Speaker Level Signal Electrical Safety

Is there an electrician-home theater expert that can answer this
question?

Can you get a dangerous or fatal shock from the wire (if exposed, like
at the binding posts) from your amplifier to you speakers?

According to what I've read, shock danger depends on current through
your body, which depends on source voltage and you body's resistance.
The voltage you could have on a speaker level signal could be 28 V
from a 100W channel running an 8 ohm speaker. And the body resistance
with wet hands could be as low as 100 ohms (although much higher for
dry hands--in the area of 500,000 ohms). Thus, you could get as much
as 28 V / 100 ohm = .28 A current. But as little as .05 A can be
fatal and .1 A is nearly always. (I'd be most concerned about
children with slobbery hands playing with the shiny gold binding
posts.)

I have also read that current must be delivered for many milliseconds
to be dangerous, so the frequencies of alternation for sound signals
might mitigate the danger since most output is above the 300 to 1000
Hz range (corresponding to 3 to 1 milliseconds of voltage before
reversing with the time of peak voltage even less).

Well ... it does not seem to me that the binding post design would
have become so popular if it was dangerous, so what gives? Is a
dangerous shock possible, but unlikely? If you touch a binding post,
does the current find the path through the speaker less resistant?
What if you undo screw the binding posts, and then touch the wires?
Does the higher (relative to powerline) frequency signal to a speaker
in fact mitigate the danger? Is my information wrong?

It would seem to me that a dangerous shock would be possible but
unlikely with usual speaker level output from music or dialog. But I
would think there could be a significant hazard with a high volume,
low frequency test signal. ***** BUT I AM JUST GUESSING BASED ON THE
IDEAS I PRESENTED ABOVE ****

Can someone break it down clearly and authoritatively (that is,
educate me)? Any stories of acutal shocks?

I think the likelyhood of shock resulting in serious injury or death falls into the possible, but highly unlikely category.

This certainly is not a definitive and "scientific" explaination, but...

First, you need a fairly powerful amp to generate any serious voltage. Second, you have to touch the terminals when the amp is operating at high power output level. Third, the little "cookie-muncher" would need to be messing around with all the factor simultaneously combined plus have the wet, slobbery hands.

So, you would need to have your massive setup cranked very loud, while your young child was in close proximity fiddling around with it, just after pulling hands out of mouth.

You would have to touch both terminals for current to flow, as, it's not likely that the negative terminal is at ground.
A few milliamps travelling between your fingers won't kill you...across the heart, maybe.

You would have to touch both terminals for current to flow, as, it's not likely that the negative terminal is at ground.
A few milliamps travelling between your fingers won't kill you...across the heart, maybe.

If you touch one terminal with one hand, and one terminal with the other, and are wearing rubber-soled shoes, then the path would be across your chest, right?

I think the likelyhood of shock resulting in serious injury or death falls into the possible, but highly unlikely category.

This certainly is not a definitive and "scientific" explaination, but...

First, you need a fairly powerful amp to generate any serious voltage. Second, you have to touch the terminals when the amp is operating at high power output level. Third, the little "cookie-muncher" would need to be messing around with all the factor simultaneously combined plus have the wet, slobbery hands.

So, you would need to have your massive setup cranked very loud, while your young child was in close proximity fiddling around with it, just after pulling hands out of mouth.

I'm not so sure about your claim about voltage. Large voltages are easily created--even in static electricity. It's just large voltage with large current, that is, large power, that takes more effort. 100 W = 10 V * 10 A, plenty to be dangerous, according to what I wrote above. Another way to think about it, you can easily burn yourself off of a 60 W light bulb. The electrical energy converted to heat is dangerous by itself. I would measure, but I do not think my cheap-o multimeter works with AC other than powerline frequency.

Otherwise, this is basically the way that I am thinking. As long as the posts are screwed onto the wires, the path of least resistance will be the through the speaker, right? (Although, I'm not 100% sure of relationship between 8 ohm impedance (of speaker) and 100 ohm resistance (of body). Is impedance the same as resistance for calculating current?)

Here's is my theory about what has to happen to get hurt:

1) At least one post has to be unscrewed, and you must be touching bare speaker wire with one hand. (Is one post (side of speaker wire) considered "hot", and the other not, as in AC power?)
2) Other hand must be touching other post or other path to ground.
3) Hands must have low resistance (most likely due to moisture).
4) Audio signal must happen to have high enough voltage (likely).
5) Audio signal must have low enough frequency (not sure).

Experts? Shock stories?

Other hand must be touching other post or other path to ground.


If the amplifiers output is not referenced to ground (and most are not), then you could touch it all day long, with a ground wire attached to your head, and you wouldn't get a shock.

Frequency has nothing to do with it.

I'm not so sure about your claim about voltage. Large voltages are easily created--even in static electricity.

Sure, dangerous voltages lurk in any given household.

My point was that you have to be cranking the stereo to very high levels of volume to get significant levels of voltage on the output terminals.

In your example of a 100W amp, that's around 28 volts present at the terminals if considering an 8 ohm speaker. However, that 28 volts is there ONLY when the amp is putting out the full 100W. At "normal" playback levels, the voltage at the speaker terminals is only going to be a few volts, give or take.

I guess I have to wonder why a young child would be close enough to a system at that level of output to be messing around with speaker or amplifier terminals?

I'm not really sure what's generated this level of concern, but I really think there are much greater risks to ponder.

the amount of current flow is really quite small unless you have it turned up ridiculously loud with a ridiculously powerful amplifier. In all likelyhood, you'd need to lick the terminals with your tongue to feel anything.

For this to even be remotely possible you'd have to touch positive with one hand, negative with the other.

The problem with the equation you used is that you only used the Per inch ohms. You forgot that the shock has a very long way to travel.

I believe that the 100~1000ohm(depending on saturation) of the wet skin is only stated per inch. So you have to factor in how many inches the current must travel. You'll find that the amperage is very, very low.

When I was younger (and dumber) I used to DJ at a rec-center. One fine afternoon we had the PA system outside to play music and for announcements during a volleyball tournament.

During setup we were cranking some tunes, there was some issues with one of the speakers. I walked over in my bare feet and standing in slightly damp muddy ground grabbed one binding post on the speaker to make sure it was tight.

Felt like I grabbed an electrical fence! :eek:

So possible, yes, likely, no.

For electrical safety, anything under 50 volts is considered as a Class A hazard. A 100 wrms amp peaks at 40 volts, and has rails around 45 to 50 volts.

Class A devices do not need protection against exposed energized conductors.

You can be bit, but it is not considered lethal in the workplace.

A 300 WRMS puppy rails at about 75 to 80 volts, definitely worthy of respect, as it is class B, with more stringent safety requirements.

2 milliamp used to be considered lethal across the chest. Data was from the 50's, I believe they upgraded to 10 milliamps..what bugs me is, where did they get the data on lethality from...test subjects???? (like the data on how much force is required to pull an ear off the head...7 lbs...who tried that???)

Skin resistance is typically point to point, not by the inch. It can vary hugely. You can get jolted by a 12 volt battery if you really try...like disconnecting the positive terminal of a boat battery while outside the boat standing in the salt water..

The best rule it, don't touch it if you don't know how much voltage is present..

Cheers, John