21 replies to this topic

[heynow]

why do I not shocked when working with a live 24 volt system in our lifts?

[lastchair_44]

Because you're being safe?

[EagleAce]

because usually the current in such a system is low

[Emax]

heynow, on Jan 24 2008, 09:01 PM, said:

why do I not shocked when working with a live 24 volt system in our lifts?

It's a matter of conductivity... your own. You are an electrical conductor with a hand-to-hand resistance (dry hands) of about 1,000,000 Ohms (1meg). Depending on moisture, your resistance might be much lower - down to about 15,000 Ohms (15k) with sweaty hands.

1. At a current flow of 1 to 5 milliamps (thousanths of an Ampere), you nervous system will barely perceive the shock.
2. At a flow of 10 milliamps, most people's pain threshold is breached.
3. At 100 milliamps, muscular contraction will probably occur.
4. At 300 milliamps, electrocution is probable.

Let's apply Ohm's Law: E = IR or Voltage = Current (Amps) x Resistance (Ohms).

Solving for current (the factor that you feel... or even kills you), we get: I = E/R.
Substituting 15,000 Ohms for R, and using 24 volts for E, we get .0016 Amps - or 1.6 milliamp. Most people would feel nothing.

Now let's try it with 120 volts: 120 / 15,000 = .008 Amps - or 8 milliamps. It probably hurts (unless you're me).

One more time with 480 volts: 480 / 15,000 - .032 Amps - or 32 milliamps. Major ouch.

AC voltages hurt more than DC. 60 ma DC feels about the same as 9 ma AC

Body resistance is not a fixed quantity. It varies with many things like skin thickness and moisture content, body fat, electrolytes, etc. Compared to the rest of you, your skin is a fairly good insulator. Anyone who has been shocked through a small cut - or in my case via a sweaty armpit (armored cable - working on live 120v circuit) will attest to this.

If you have reason to think you're a particularly good conductor, you'd probably be better off working for the railroad than working on electrical equipment.

This post has been edited by Emax: 24 January 2008 - 09:11 PM

[Emax]

Back on top. I think that "heynow" deserves his answer.

[Lift Dinosaur]

Emax, on Jan 25 2008, 11:38 AM, said:

Back on top. I think that "heynow" deserves his answer.

Because you're not trying hard enough.

Seriously, great information, Emax.

Dino

[Allan]

I've been shocked lots by our 24v control systems (around 40 - 100 dc ma) It doesn't hurt, just surprises! :)

[Emax]

Allan, on Jan 25 2008, 07:23 PM, said:

I've been shocked lots by our 24v control systems (around 40 - 100 dc ma) It doesn't hurt, just surprises! :)

Perhaps - but don't delude yourself into thinking that you drew anything like 40 ma.

The installed circuit might be operating at that current, but the circuit through you was much, much less. See Ohm's Law again.

By the way - if you can feel a 24 vdc circuit, you're a pretty thin-skinned guy. I'll bet that testing a good 9v battery with your tongue really sets you off!

This post has been edited by Emax: 26 January 2008 - 07:19 AM

[lastchair_44]

We were slipping grips on a Yan triple two summers ago and the deropement switch on the guide sheave made contact with the haul rope. As I was inspecting the grip for cracks I leaned against the haul rope and my skin made contact with the rope through a hole in my shirt and I received a decent shock that kind of felt like a bee sting.

[Emax]

lastchair_44, on Jan 26 2008, 08:56 AM, said:

We were slipping grips on a Yan triple two summers ago and the deropement switch on the guide sheave made contact with the haul rope. As I was inspecting the grip for cracks I leaned against the haul rope and my skin made contact with the rope through a hole in my shirt and I received a decent shock that kind of felt like a bee sting.

That was probably wind charge. If you play your cards right, you can get a hell of a shock from a totally unconnected com-line. Just make sure that the cable is connected to nothing at each end of the lift and that the wind is blowing briskly - preferably on a very dry day. Now go to the middle of the lift and cut any one conductor with an uninsulated pair of cutters. Make sure your harness is tied off.

[lastchair_44]

Does the wind cause a build up of static electricity?

[Emax]

lastchair_44, on Jan 26 2008, 09:19 AM, said:

Does the wind cause a build up of static electricity?

