Engineers know they’ll get a poke if they touch household ac wires. But RF current typically is not felt as electric shock because the frequency is too high to depolarize nerve membranes. However, RF current can cause both internal organ damage and surface radiation burns, so care must be taken to avoid high exposure.
It’s well known that high levels of RF radiation can cause heating effects in tissue, which is why microwave ovens work. The effect is the result of an induced current flow. But cases of outright direct RF electrocutions are rare, so rare that researchers often write case studies when they encounter one. So it is interesting to review a few of the instances where unfortunate individuals fell victim to high RF.
In one case, A 53-year-old technician received two brief exposures to both hands of 2-A current at 196 MHz. This happened in the 1990s when he changed a switch (U-link) in a Phase Alternating Line (PAL) TV transmitter. He was removing the switch with both hands when a couple of blue flashes happened. He felt no shock or heating, and he wasn’t burned. He was shook up but able to walk away.
But his problems were just starting. He reported that the next day a wart on his hand split open but eventually healed. He also noticed a 1.5-in-diameter rash over his lower rib cage which lasted about day. He figured its position corresponded to the position of his elbows against his chest as he removed the switch. A few days later, he noticed aches and swelling on some of his hand joints which eventually affected his wrists and elbows. The pain worsened over the next few days to a point where it was painful to press buttons.
Then something interesting happened. He went in for an ultrasound of his wrists and hands. The result showed no abnormalities, but the ultrasound itself gave him some relief for a day or so.
Over the coming months, this unfortunate technician experienced headaches, dexterity issues, sensitivity to temperature changes, and more swelling problems. These woes had persisted for 20 months after the incident, at which point the medical report was written. Notably, his blood work was normal, and tests of his major nerve functions were normal as well.
Researchers made a few assessments that shed light on what might be behind the technician’s symptoms. They noted that his forearms, with both hands clenched on the switch, would be close to 40 cm long, which corresponds to about a quarter wavelength at 196 MHz (1.5 m) in free space. This would lead to maximum voltage near the spot on his chest where he positioned his elbows. It was this voltage that probably caused the red rash he noticed.
To estimate the current that would have passed through the technician’s hands, researchers noted the switch had a potential of about 700 V (196 MHz). They figured 100 V of this was lost in arcing and another 100 V was lost in capacitance. This meant his arms saw 500 V. Researchers measured the technician’s resistance ‘in situ’ using a network analyzer with low voltages applied and found it to be about 220 Ω. By Ohms law (1 = V/R) the current flow around his arms and chest would have been about 2 A. (The power along his arms was approximately 1.1 kW.)
Researchers noted that at 196 MHz current flow is mainly over the surface of a conductor because of the skin effect, and the likelihood of burn marks depends on the amount of surface area touching the conductor. (Surface-effect current flow probably also explains why the technician’s heart was unaffected.) In this case both the technician’s hands presented a large surface over the metal surfaces of the switch which explains why there were minimal burns.
Most of the studies examining RF shock have pertained to frequencies much lower than 196 MHz. Conceptually, this makes sense; current flow at lower frequencies would be much less likely to propagate via the skin or surface effect. There were several studies in the 1980s, for example, pertaining to RF currents induced in large metal structures of ships due to the proximity of the ship’s communication antennas.
In one such study, researchers determined current levels that yielded a barely perceptible sensation (called a perception current) and that caused discomfort (a let-go or hazard current) for frequencies in the MF (0.3-3 MHz) and HF (3-30 MHz) bands. As in the case of the CATV technician, one lesson learned seemed to be that a larger skin contact area lessened the likelihood of a shock sensation. Researchers said the perception current and let-go current for the tip of the forefinger were both about twice that for contact with the back of the forefinger, and were even higher for large-area contact with the palm. A test of 50 people (with the back of the forefinger) showed a mean hazard threshold current of about 200 mA for the 2-20 MHz band.
In another 1980s-vintage test, researchers measured threshold currents for perception and pain among about 350 people for the 10 kHz to 3 MHz range. One interesting finding: The measured impedance of female subjects was a lot higher than that for males. At 10 kHz, for example, the mean values for women and men were respectively about 630 and 520 Ω. The difference in mean phase was not significant.
Reading between the lines, you can guess why the researchers became interested in impedance differences between males and females: The two sexes exhibited different levels of perception and let-go currents. For men, the mean threshold-perception currents for finger contact rose linearly with frequency from about 4 mA at 10 kHz to about 40 mA at 100 kHz and then leveled off up to 3 MHz. The curve for women was parallel to that for men, but about 25% lower. This result came out of having test subjects grab a live metal bar for the test. Researchers then tried the same experiment but had test subjects only touch their finger to the RF source. The curves of finger-contact threshold currents for pain vs frequency also rose roughly linearly to about 100 kHz but dropped slightly from 100 kHz to 3 MHz. At 10 kHz, the mean pain-threshold currents for men and women were respectively about 10 and 6.5 mA, but at 100 kHz they were both about 14.5 mA.
For frequencies below 100 kHz test subjects reported noticing a tingling or pricking sensation around where their finger or hand touched the live surface. For frequencies above 100 kHz the sensation was warmth or heat in the area touching the RF source. The transition point from tingling to warmth seemed to be around 70 kHz. Finally, researchers also noticed that for frequencies above 100 kHz and current levels at which test subjects felt warmth, pain typically came within 10-20 seconds, something not seen for frequencies below 100 kHz.
It’s clear that most of the interest in RF current effects on humans lies in the area below 200 MHz. Standards for limits on RF contact currents are pretty much written with that frequency range in mind. You’d be hard-pressed to find studies like those mentioned here conducted for higher frequencies, and the reason is easy to discern: The wavelengths involved are far shorter than the neurological frequency response of the body. So the main problem at higher wavelengths is that of tissue heating. This is a long way of saying, keep your hand out of your microwave oven when it is operating.