I measure 112VAC on ground pin of LCD-monitor. Does that mean it's broken? - Page 1
Forum section: electronics / beginners

author: king.oslo
author_rank: Frequent Contributor
timestamp: August 05, 2018, 12:03:32 am
content: Hello there,I just wanted to ask this question. I have a special hardware (color-)calibrated monitor. I'ts 10 years old, but replacing it is going to be expensive. I got a shock by touching the shielding on the HDMI-port, when I measured the earth-ground pin, it measures 112VAC and can deliver 2mA according to my multimeter (mA-range). I am on 230VAC power-grid in europe. I tried to measure the resistance between the live and mains earth ground pin, and also the neutral and mains earth ground pin using my multimeter. After 5 minutes I attempted the measurements again, and this time the earth pin can only deliver 200uA. Both measurements showed Open Loop. I have the following questions:1. I am correct in assuming that this is a fault of the monitor?2. Should I bite the bullet and replace the monitor?3. What do you reckon has happened to it?Thank you for your time.Kind regards,Marius

author: JS
author_rank: Frequent Contributor
timestamp: August 05, 2018, 12:11:23 am
content: Good news, it's likely to be a lifted ground, or missing somewhere between the groubdong bar on your installation and the monitor.To be sure you could measure it loading it a bit, 100k+ resistor might do. Useful tp have a eevblog meter or some other with lowZ measurement for this.When ground is faulty a voltage about half or a bit less than mains appears at ground, as it has some coupling through the circuit so its better not to have a ground connection than one that's not ptopperly wired. But always, to be safe, have a proper ground connection.JS

author: helius
author_rank: Super Contributor
timestamp: August 05, 2018, 12:24:16 am
content: How did you measure the earth-ground pin, and in relation to what?JS is correct, the normal behavior of the chassis ground when the (low-impedance) path to earth is not present is to sit at half the supply voltage. On Class 2 equipment this is always the case!See this thread:https://www.eevblog.com/forum/beginners/new-2017-macbook-pro-15-inch-giving-electro-static-shock-is-it-normal/

author: king.oslo
author_rank: Frequent Contributor
timestamp: August 05, 2018, 01:02:43 am
content: Thanks for your reply JS and Helius. Here is some more information:REGARDING MAINS EARTH IN MY APARTMENTThe wiring in my apartment is 70 years old. Behind the power outlets there is a green/yellow wire, but i do not know whether that wire is connected to anything. How do I correctly measure if it has good connection to earth? I have a fluke 87V and Agilent U1272A and agilent U1733C LCR-meter. Can I use any of these to measure it? The reason I suspect that it could have a poor/no connection to earth is two-fold: (1) none of the outlets had earth grounding tabs when I moved in. I have since added one power outlet with grounding tabs to which I connected the green/yellow wire. (2) I have had electrical shocks by touching the green/yellow wire whilst the chassis ground of some equipment I own was connected to it a couple of times. I’d really like to measure how well it is earth grounded.MEASUREMENTSWhen I measured the monitor chassis voltage of 112VAC , the mains earth ground tabs were floating, and only the live and neutral was plugged in. I conducted a few measurements: I conducted the measurement with standard fluke probes on the AC range by first connecting one probe to a water pipe in my kitchen and the other to the chassis ground on the monitor. The 87V showed 112VAC. I also checked the current, which was 4mA. I conducted a second papir of measurements by connecting one probe to the (potentially not grounded) green/yellow wire behind the power socket, and the other to chassis ground on the monitor. I again measured 112VAC/4mA.Thank you for your time.

