# Electrical Safety

Discussion in 'Electronic Basics' started by phaeton, Aug 14, 2007.

1. ### phaetonGuest

Hello again from the dungeon...

As I start to get into building line-driven power supplies and such,
where should I start REALLY being careful? 40V? 60V? 110V? I realize
that current is what does the damage, but you don't have it without
voltage. (I also realize that other factors will factor).

What is the realistic neighborhood of where I shouldn't be grabbing
wires and/or switching out caps or resistors in a live circuit?

The physics and shop teacher in HS talked about 110 AC Line current
like it would knock you dead instantly. An electrician that was
installing a 208v-->480v step up transformer said "110 won't hurt you
at all. It's just enough to scare you a little bit".

I'm not about to strip a power cord and bite down on it, obviously,
but he's not the first to say 'I've been bit by 110 a bunch of times.
It's no big deal'..

Thoughts?

2. ### David L. JonesGuest

There should be no need to be doing that at all.
But if you want a ballpark - anything mains powered (except plugpacks
which are ok).
If you don't know the answers to these questions then you shouldn't be
playing with mains powered stuff at all. The mains can kill you.

Generally speaking, anything over 50V or so is potentially dangerous
and should be treated as such.

Dave.

3. ### DJ DelorieGuest

IIRC, the cutoff for "safe" is somewhere around 60v, which determined
the rail voltage for telephone switches or something.

I've gotten bitten by 120v before. It hurt, but caused no permanent
damage, not that I'd want to do it again. "Safe" is different than "I
got away with it" too - a little current in the wrong place is fatal,
especially if it has to cross your chest to get to ground.

As for lower voltages, I've destroyed ICs by inserting bypass caps
without discharging them first. Safe for me, not safe for the chips.

48 volts.

John

5. ### Don KlipsteinGuest

I think do some bit of research into body count or annual body count as
a function of voltage.

110-120 volts AC is "up-there", maybe 2nd place to industrial 460-480
volts AC or so. Body count of 110-120 volts AC exceeds that from 440-480
volt AC range on US Navy ships among those where both of these voltage
ranges are present.

I suspect that 110-120 volts AC at 50-60 Hz has a high body count due to
high carelessness due to survival rate of shocks from 110-120 volts AC
50-60 Hz being high, though short of 100%.

- Don Klipstein ()

6. ### Bob MastaGuest

Sorta like the drunk: "I've driven home from the bar while totally
sloshed a bunch of times. It's no big deal."

Or the guy who never wears his seat belt.

Or the cyclist or biker who never wears a helmet.

It only has to be a "big deal" one time.

But even at low voltages, there is a good reason
to switch things off: If you work on live equipment,
sooner or later you will slip and short something and
let the magic smoke out. (Ask me how I know this!) <g>

Best regards,

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator

7. ### ChuckGuest

I was surprised by some of the answers
given.

First off, disregard the notion that 120
volts can't kill you. The effect it has
on you depends on many factors, such as
whether the current passes through your
chest (i.e., heart), and how much
current passes through your body. The
latter depends, for example, on whether
you are an older tradesperson with
calloused, dry fingers.

But if you have soft, moist fingers and
contact high enough voltages (definitely
less than 120 volts), you may easily
pass enough current to kill. The point
here is not that you need to measure
your skin moisture, but rather that you
need to have a good understanding of
Ohm's law and why someone might escape
direct "contact" with lethal voltages.

Statistics show that even among
experienced industrial electricians, the
greater cause of injury and death is
the consequence of massive muscle
or being thown violently into some

I would try to learn a great deal more
about the dangers of working with high
voltages before attempting to do so. Do
a Google search on some of these issues
and consider soliciting opinions from
the deceased to balance those available
from the fortunate! ;-)

Chuck

8. ### NobodyGuest

Bear in mind that it isn't just a case of being directly harmed by the
electricity itself. Even 50V can give you enough of a "surprise" that you
reflexively pull your arm away and ram a soldering iron into your
face or send a tank of FeCl3/NaOH/etc flying.
I've been bit by 240 a bunch of times (in the UK, where 240V is 240V
above ground, not +/-120V). At least one of those was the "bad" case:
right hand on the live, left hand resting on the grounded chassis. Mind
you, that was when I was 16-17, young and healthy; the consequences might
be a bit more serious these days.

9. ### Anonymous.Guest

Here in Britland we have an old saying,

"It's volts that jolts
But mills that kills"

10. ### Dr. Leonard H. McCoyGuest

I've been bit by 240 a bunch of times (in the UK, where 240V is 240V
Ah, we do tend to forget that fact (it's not 240 a.g. here in the USA). But
touching 480 3-phase (what's that a.g.?) "got my attention".

11. ### Tim WescottGuest

And if it has high current capability it can have nasty interactions
with jewelry. By brother bears a scar on his wrist from a metal watch
band that got shorted (thanks to a wrench) between a truck battery and
ground. That was just 12 volts, but it stripped the skin off his wrist
underneath the segmented metal watch band.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

12. ### Doug MillerGuest

Ours are still 240V line-to-line, though -- grabbing one hot lead of a US 240V
circuit in each hand is every bit as dangerous as grabbing the hot lead of a
UK 240V circuit in one hand and the neutral in the other.

13. ### Michael A. TerrellGuest

In the US, it is:

There are some old electricians,
There are some bold electricians,
But there are no old, bold electricians.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

14. ### Sjouke BurryGuest

Just today i was bitten by 240v twice.
As long as you dont actually grab hold of a wire,
its fells and acts like a warning to to stay away
from the hot wires.
Any time I have to work on a hot circuit, I have
learned not to grab things, and use proper insulated tools.
And any time you let your attention slip a little bit,
you get a stinging lesson to re-train your reflexes.
It also sometimes causes nice sparks.
Oh, and make sure your footware is isolating you from ground.

15. ### JamieGuest

Here in the states, 50 Volts or more, you're in a whole new world.

Now.

Good Luck!
Rich

17. ### Rich GriseGuest

Well, if he can drive safely, it's not.
That's trivial: Look in front of the car. If there's something there,
don't go there.
Oh, feh. This is just too much. A helmet on a freaking BICYCLE?

You're supposed to learn how to ride the bike so that you don't fall

Cheers!
Rich

18. ### Don KlipsteinGuest

Since I do a lot of bike riding for delivering delivered restaurant
food, I have to ride bikes while being a restaurant worker. That is my
"day job" and I am an engineer as my side job since I have some need to
make my living from my brain only about 15 hours per week, so I have to
make my living from my feet quite a bit.
Restaurant workers and those juggling 2 lines of work due to brain load
issues should be expected to have "senior moments" at any age.

I expect that most who do major work or major commuting with a bicycle
are only a step or 2 better off - as in still having *some* vulnerability
to a crash where a helmet makes a difference in brain injury outcome.

For this matter, about a year and a half ago I did suffer a crash that
had me landing on my head. First time after maybe 1/4 million miles of
cycling. Prior to that I had a crash resulting from a minor brief brain
lapse where I failed to avoid a smallish known obstacle and went flying

- Don Klipstein ()

19. ### Bob MastaGuest

Then again, I guess there's no sense protecting anything that
isn't getting used anyway... <g>

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator

20. ### Al in DallasGuest

I work around 480V, but I've heard 12V can kill if there's enough
current available. I had to spend a whole day watching videos of
arc-flash accidents.

When I was a kid, I got my pinkie stuck on 120V. Since it was just a
trickle along one finger, it didn't hurt me, but DJ's advice is
important: "Safe" is different than "I got away with it" too - a
little current in the wrong place is fatal, especially if it has to
cross your chest to get to ground.