Electronics > Beginners
I measure 112VAC on ground pin of LCD-monitor. Does that mean it's broken?
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king.oslo:
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
JS:
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

helius:
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/
king.oslo:
Thanks for your reply JS and Helius. Here is some more information:

REGARDING MAINS EARTH IN MY APARTMENT
The 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.

MEASUREMENTS
When 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.
JS:
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

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