voltmeter question

[electricalperson]

today i went to my uncles store to fix a display case that was shocking people. i measured volts to the other case right beside it and had 90VAC

he said when people touch both cases at the same time they feel a slight zap

i used the loimpedance function on the 117 and had 0 volts

i opened the junction box and found the wire nut missing off of the hot and the case wasnt grounded at all. the hot was barely touching the cover but there was still enough voltage to be felt.

i repaired everything and all is well

why did i measure 90vac with the high impedance meter and nothing with the loz function? i figured it was because the connection was so light that the meter couldnt pick it up because of the meters load.

am i right with my thinking?

 

[charlietuna]

Maybe the case was dropping 17 volts on another path to ground?

 

[iwire]

Does the case have florescent lighting?

 

[electricalperson]

Does the case have florescent lighting?

it had a 4 foot lamp inside and a ballast in the bottom. after i fixed it the voltage wasnt there anymore

 

[iwire]

Here is what I think.

The wire nut missing had noting to do with it, if you had a bare hot against the metal people would be getting really whacked, you meter would have said 120 on both hi and lo z.

I be what you had was capacitance coupling of the high voltage from the ballast secondary leads to the ungrounded case. Enough current for people to feel but not enough to drive a lo z meter.

Just fixing the lack of grounding would have fixed it ............ of course none of us would have left the missing wirenut off.

 

[electricalperson]

Here is what I think.

The wire nut missing had noting to do with it, if you had a bare hot against the metal people would be getting really whacked, you meter would have said 120 on both hi and lo z.

I be what you had was capacitance coupling of the high voltage from the ballast secondary leads to the ungrounded case. Enough current for people to feel but not enough to drive a lo z meter.

Just fixing the lack of grounding would have fixed it ............ of course none of us would have left the missing wirenut off.

the fixture still worked even when it wasnt grounded. its hard to proove if the hot was actually touching or if there even was a good enough connection to all the metal parts. but that bare wire was very very close to the front cover of the junction box.

its possible it was the capactice i was reading since i didnt get any on the lo z. i didnt feel the voltage at all so i dont know if it was actual line voltage or off of a ballast i used a meter. the guy in the store used his body to tell me it was live. i had 88 volts to one case and 90 to the other.

 

[electricalperson]

i feel like i should know basic theory off of the top of my head like you guys. i havnt done theory calculations since freshman year of highschool around 1998-99. i feel pretty stupid sometimes. :-? thanks for understanding and not making fun of me

 

[LLSolutions]

The connection may have been so slight that as soon as a small load was put on it it couldn't read voltage. I did a service change and my drop light wouldn't work so i metered it at 90 volts, surely that would light a halogen at least a little bit but the connection at the pole was just that loose.

 

[electricalperson]

will a analog volt meter detect this kind of voltage? sometimes the DMM isnt the best choice of meter IMO.

 

[LarryFine]

I tend to agree with Bob: capacitive coupling, not direct contact.