Voltage potential between scope ground and unconnected circuitry issue - Page 1
Forum section: electronics / beginners

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: February 15, 2015, 01:18:24 am
content: Hello,I've watched Dave's video regarding the dangers of placing a scope's probe ground and tip however one wants without taking into consideration the ciruitry one is trying to measure.I've found to be hundred of volts of potential between the ground pins on the scope's probes and a lot of other object connected to mains such as the outer shell of a usb connector and the case of a power supply.I've tried connecting the scope to a DC to AC power inverter to isolate it from the mains but the high voltage potential was still there.None of the electronics are grounded and there is no physical connection between the scope's ground and the hot or neutral wire so how the hell am I getting shocked to pretty much every metal surface I touch. I just can't seem to understand where the voltage potential is coming from.How can I avoid getting electrocuted by touching the ground and the bare metal of other electronics devices?Thanks.

author: David_AVD
author_rank: Super Contributor
timestamp: February 15, 2015, 09:31:02 am
content: The question you ask comes up all the time on t his forum.The usual explanation for high voltages is to do with the equipment not having a grounded case.Double insulated devices often have a small value capacitor going from mains active to the chassis and another from mains neutral to the chassis.This is why you'll be able to measure (typically) half the mains AC value on the case with respect the mains ground.The current through those capacitors is very low, but the impedance of the multimeter is very high so doesn't dampen it down much at all.If you connected a 100K resistor from the metal case to mains earth you'd see the voltage drop to quite a low level.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: February 15, 2015, 10:22:36 am
content: Quote from: David_AVD on February 15, 2015, 09:31:02 amHow can I do this if we have no mains earth in my country?

author: rob77
author_rank: Super Contributor
timestamp: February 15, 2015, 10:34:35 am
content: Quote from: drummerdimitri on February 15, 2015, 10:22:36 amwhat country is it ? in every country there must be a mains earth... ok. there are countries with many old installations but the new installations got mains earth.if i would be in a situation without mains earth i would simply route a earthing wire (1,5mm2 minimum) across the whole place and replace all the sockets to ones with a mains earth (connecting to that earth wire) and finally i would earth that "artificial mains earth" by burying a sufficiently sized metal plate deep enough near the house to have small ground resistance and connecting that grounding plate to my brand new earth wiring. - at least all the socket’s earth pins would be on a same potential as the ground.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: February 15, 2015, 10:36:55 am
content: Quote from: rob77 on February 15, 2015, 10:34:35 amI'll look into that. Country is Lebanon.What if I were to connect the ground leads of the oscilloscope and say a metal frame power supply together. Would that eliminate the shock?