Voltage potential between body & earth wildly different in different rooms  - Page 1
Forum section: electronics / projects

author: brucew268
author_rank: Newbie
timestamp: May 03, 2014, 07:37:45 pm
content: I'm measuring the voltage potential between my body and earth and getting some unusual variations that I can't understand:Room A: 4 VAC, dropping to 1.5-2 VAC when I put my hand on my laptop, or 0.5 VAC when touching the metal of HDMI plug coming from monitor.Room B: 9 VAC, increasing 10x to 90 VAC with hand on laptop,or HDMI plug.I cannot seem to identify why it decreases in one room and increases 10x in the other room. What is implied electronically/electrically here? Does it imply that the sockets in Room B are improperly earthed? Rooms A & B are on the same circuit which goes through an RCD. All PC electronics are plugged into the same power strip, L-N-E are wired the same for the sockets in both rooms. I guess I could test for a ground fault with an RCD-plug tester? Like:http://www.diy.com/nav/garden/garden-power-tools/garden-power-accessories/timers_rcd_s/B-and-Q-Safety-Rcd-Plug-PRCDNB-Black-10278510?skuId=10709167The laptop increase in Room B does not occur if the laptop is unplugged from everything. But if I plug in the PSU, or USB hub, or HDMI to monitor, it increases. I've tried unplugging everything from the power strip, unplugging one at a time.

author: IanB
author_rank: Super Contributor
timestamp: May 03, 2014, 07:52:01 pm
content: Am I to assume you are just holding a meter probe and touching the other probe to earth? If so, you are not measuring anything meaningful, so any differences in the numbers you see are meaningless. (They are meaningless because you are not completing a circuit with your measurement, and an ordinary multimeter is not designed for measuring electric fields. It is designed only for measuring closed circuit voltages.)What are you trying to achieve?

author: brucew268
author_rank: Newbie
timestamp: May 03, 2014, 08:00:17 pm
content: I've read various blogs and sites (including one by an EE) who claim that if your circuit's 'earth' is good, that doing the above does meaningfully measure 'body voltage' which has to do with electrical charge (static) on the skin. The positive probe is meant to have a short lead (12").

author: IanB
author_rank: Super Contributor
timestamp: May 03, 2014, 08:16:30 pm
content: Quote from: brucew268 on May 03, 2014, 08:00:17 pmI can tell you with some assurance that what you have read is complete nonsense. It is pseudo-scientific mumbo-jumbo.

author: IanB
author_rank: Super Contributor
timestamp: May 03, 2014, 08:18:55 pm
content: In other words, yes there are "voltages" on your skin, and yes, your meter is showing "something" related to those, but what you measure has nothing to do with how good your household wiring is, and has nothing to do with static charge.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: May 03, 2014, 09:31:06 pm
content: If you are measuring AC, it is not static charge.

author: Nerull
author_rank: Frequent Contributor
timestamp: May 03, 2014, 11:13:57 pm
content: The impedance of a meter is low enough to dissipate any static charge you have in a few fractions of a second as well.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 03, 2014, 11:15:55 pm
content: You body is simply a big, amorphous, imprecise "antenna". What you are "measuring"(?) says much more about the meter than about "body charge" (whatever that is?)

author: Kremmen
author_rank: Super Contributor
timestamp: May 04, 2014, 05:25:57 am
content: Quote from: brucew268 on May 03, 2014, 08:00:17 pmYou can only meaningfully measure the "goodness" of your earth by a proper continuity measurement. That is easiest done at the initial build time of the wiring but can be done later as well. Exactly how you do it depends on the power distribution system in use. On general terms, you open the feeder wiring at the distribution panel end and short all wires together at the consumer end (wall outlet, light fixture, whatnot). You then measure the resistance of wire loops. Measured this way, the acceptable resistance in the wiring between live and earth for 230VAC TN-S distribution standard are 0.1 - 3 ohms max. Though the 3 ohm value is already uncomfortably high.Of course there is still the wiring from your distribution panel to the real planet Earth. Enter the grounding rod or other arrangement. Personally i have always found the traditional concept of a stick of metal buried in a hole slightly amusing if you attempt to connect anything properly to earth potential. Ring cabling embedded in the foundation circumference, connect all concrete rebar to ring and bring heavy riser cables to panels and roof furniture and lightning conductors. That is the way to do it.Whatever you do, forget about any "body static" gimmicks. Nothing whatsoever to do with proper wiring practices.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 04, 2014, 08:13:56 am
content: lets just assume for simplicity... earth (soil beneath your house) is at 0V and space (just right above the ozone layer is a fixed 1MV, just for simplicity... and then anything in between are very complicated network of circuitry, the air, the humidity, the concrete the stones, the wind the clouds etc and the you, the room the carpet everything. you yourself is also a very complex of circuitry, the food you ate, how dehydrated are you, are you sick how wet is your feet and hands and so on everything will affect the stray L,C and R values in the complex network, let alone electromagnetism and gravitation force, and mostly in a very high impedance network. you move to different room you formed another different circuit network and the cloud above you. and you build different voltage across you, hows that? meaning full? as someone said, its science (or nature?) mumbo-jumbo. why its meaningless? because you dont put all the equations into your consideration.

