Shocking Experience

[jerjwillelec]

I went on a service call to an older Highschool. One of the teachers there claimed she was shocked while using a mouse on a computer. Immediately, I thought surge but thought I would take some readings anyway. At the outlet is 120 vac H to N, 120 vac H to G, and 0 vac N to G. I left the red probe in the hot side of the plug and touched a metal table, and after scraping the paint off I had around 60 vac from the hot side of the plug to the table. I touched a chair and had 86 vac. I touched a filing cabinet and had 106 vac. I then grabbed the probe with my fingers and touched the filing cabinet with my other hand, my meter read around 96 vac. Not once during this whole time did I feel even a tingle. The floor is vynal on concrete. As I said it is an older school and I am quite certain they used conduit grounds. I left it as a surge but am still a bit baffled with the whole situation. Any thoughts?

 

[ultramegabob]

if your using a digital meter you may be reading noise, get an analog meter and check it again and see if you get the same voltages.

 

[jerjwillelec]

Good point! I will try that.

 

[ultramegabob]

i just read this again....

i just read this again....

I left the red probe in the hot side of the plug and touched a metal table, and after scraping the paint off I had around 60 vac from the hot side of the plug to the table. I touched a chair and had 86 vac. I touched a filing cabinet and had 106 vac. I then grabbed the probe with my fingers and touched the filing cabinet with my other hand, my meter read around 96 vac.

you need to check from neutral to the metal surfaces your testing to check for stray voltages, you will definatly get odd readings running power through a digital meter and touching things. and grabbing the probe with your fingers with votage going through it is hazardous.

 

[ptonsparky]

Don't touch the leads of an analog meter. Not a good practice. Some may pass enough current to at least make you fall off the ladder.

 

[gar]

080515-0823 EST

jerjwillelec:

Assuming the water supply to the building is with a metal pipe, then attach a wire, like #20 insulated, at the water pipe entry point. This will be your reference test point for earth ground. Obviously the wire has to be long enough to go wherever you want to make voltage measurements.

A Simpson 260 meter should a good analog meter to use. My newest 270, about 40 yrs old, is 5000 ohms/volt on AC. That is still not much current, 0.2 MA full scale. My oldest 260 was 1000 ohms/volt or 1 MA full scale, circa 1948. The 270 was just a higher accuracy 260, and I do not believe the 270 is made anymore.

On each measurement always start on the 250 V range. Check your main panel first for nuetral and the ECG to the water pipe. Could be the grounding wire from the panel to earth is open or a high resistance.

Any conductive object in the building should be near zero volts to the water pipe reference. If not, then there is a current flowing to it relative to ground. What is zero volts? You will have to judge this based on what you find. You can add resistance in parallel with the meter, like a 25 W 120 V bulb. Cold this will be about 45 ohms, based on 11 ohms for a 100 W bulb. Hot a 25 W bulb should be about 575 ohms. If there is substantial voltage and current the lamp will glow.

.

 

[Lxnxjxhx]

it's romex cable capacitance in series with 120v

it's romex cable capacitance in series with 120v

in series with a floating conductor.

Regarding the value of this impedance, I must say that one time I tried to calculate the upstream resistance value of refrigerator leakage current by loading it with different resistance values.
The resistor value came out negative, probably because the leakage resistance depended on the current through it, something which is a little more difficult to model than E=IR.
You won't have this problem with 500k of capacitive reactance.

Can, and should, this Forum be modified to easily retrieve archived posts that give unanimous answers, like to the problem of floating neutrals?

 

[coulter]

... Immediately, I thought surge ... I left it as a surge ... Any thoughts?

Just one. Tell me about this "surge" thing causing one to get shocked. I don't understand that.

carl

 

[crossman]

I am thinking it is one of those "nerve ticks" that are experienced every now and again, at least it has happened to me. Feels like a shock in the wrist or wherever, when there is no electrical equipment in the vicinity.

A purely physiological function.

 

[Lxnxjxhx]

read around 96 vac. Not once during

read around 96 vac. Not once during

If no tingle, < 1 mA current.
If 96v at the same time you were touching the wires your skin resistance was 96/(<1mA) > 96 K.
Current passing through the hand is better than current passing through your chest.

 

[K8MHZ]

Is the school close to a radio or TV station?

 

[jerjwillelec]

one of those "nerve ticks" that are experienced

I have experienced these as well. That too is one thing I was thinking. I have a hard time telling some customers that they didn't get shocked even if I know they didn't. They are determined that they did.

 

[SiddMartin]

I am thinking it is one of those "nerve ticks" that are experienced every now and again, at least it has happened to me. Feels like a shock in the wrist or wherever, when there is no electrical equipment in the vicinity.

A purely physiological function.

I hate them, i wonder if the more you get zapped, the more often these occur?

 

[TwinCitySparky]

I hate them, i wonder if the more you get zapped, the more often these occur?

That would suck. If thats true, we would need invent a wearable data logger just to tell if it was nerves or current.

 

[DanZ]

One of the teachers there claimed she was shocked while using a mouse on a computer

It could also be a medical problem, similar to carpal tunnel or tendinitis. Realistically, plastic shouldn't be that good of a conductor, and if she's using a mouse pad, that shouldn't be that good of a conductor, but if she has a metal desk or metal inbox that's close, it could have been static electric discharge. Just throwing some odd ideas out.

 

[sparky59]

i don't trust a digital meter. they give a lot of false readings