Voltage potential between earth/ground and human body - Page 1
Forum section: electronics / projects

author: ajc
author_rank: Contributor
timestamp: November 18, 2013, 09:19:59 am
content: Does anyone know what 'typical' voltage potential (AC rms) will be observed between a ground/earth connection and the human body?I've measured up to 40V AC rms in my apartment which I suspect means the earth is not properly connected. In other buildings I've measured something much lower (around 1V RMS) which I suspect is closer to what it should be for a proper earth connection.Obviously a key variable here is the human body and how effective it is as an antenna.Reason for question is that I believe the faulty earth connection could be behind problems with death of some components in a design I was working on (I'm guessing my CMOS devices probably don't like seeing 40V rms AC potential difference between my body and grounded ESD mat...)

author: Zero999
author_rank: Super Contributor
timestamp: November 18, 2013, 09:23:56 am
content: The solution is to wear an earthed wrist strap.

author: ajc
author_rank: Contributor
timestamp: November 18, 2013, 09:30:44 am
content: Quote from: Hero999 on November 18, 2013, 09:23:56 amYep, I am doing this (now!).Now I am trying to figure out if the ~40V AC rms is a clear indication of a ground connection fault in my apartment or not. Anyone know?

author: Kjelt
author_rank: Super Contributor
timestamp: November 18, 2013, 09:32:17 am
content: You can't quantify that in volts. You need to know the resistance between a known good earth point (perhaps where it enters the home) and the unknown earth point.A well isolated human can have any voltage with respect to earth.

author: ajc
author_rank: Contributor
timestamp: November 18, 2013, 10:41:54 am
content: Quote from: Kjelt on November 18, 2013, 09:32:17 amMy issue is that I am pretty sure that the earth is not connected (ie. floating). As a result, any wiring associated with the earth just becomes a big antenna. Hence the measurement of ~40V AC. I agree that the human body can have any potential difference, but the question is what the typical AC potential difference. My guess is that this primarily comes down to how effectively our body is acting as an antenna, but I'm not sure what the typical figure for this is (assuming a typical figure exists of course!).

author: Kjelt
author_rank: Super Contributor
timestamp: November 18, 2013, 11:10:04 am
content: If your electrical installation has a RCD installed with human safe values (35mA here in Holland) then you can test the suspicious earth point by tripping the RCD in a safe way.For instance build inside a little box (these values are for 230VAC , 40mA) a 5k6, 10W resistor between fase and earth and a remote relay or switch and see if it trips.If it does not trip the RCS you are sure that earth point has some serious issues.Don't wait too long to test if it trips (it should be instanteneous) or your resistor starts smoking

author: andtfoot
author_rank: Unknown
timestamp: November 18, 2013, 12:11:01 pm
content: Just out of curiosity, I shoved one lead of my Fluke 117 meter into the earth part of a kitchen outlet (after checking the power point was wired correctly!), and grabbed the other lead with my hand. Here are the rough average AC readings (wearing socks, pants and t-shirt on a tiled floor):- Standing, both feet on ground: 170mV- Standing, one foot on ground: 290mV- Standing, one foot on ground, shaking the other leg: 400mV- Standing, both feet on ground, grabbing steel kitchen sink with other hand: 500mV (incidentally, there is ~1.1V across ground and the sink)- Sitting on melamine benchtop: 1.63VIf I just grab the lead without the other lead connected I get ~30mV. With both leads floating I get ~3mV.

author: ajc
author_rank: Contributor
timestamp: November 18, 2013, 12:18:33 pm
content: Quote from: andtfoot on November 18, 2013, 12:11:01 pmThanks andtfoot. This kind of matches the measurements I get for a properly earthed connection which is also why I'm pretty sure measuring ~40V AC is an indication that something isn't right...Of course every location you will get a different result too since the amount of background signal will differ. I'm assuming 50Hz is the predominant source though in this case.

author: G7PSK
author_rank: Super Contributor
timestamp: November 18, 2013, 12:19:55 pm
content: If your ESD mat and your ESD wrist strap are connected to the same earth point the will both be at the same potential.I have never tried it but I suspect that as long as the mat and wrist strap are tied together so that they are at the same potential the grounding is not realy required as anything on the mat and you are at equal potential no current can flow.

author: ajc
author_rank: Contributor
timestamp: November 18, 2013, 12:30:20 pm
content: Quote from: G7PSK on November 18, 2013, 12:19:55 pmI think you are right here, but the issue I've had was related to a sensor circuit detecting human ecg signals. Because there was such a voltage difference between ground and the body, I was saturating the input of the amplifiers in the circuit which led me to look into the actual behavior of the ground connection. For some reason, even shorting my body with the ground in this case isn't eliminating the problem.