field.

Now, introduce a human into that field, and you're going to get conduction. The current won't be high, because the capacitance of an isolated body isn't large, but think of getting a spark off of a door knob, but on a continuous basis, and at a probably higher voltage. At the least it wouldn't be fun, and at the most it would be painful, possibly disabling, and even harmful.

So, comfort and safety dictate that they wear conductive suits.

• youtube.com/watch?v=x94BH9TUiHM Scroll to 2:40. Yeah, kind of like touching a doorknob, only more so. – Solomon Slow Jun 28 '15 at 3:18
• The entire helicopter provides a much higher capacitance, and so much larger currents (and sparks), but yeah: that's a very graphic demonstration. (Way cool video, BTW. Those helicopter pilots are nuts.) – Daniel Griscom Jun 28 '15 at 11:35
• "The electric field around high voltage transmission lines is extremely high" why? electric field is high when the potential steeply, the lines are at maybe 100kV, there is no grounded object near to result in a high potential gradient. – eimrek Jun 30 '15 at 22:03
• "High voltage" lines start at 110kV and head up past 700kV. And, if you have a single wire at high voltage, then the gradient is extremely high next to the wire. Check out the "Corona discharge" Wikipedia article. – Daniel Griscom Jun 30 '15 at 23:14