Yes it does. As an example, the whirling blades of a helicopter in dry air accumulate enourmous charge that sometime causes mishaps.

http://stinet.dtic.mil/oai/oai?verb=getRec...ifier=AD0463268

[aug]

In my previous life as an electrostatic paint equipment specialist I worked with unusually high voltages. It was not uncommon to be in the vicinity of 100 kv dc voltage. was I in any kind of danger of being electrocuted ?? no as the current measured in these devices are measured in the microamp range. one of my main objectives in this job was to eliminate any kind of capacitive charge capability in the vicinity of the spraying operation. as you well may know most paint that is sprayed electrostaticly is a solvent based material and the abatement of any spark in the spray area is of the utmost concern due to the explosive nature of finely atomized paint and solvent that is present in the spraying area. All items in the spray area must be grounded to disapate the capacitive charge that could accumulate in any un-grounded item. the shock you experinced from the haul rope on your lift was a capacitive charge that accumulated on the haul rope from the electron transfer from the air to the haul rope . since the haul rope is not grounded and is isolated from ground by being supported by an insulated material , when you touch the rope ,even if you are not grounded and do not have the same potential as the rope the charge will seek balance in your mass and then you become part of the capacitor. if you are grounded the potential will travel though you to get to ground. I have been told that any material can be a conductor as long as the voltage is high enough.

This post has been edited by aug: 26 January 2008 - 09:58 AM

[Allan]

Emax, on Jan 26 2008, 08:07 AM, said:

That was probably wind charge. If you play your cards right, you can get a hell of a shock from a totally unconnected com-line. Just make sure that the cable is connected to nothing at each end of the lift and that the wind is blowing briskly - preferably on a very dry day. Now go to the middle of the lift and cut any one conductor with an uninsulated pair of cutters. Make sure your harness is tied off.

So that's what causes that... I've had the same thing happen on our long YAN... Removing grips in our "bird cage" with one hand on the guide sheave arm and leaning some other part of me on the rope and getting one heck of a whack that made my arms feel stiff.

[mthornton]

heynow, on Jan 24 2008, 10:01 PM, said:

why do I not shocked when working with a live 24 volt system in our lifts?

Procedure for locating an intermittant tower-fault (in the old days)
Go sit on top of a tower from an old Mueller or Hall, the ones with a bare insulated messanger as the tower-loop. Pick a wet day. Better clip in. Make sure your ass is good and wet first. Now grab the bare messanger with one hand. It might tingle a wee bit... but you just wait!

Lift stops... tower-fault
If you get a big jolt... just as the lift stops, the intermittant tower-fault is above you. If you don't get a jolt, then the intermittant fault is below you.

I s*it you not.

We have it much better now.
MitchT (Pano)

[Emax]

mthornton, on Jun 26 2008, 12:13 AM, said:

Procedure for locating an intermittant tower-fault (in the old days)
Go sit on top of a tower from an old Mueller or Hall, the ones with a bare insulated messanger as the tower-loop. Pick a wet day. Better clip in. Make sure your ass is good and wet first. Now grab the bare messanger with one hand. It might tingle a wee bit... but you just wait!

Lift stops... tower-fault
If you get a big jolt... just as the lift stops, the intermittant tower-fault is above you. If you don't get a jolt, then the intermittant fault is below you.

I s*it you not.

We have it much better now.
MitchT (Pano)

Very good "hands-on" test! I like that kind of thing.
However, I'm inclined to think that the jolt that you describe has a lot to do with the flyback voltage from the tower relay coil.

[Allan]

Emax, on Jan 26 2008, 08:12 AM, said:

Perhaps - but don't delude yourself into thinking that you drew anything like 40 ma.

The installed circuit might be operating at that current, but the circuit through you was much, much less. See Ohm's Law again.

By the way - if you can feel a 24 vdc circuit, you're a pretty thin-skinned guy. I'll bet that testing a good 9v battery with your tongue really sets you off!

I can grab a fork and the tower and not feel the voltage unless some other part of my body like my armpit or the inside of my arm is contacting the assembly/rope... learned that yesterday!

[Emax]

Allan, on Jul 9 2008, 11:06 PM, said:

I can grab a fork and the tower and not feel the voltage unless some other part of my body like my armpit or the inside of my arm is contacting the assembly/rope... learned that yesterday!

Unfortunately, armpits are excellent conductors. If you're shock sensitive, be sure to keep your crotch dry!

[mikest2]

I had my runny nose a little to close to a pair rocker one time when pulling a fork, that was a nasty sensation.

[Emax]

mikest2, on 10 July 2008 - 05:21 PM, said:

I had my runny nose a little to close to a pair rocker one time when pulling a fork, that was a nasty sensation.

'Snot funny, is it?