author: brucew268
author_rank: Newbie
timestamp: May 04, 2014, 01:03:12 pm
content: OK, let's take the controversial human body out of it, and measure the electronic equipment:From AC Earth to the neg DC barrel coming from the laptop A's PSU, I get 90 VAC.From AC Earth to USB neg coming from the printer, I get 145 VAC.From AC Earth to USB neg coming from the powered USB hub, I get 117 VAC.From AC Earth to HDMI neg coming from monitor A, I get 87 VAC.From AC Earth to the neg DC barrel coming from the laptop B's PSU, I get less than 500 mVAC.From AC Earth to HDMI neg coming from monitor A, I get less than 200 mVAC.Is this supposed to be normal or irrelevant? If you were working on the output of a PSU and found this on the -5V rail, would it cause you concern? BTW: The above measurements occur regardless of which room we are plugged into.

author: Jeroen3
author_rank: Super Contributor
timestamp: May 04, 2014, 02:39:30 pm
content: Yes, that is supposed to happen.Isolated supplies tend to float at half the mains voltage.Unisolated supplies can also do that, but they won't float, they are hard.Connecting ground of your laptop charger to mains earth can either remove the voltage potential, or create fireworks. But most likely the first one.The double square symbol on the supply indicates galvanic isolation if I'm correct.

author: brucew268
author_rank: Newbie
timestamp: May 04, 2014, 03:09:40 pm
content: Hmm. That seems to correspond with the measurements: The following devices all have the square within a square which, if you are correct, mean galvanic isolation: Laptop A / Monitor A / Hub A / Hub B. All of them show 85-120 VAC on DC neg.Printer has no connection to ground (measures 145 VAC).Laptop B's PSW uses a cloverleaf connection and says to only connect to a grounded socket.Monitor B has the PSU built into the back of the monitor, IEC plugged into the back, meaning access to earth.So, these are the only two with 500 mVAC or less and earth connected to the PSU.

author: Kremmen
author_rank: Super Contributor
timestamp: May 04, 2014, 05:29:02 pm
content: Quote from: Jeroen3 on May 04, 2014, 02:39:30 pmMore than that, it indicates a class II device with double insulation. All exposed parts of consumer electric/electronic devices must be galvanically isolated in any case.Protection classes (YMMV based on what code governs your region. This applies to EU):0: Basic isolated. Connection using ungrounded plug/socket (only Live and Neutral wires). Not permitted for new installations any more.I: Protective grounded. All touchable metal parts of the device are connected to the PE (Potential Equalization, i.e. earth) wire. The wall socket has corresponding PE prongs or similar to facilitate the earthing. The most common plugs in EU are the various versions of Schuko and the british 3 pin plug.II: Safety insulated. Basic and additional isolation tht together prevent the outer parts from becoming live shoud an internal insulation fail. In EU class II devices use the flat "Euro" plug that has no ground connection and can used with both class 0 and I wall outlets.III: SELV or Safety Extral Low Voltage devices. Not applicable for domestic installations directly, as the supply voltage is below 120V DC or 50V AC.So, 2 squares indicates a device where the live parts are doubly insulated and no PE wiring is used. These devices are mostly (but not always) plastic housed so as to avoid any chance of a metal part on the outside becoming live under any circumstances.

author: Jeroen3
author_rank: Super Contributor
timestamp: May 04, 2014, 08:34:25 pm
content: Monitors also get to mains earth though hdmi shield, which connects to your pc's chassis, which must be mains earth.@kremmen: Please point me to the relevant ISO/NEN specifications for that if you can.

author: Kremmen
author_rank: Super Contributor
timestamp: May 05, 2014, 01:28:32 pm
content: @jeroen3: Check IEC 61140, that should have the info.

author: David Hess
author_rank: Super Contributor
timestamp: May 06, 2014, 03:18:13 pm
content: Quote from: Jeroen3 on May 04, 2014, 08:34:25 pmWhich is annoying when the monitor and computer connect the HDMI chassis shields in parallel with their ground returns blowing a trace on one or the other.I have used my good multimeters before to find hot and neutral by holding one lead in my hand and probing with the other. The capacitive coupling to earth ground is almost always enough to reveal hot, neutral, or a floating AC source.