Electric Shock
Electric shock is something to be avoided. Navy training gives an example of one person being shocked to death with a Simpson 260 multimeter after jabbing the probes through the skin. He was trying to measure his internal resistance. Under the right conditions a current as small as 21 mA for 30 seconds can kill. A 120 volt shock through the average resistance of the body will typically draw a current of 80mA.
I have been shocked several times. Mostly when I was a youngster and conducting my own study of basic electricity down in the damp basement of the house at Lake Lansing. Most techs have a brush with death, and if they survive that they become much more careful going about their work.
One time, replacing a fan in the back of a 19 inch rack at NAVCOMSTA Stockton, I noticed sparks as I touched my soldering iron to the wires to remove them. Disconnecting the 120 volts to the fan made the job much safer and easier. It was late, I was tired. But that is no excuse. ET’s work in pairs for safety, for some reason I was alone. Probably taking a shortcut, can’t wait for a shadow, gotta fix the gear right now. Slow down, do not take short cuts, ask questions if in doubt.
At the shipyard calibration lab in Hawaii a positive evaluation comment was that “he does not hesitate to ask good questions”. Imagine the damage caused by someone who does not know, does not ask, and continues on anyway.
I remember removing a large cover over a circuit breaker panel at Stockton. The panel was heavier than I expected and it dropped and cut a live wire going to one of the breakers. Again, I should have had a second person there. Should have checked with my supervisor for advice. Should have been more careful.
When working on a circuit where you can secure the power you should secure the power and lock the breaker in the off position with a padlock, the key to which stays in your pocket. Even so, every time you start to work you need to check again to be sure the power is still off. First thing in the morning, after your break, after lunch, check it again.
While at Stockton a contractor was killed drilling into a live conductor while installing some new equipment. After that they decided to secure power first and do the work.
To determine if the power is off (or still off) it is necessary to ”frisk” the circuit. It is best to use an old meter such as the Simpson 260 to do this test. Modern digital meters have such high input impedance that they will give a false positive result. It may indicate a small voltage even though the power is off. Using an old analog meter will give a true indication.
First test the meter on a plug or other circuit known to be live. Then check the circuit that is supposed to be off. Finally check the meter again on a known live circuit to make sure you still have a working meter.
The worst situation is that which causes the current to pass through the center of the body. Hand to hand by way of the heart. For this reason when troubleshooting on a live circuit develop the habit of only using one hand. Keep the other in your pocket. Also wearing special electrical safety boots will insulate your feet from the ground. In high voltage situations additional precautions are applicable such as rubber gloves, leather gloves over those, a insulating safety mat and a safety observer, maybe with a rope around you ready to pull you out. Dry rope or a wooden cane can be used to pull a person being shocked to safety. Do not touch them with bare hands or you will also be a victim in need of rescue.
At Verdin school in Virginia we were working on a CRT monitor for the MOD-40. The MOD-40 is a sort of an electronic teletype that looked more like a computer. A CRT monitor contains a high voltage transformer aptly named the “flyback”. Because when you touch it, it makes you fly back across the room. The instructor took a shock trying to put the cover back on. Freaked him out, I had to finish it. He refused to touch it again. But all I did was follow the one hand rule and no problem. Well, that and he probably bleed off most of the voltage for me. Ouch.
ESD damage
When replacing modern circuit boards and components wear a grounding strap and also ground the gear chassis. Work on an ESD safe surface. Store parts and circuit boards in ESD safe bags and packing materials.
Radioactivity
Some electron tubes contain radioactive materials. They are safe unless you break them. Best to keep a radiation survey meter handy.
Also some smoke detectors have radioactive components.
Heavy metals and toxins
If you solder everyday, be careful about ingesting lead. Wash your hands every time you get up from the bench. Do not eat (or smoke) where you solder. Use a fume extractor with a HEPA filter to safely remove the smoke and fumes.
Mercury is found in some old switches, batteries and even modern florescent tubes. Which reminds me of a story:
On a schematic some lamps and indicators are labeled with the component symbol DS as in DS1, DS2… Since most schematic symbols have a symbol that makes sense (such as R for resistor) I wondered about this and inquired of the Chief why the lamps are called DS and not something more sensible. Chief looked up, as if to remember, and then he called my attention to the florescent lamps in the overhead.
“See how the ends are beginning to darken?” he asked.
“Yes, Chief, I can see that clearly”.
“Well, he said, most people think these lamps emit light”.
“Uh huh, I nodded”
“The truth is that they actually work by sucking all the dark out of the room. And as they age they start to fill up with dark, so you see the ends starting to darken. And that is why the schematic symbol is DS”
“Why Chief?”
“Dark sucker, it stands for dark sucker”.
Anyway mercury is bad. Can be absorbed through the skin. Spilled mercury can evaporate and make everyone on the boat crazy. Submarines are “mercury exclusion zones”. The batteries that go in the “Leeds and Northrup” temperature measuring bridge had to have a special green label. The label had to be signed, signed by someone to indicate it was ok to come on the boat as an exception to the mercury exclusion zone. How did we ever get anything done?
Noise: Sid in the Pearl Harbor calibration lab has probably forgotten more than I ever knew. He knows everything about the old HP gear. He fixes TDR’s! He always wears ear plugs at work because of all the white noise created by the cooling fans of all that gear. The noise does not seem loud because it is constant but it will damage your hearing over time.