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Is ac current more dangerous than dc?
November 26, 2007

From a physics viewpoint, the difference between ac current and dc current is its frequency. The frequency of a dc current is zero. The frequency of ac current is something higher. So, a better way to ask the question is: “Does the danger from electric current vary with frequency,” and the answer is “Yes.” This fact was driven home to me one day in the mid-1970s when I had a summer job repairing two-way radios for the City of Cambridge, MA police department.

My buddy at the other end of our work bench specialized in the legacy tube-type radios. He managed to get nailed by a high-voltage power supply every Monday morning.

He’d show up for work on Mondays groggy and mumbling and not yet awake. He’d sit there on his end of the bench like a zombie in a fog, poking metal tools around the innards of a live panel-mount tube radio while trying to figure out what was causing it to exhibit whatever evil behavior it was exhibiting.

About 10:00 am, I’d hear a loud “Bzzzt!” followed by the stool crashing over as my buddy flew backwards out the door. He would immediately reappear, shaking his head. Then he’d pick up the stool, smile and say: “Now I’m all set!” It took a jolt of several hundred Volts from a dc power supply to get him started for the week.

I, on the other hand, specialized on the brand-new whiz-bang transistorized hand-held units. No problem! The highest voltage anywhere in those things was 6 V. Touch 6 V dc with your finger, and you won’t even feel it.

Hah!

Poking around a live one of these things with my finger, I happened to press on the wrong little bit of solder and got walloped, myself. It didn’t have the nice, crackly “Bzzzt!” sound made by the high-voltage arc. Instead, it was like quietly being jabbed in the fingertip by a white-hot needle. I was so surprised that I did it again, just to make sure! (Hey, I was a budding test engineer. We TE’s calculate hazards differently than other folks.)

What I found hard to believe was that I could get zapped by that puny little 6 V signal. The voltage isn’t what’s important, it’s the current.

Electrically, a human body is equivalent to a plastic bag filled with salt water. The plastic bag represents your skin, which has a very high resistance. Think giga-Ohms!

The impedance presented by a composite object, such as a finger, depends on the frequency of the signal.

The rest of your body is pretty much low-resistivity salt water. If you put the probes of an Ohm meter on your finger, it measures the resistance of two layers of skin (one on the way in and one on the way out for the current) and the effective short circuit through your body fluids in between. You’ll get the same resistance (providing the Ohm meter registers anything at all) if you hold one probe in one hand and the other in the other. It still sees two layers of skin with an effective short circuit between.

Your skin is the only thing between you and any electric current that cares to look your way. A 6 V dc supply introduced under your skin can kill you.

Now, we crank up the frequency. To duplicate my little walkie-talkie zap, we have to crank up the frequency to VHF levels somewhere North of 100 MHz. At those frequencies, everything changes. Wires become RF-blocking inductors, and thin insulating sheets (like skin) become RF-passing capacitors.

At dc, your skin is a high resistance insulator. At VHF, your skin is a low-impedance capacitor. That 6 V VHF radio signal was much happier grounding itself through my skin capacitance (and the salt-water bag underneath) than trying to fight its way through the inductance presented by conductors on the circuit-board. Hence, I got zapped!

Nice story. What does it mean to practicing control engineers?

Well, control engineers deal with digital electronics, and modern digital electronics operate with signals at high clock speeds and sometimes with some oomph (spelled C-U-R-R-E-N-T) behind them. High clock speeds translate immediately into high frequencies.

Gigabit Ethernet (gig-E for short) { http://www.hardwaresecrets.com/article/231] running through CAT-5 [http://en.wikipedia.org/wiki/Category_5_cable] cable are not VHF signals at a few hundred megahertz. They’re UHF signals at about a thousand megahertz! To transmit a bit stream at 1 Gbps takes a fundamental signal frequency of 1 GHz—plus harmonics! That’ll go through your skin like it wasn’t there!

The take-home lesson is to give serious respect to digital signals running around inside your computers, controllers, PLCs, and drives. Some of those low-voltage ac signals can give you a bigger jolt than power lines carrying 120 V at a measly 60 Hz.

Posted by Charlie Masi on November 26, 2007 | Comments (0)