Some Thoughts on Mains Safety

Thread Starter

Yaakov

Joined Jan 27, 2019
1,611
If you’ve been working with mains power for any length of time, you’ve almost certainly gotten a shock or two. It probably scared more than hurt, and it also probably sobered you up concerning care handling wires at mains potential.

In the US, our 110-130V mains isn’t as scary as the UK and other places with about twice that potential, and I have even known electricians who routinely tested for hot circuits by touch, which they feel safe in doing because they hadn’t yet encountered conditions that would lead to a very unhappy outcome from that really foolish practice.

US, UK, or wherever, it is important to be very clear: mains voltages have the potential to kill you. (No pun intended but if you like it then, sure, I intended it.) This is a really important idea because there will be only a small number of cases where you could be killed or seriously injured by the mains but they really do exist. Even if 999 times out of a thousand no extreme adverse outcomes will happen, it’s the one time that matters.

Seatbelts are a good analogy. It is very likely you will drive for your entire life and never need seatbelts to save you. But, when they are needed, they very well may be the only thing that stands between you and dying. So, when you calculate the risk-benefit of wearing seatbelts you have the inconvenience and improbability on one side and death on the other. Death wins, the outcome is too negative to put the cost against.

Similarly, doing what is necessary to protect against electrocution from the mains may be an inconvenience, and you may never actually need it, and, you may have experience of not dying many, many times even when getting shocked. But the fact that you could die from it needs to remain in your mind, and you really need to do what is needed to reduce that risk to near zero, which, in the end actually costs very little.

There are two reasons that people ignore mains safety issues: ignorance and complacency. The ignorance may be from a lack of experience, as in a new person who has never been zapped, or it may be because someone has actually been shocked so they think they survived the worst.

Then we get complacent as our safety measures work, and we don’t get even shocked—until that screwdriver shorts out an exposed terminal block with a big flash, and we see the burns and pits in it, and then we renew our vigor.

But we really need to build into our thinking, “this is dangerous stuff, and must be handled safely”, and inculcate it into those we have the privilege to teach. Electrical safety, chemical safety, firearms safety, shop safety—when things have the power to maim or kill us, we need to respect that, and “build a fence” of safe practices around them. Violating our rules only when it can’t be avoided and we’ve got alternative procedures for those cases.

We should never knowingly allow an ignorant person to set up a situation which is dangerous to them or others, and we should always keep our attention on safety first—scaled to the danger of what we are working with.

This is just my view, and certainly some will disagree, but I’ve thought about it a lot and I believe it is, for me, an ethical imperative.
 

nsaspook

Joined Aug 27, 2009
6,607
Even if the voltage shock doesn't directly kill or injure, the human bodies muscular response to the electrical energy might harm you. I fell off a ladder once after a pretty mild shock. For just a split second I could fly.

One of my equipment training videos for higher voltage electrical danger. 90KV @ 100mA power source. Corona ring incorrectly assembled.
 

Thread Starter

Yaakov

Joined Jan 27, 2019
1,611
Even if the voltage shock doesn't directly kill or injure, the human bodies muscular response to the electrical energy might harm you. I fell off a ladder once after a pretty mild shock. For just a split second I could fly.

One of my equipment training videos for higher voltage electrical danger. 90KV @ 100mA power source. Corona ring incorrectly assembled.
The sound from that scary video reminded me of this one, which is an old internet goody: https://kovaya.com/movies/arc.mpg
 

MaxHeadRoom

Joined Jul 18, 2013
19,442
Early in my Electrical career in the UK I felt the wrath of 240v grabbing two conductors I thought I had pulled the fuse on, I believe the unconscious automatic spasm reaction of my hands being flung off save the day.
Incidentally the majority of electrical injuries are due to arc flash over not shock, per-se.
Max.
 

Thread Starter

Yaakov

Joined Jan 27, 2019
1,611
Early in my Electrical career in the UK I felt the wrath of 240v grabbing two conductors I thought I had pulled the fuse on, I believe the unconscious automatic spasm reaction of my hands being flung off save the day.
Incidentally the majority of electrical injuries are due to arc flash over not shock, per-se.
Max.
Yes, flashover is a really terrible industrial hazard but most people will never encounter potentials high enough to be at risk for that.

You remind me, though, of walking into the electrical vault at the Boston Museum of Science where the 7.5KV feeder terminated to serve the building. Even with all the panels close, it felt scary to be near that power.
 

MaxHeadRoom

Joined Jul 18, 2013
19,442
The highest I have actually worked on in the distribution sense is 4kv.
Requires a 8ft extension from the disconnect and rubber gloves!.
Max.
 

