measurement of voltage on open circuit

[EEJaime]

Gentlemen, (and ladies if applicable),

Perhaps one of you can assist me in formulating an explanation for on of our clients that is clear enough for a layperson to grasp. In the process, helping me understand better the operation of a tool we use all the time. This is not my field of specialization, but I feel I am missing something very basic here.

The issue started as a school district's concern over what they viewed as a safety issue. It turns out not to be so, however in trying to explain it to the district staff, we ran into a misundestandng that we could not easily explain to the district.

We have a hand dryer in a restroom that was reportedly delivering a mild shock to a student. The contractor re-circuited the dryer with a dedicated 120 V circuit and equipment ground and the dryer was replaced. The same result was reported, (by the same student-we could not reproduce the "shock"). In attempting to demonstrate that there was no "stray voltage", we used a digital multi-meter to show that there was no voltage present. Part of the field tech's demonstration involved showing the district the meter's operation. This is what he did:

Used meter on a receptacle with probes to demonstrate that with the probes on the Hot and Neutral contacts, the reading as expected was 120V +/-. With the probes on Hot and Ground contacts the voltage was showing as a minimal 0.3 Volts. But when the probe was connected to the Hot and the second probe was held to a floating piece of 500kcmil cable, the meter read 30 Volts.

Can someone explain what we are seeing here? Your assistance in educating myself and helping us explain it in turn is much appreciated.

Thank you and Regards,
EEJaime

 

[djs]

Voltage between the hot and ground was 0.3 volts? It sounds like your hot and neutral are reversed somehow.

However to answer your question, modern DVMs (Digital Volt Meters) have a very high input impedance. For this reason they can and will act as "radio" recievers. The more "antenna" used, the higher the voltage. So what you are reading is in effect the ambient electromagnetic field.

 

[itsmoked]

With the probes on Hot and Ground contacts the voltage was showing as a minimal 0.3 Volts.

This is wrong! THIS CANNOT BE THE CASE unless the ground is is broken.

It does however look exactly like what would happen when you measure between the NEUTRAL and the GROUND.

So either your NEUTRAL and your HOT are reversed or the ground is NOT actually a ground but a floating piece of wire.

Next:

But when the probe was connected to the Hot and the second probe was held to a floating piece of 500kcmil cable, the meter read 30 Volts.

Here you are looking at the what a capacitively connected piece of cable's voltage relationship is between your HOT and a floating piece of metal. It can be all over the place.

Generally measuring from HOT to some random insulated piece of metal is pointless and confusing because your reference is a voltage potential. More logical is measuring between GROUND and floating metal. That then shows you how much capacitive coupling your floating metal has to the electrical fields that are nearby.

Seems like your demos were poorly conceived and/or the wiring is faulty. The express train to confusion!

Unless you want to go into a side discussion of capacitive coupling and stray fields, using a modern DMM for a demonstration to noobs is a bad idea. Get out any old swinging needle volt meter and use it. You would not then see these little differences like the 0.3V or the 30V you mentioned at all and confusion would not rear its head. The meter itself would consume these two voltages just by their loading.

As for the shocks: air rushing past isolated metal will often deposit charge on it just like a Van de Gaff generator. If, say, the metal nozzle of the dryer was electrically isolated from the grounded metal body of the dryer it could certainly be charged up and occasionally discharge into a victim this static shock. So it would be a good idea to check from the HOT to the metal contact points on the dryer. This should give a ~120V reading. If not, that is likely your problem.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

The test would be if he can be repeatedly shocked within a short time frames, i.e., touch, wait 1 second, touch again, to distinguish static shocks. If there's AC on the nozzle, a gentle brush with your fingers will often detect a "stickiness" or light tingling; I can't really describe well, but on older, non-grounded appliances, you can often "feel" an AC buzzing when lightly touching metal surfaces.

TTFN

FAQ731-376

 

[rodneysab]

I think itsmoked is correct. One suggestion is to use one of those voltage detector pens, like the one from fluke. You don´t have to touch the surface you suspect to be energized, just approach the probe of the pen and lights or beep will start.

 

[EEJaime]

Good morning, (well it is here),

I am told I misspoke in my OP, the 0.3 Volt measurement was in actuality read when the probes were on the Neutral and the equipment ground.

Thank all of you for your astute comments and observations. As always a big help. Kind regards and have a pleasant day.

EEJaime