author: JS
author_rank: Frequent Contributor
timestamp: August 05, 2018, 01:54:20 am
content: You could measure the voltage reference to the yellow green wire to live and neutral, with high and low impedance and know if its connected to anythining. If its fine itnshould read a few volts or less to neutral and the same to theblive as if you measure from neutral, independent of the impedance. If the highbimpedance measurements reads about 100V from each and the low impedance measurement are much lower the wire is unconnected.If it zaps you it's likely to be open. If the wiring is 70yo the pipes are likely to be made from metal so there you have a good grounding you could use. You shouldn't connect it to the grren and yellow unless you check or mod it so it's not connected outside your appartment so you don't short anything.Meaauring voltage between yellow and green to the pipes should also give you an idea of what's going on.JS

author: helius
author_rank: Super Contributor
timestamp: August 05, 2018, 03:45:44 am
content: The permissible earth leakage current for computer equipment is 3.5 mA per IEC regulations, and you measured 4 mA. This is not dangerous.If you are so inclined you can investigate the reason for the increased leakage, but I wouldn't lose sleep over it.

author: JS
author_rank: Frequent Contributor
timestamp: August 05, 2018, 05:25:29 am
content: 4mA is fine, won't trip the standard 30mA gfci we use here but ground path should be low impedance and meassure pretty close to neutral or the pipe. Also, measuring current in such conditions is kind of dangerous, if it were correctly conncected the current could blow at least the fuses if you have a proper scope like the 87V.To solve this I keep my mind with using the pipes, is not new to do so. Many structural parts are used as ground and pipes are usually the easier to get to. Avoid gas pipes and try to disconnect from the outside as all your neighbours in the buildong dumping the leakage to your installation isn't nice either.I've tried to replace an structural ground with a proper one in an industrial enviroment, the structural one showed 10Ω being a tad high for an industrial installation. A 3m rod measured well over 100Ω because a really crappy dirt, kind of sand... So I would need 20 of those rods to get it better, no deal, took the rod out (you would normally leave it there but we had the elevator). Luckily there was an unused train rail right next to the building, less than a meter away, so there was my better grounding measuring less than 3Ω but measurements are kind of hard to get as the dirt, as said, is crap.JS

author: larsdenmark
author_rank: Regular Contributor
timestamp: August 05, 2018, 08:19:52 am
content: Funny, I ran into the same problem the other day when I was shuffling some equipment to make room for my new (used) TTI power supply. Apparently, not all my extension cords have a proper ground connection. Hence my switched mode power supply and my computer’s USB connector would give me an unpleasant schock, but the voltage between the positive and negative terminal are OK. When not grounded these supplies have 120 V AC on the terminals.I now have two linear supplies. They both have about 40-70 V AC when not grounded, but I can’t feel it at all when touching the terminals (why?). I guess it is a good idea to ground them anyway!?

author: king.oslo
author_rank: Frequent Contributor
timestamp: August 05, 2018, 12:23:12 pm
content: Thank you so much for your replies. When I tried to measure between the live and green/yellow wire, the fuse in my apartment trips, and I have to reset it. I conducted some more measurements between the green/yellow wire and the live and neutral. The measurements were conducted using an Agilent U1272a in the low-Z (20kΩ) and high-Z (10MΩ) voltage modes. In the following table, FUSE! indicates that the fuse blew:Measurements green/yellow-live and green/yellow-neutralIn-ZLive wireNeutral wire10MΩ235V1V2kΩFUSE!4VThe impedance between my water pipe and green/yellow wire is Z = 1Ω with a phase angle θ = 28° at 100Hz.Measuring the water pipe and the live wire also trips the fuse. Measuring it at Hi-Z shows 235V.What is your verdict regarding the mains earthing? Can I conclude that it is a good earth connection, or is it not connected? What could be causing the fuse from blowing? The fuse is a General electric DPCA100 C16. According to the datasheet, it is a ground fault breaker with nominal current 16 amps. I think I understand what is going on here, but I'd like confirmation that I have understood correctly.

author: drussell
author_rank: Super Contributor
timestamp: August 05, 2018, 05:54:13 pm
content: You are tripping your RCD, not "blowing the fuse".The extra current draw of the 2k load to the ground wire is pushing you over the RCD's trip point.

author: JS
author_rank: Frequent Contributor
timestamp: August 05, 2018, 11:53:14 pm
content: Looks like a gpod ground, maybe your extension cord is missing the grpund connectionJS