SamR

Joined Mar 19, 2019
1,505
I've witnessed the 3Ph 15kV lines ~100' from my house grounded several times (heavily treed area). It's a sight and sound not forgotten. Standard protocol for energizing substation and motor control center feeds was to close the breaker compartment door and stand to the side in case of a failure. Had a contractor install a 3Ph 480V 1500mcm feeder and he crossed the legs (bolted dead short) luckily the switchgear didn't blow when it immediately tripped upon energizing.
 

nsaspook

Joined Aug 27, 2009
6,607
I've witnessed the 3Ph 15kV lines ~100' from my house grounded several times (heavily treed area). It's a sight and sound not forgotten. Standard protocol for energizing substation and motor control center feeds was to close the breaker compartment door and stand to the side in case of a failure. Had a contractor install a 3Ph 480V 1500mcm feeder and he crossed the legs (bolted dead short) luckily the switchgear didn't blow when it immediately tripped upon energizing.
Power lines and trees? Who wins? Power lines double tap.

 
My worst shock was when GFCI's didn't really exist and I was in high school and no one else was home. I was doing body work on my car using a sander. I was barefoot and had no shirt on. My side had contact to the ground. The extension cord I was using was flat and apparently the insulation damaged.

Well, the cord got caught between my toes. I really could not let go. I voluntarily screamed. This, I figured, might misdirect the effect of the 120 V similarly as the larger pain usually takes precedence. It worked. I was able to get my toes off the cord.

The largest voltage I worked on was capable of 100 kV @ 0.1 A. The power supply for X-ray diffraction.

The scariest was 15 kV at 1.5 Amp regulated with a tube/transistor regulator. The input to this e-beam power supply was 208V 3 phase 50 A.
Another system used 208 V, 3 phase, 90 A.

Then there was a 1000W RF transmitter (ISM band 13.56 MHz. The plate voltage was somewhere about 3 kV.

There was a machine at work that had a 6.3 Vac 2000 Amp power supply. I didn't have to work on it.

In high school, I built a 3kV power supply for a science experiment that I got an Honorable Mention in a regional Science fair. I also built an oscillator to drive an ignition coil and got got very long blue sparks on the order of 8" or more to a screwdriver.

What I found useful for ANY type of work (automotive, plumbing, electrical, electronics) is to basically perform a "Job Hazzard Analysis".
It's really nothing more than identifying the hazards and determining the right PE (Protective equipment needed). It may mean getting a hardcopy of the MSDS (Material Safety Data Sheet).

I got this stuff for fixing cracks in concrete that is excellent. The manufacture will not sell to an individual. It's like a 2-part polyurethane mix added to sand. The mix is applied with a dual-cartridge caulking gun and mixing nozzle. It's got a 10 min work time and the cleanup solvent is MEK.
Both are very yucky stuff. I messed up a little bit because I wasn't aware that you could by needle tips (16 AWG) to apply the stuff. It's thin like water.

On that extreme, I was trained to use concentrated HF, Hydrofloric Acid., where a drip on your skin could mean death.

I left the MEK, bought at a local store, in the front of the car on a hot day while I went into another store for a short time. The vapors took out the electronic electrochoic mirror (rear view mirror) in the car. So, the stuff is like Helium...leaks out of a "sealed" container. The lid was tight.

Don't get a junk yard mirror - get it re-built instead.
 

djsfantasi

Joined Apr 11, 2010
5,841
I have two stories I was working on an old house and had hold of a steam line for balance. An old metal sheathed line, which had line voltage on the shearing, came in contact with my arm. I couldn’t scream nor could I move my arm. At first. I finally broke free. Scared the crap out of me.

Secondly, I had built a house on land in NH. The previous owner had let a row of pines grow on the electric right of way for privacy. During winter, under the weight of snow and ice, the tree branches came in contact with the power lines. Several fires ensued. Two years of negotiating with the power company and two years of broken promises, I contacted the state Dept. of Public Health and Safety. The trees were removed in two weeks.
 

Thread Starter

Yaakov

Joined Jan 27, 2019
1,611
In my youth, I worked in a French bakery doing maintenance on everything. The ovens were very large, With 6-foot tall racks that went inside. Steam was a key part of the process, and "evacuation" was the step when the backer would open the oven door and let the steam escape. Production could not be stopped because it was fresh bread delivered to high end hotels and restaurants every morning, any delay was unacceptable, any repair had to be done during production if that's when something broke.

We had a typical vent fan on the tall ceiling, one that looks like a chef's hat from outside, you've probably seen them. The steam wreaked havoc on the bearings, and it needed constant attention. It seized up and I needed to free it. The tallest ladder I had was 15 feet, if I stood on the top step, I could get inside the vent and work on the bearing over my head. This was precarious, but not too bad since I had the walls of the vent around me.

I was working on it, and had managed to get it rotating when the baker, ignoring that I was on the ladder behind him, performed evacuation. The cloud of steam and 450°F air rose into the vent, and was sucked out by the fan, with me in the middle. The sudden rise in temperature made me gasp, a mistake. I breathed in steam. I remember working very hard to maintain my composure and balance. I climbed slowly back down the ladder and sat against the wall, having breathing trouble for about five minutes. No one noticed or cared. It was surreal.
 
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