Electric shocks from ungrounded equipment - Page 1
Forum section: electronics / beginners

author: new299
author_rank: Regular Contributor
timestamp: April 21, 2015, 08:39:00 am
content: I was doing a fairly simple test the other day, hooking a signal generator to a spectrum analyzer. However rather disturbingly I found I was getting electric shocks from the BNC ground on the signal generator.I tracked this down to the a faulty extension cable. The ground lead was only intermittently connected (I assume on both signal generator and analyzer).This is obviously very bad, and I threw the extension cable in the trash immediately. But I'm at somewhat of a loss as to why I should have got shocks? Is there likely another fault contributing to this? If the grounds are not connected would one or the other just be floating and effectively isolated? Why would there be a voltage between it's ground and earth ground taking a path through me?

author: DmitryL
author_rank: Regular Contributor
timestamp: April 21, 2015, 08:44:48 am
content: Google "common mode filter", find capacitors from mains N and L to your equipment Earth (case). If your device's "E" isn't connected to mains "E", you'll have mains/2 AC voltage there relative to the ground. Not a big deal, capacitors a small enough, but sometimes enough to fry your equipment when connecting one device to another

author: AndyC_772
author_rank: Super Contributor
timestamp: April 21, 2015, 09:20:43 am
content: There's a cumulative effect too, if you have many devices all connected together. Each one individually may only provide a small leakage path to earth, but add them all together and you can get quite a nasty shock.In my experience, hi-fi gear is the worst for this. Lots of interconnected components, all with 2 pin plugs.

author: new299
author_rank: Regular Contributor
timestamp: April 21, 2015, 01:13:09 pm
content: Quote from: DmitryL on April 21, 2015, 08:44:48 amThanks for your help! I googled a little. If I've understood correctly those capacitors effective work as low pass filters, removing high frequency noise on the line?They would be on the primary side of the transformer I suppose? Is there any reason they go to ground, rather than neutral?

author: TimFox
author_rank: Super Contributor
timestamp: April 21, 2015, 05:24:15 pm
content: The capacitors contained in a simple line filter: one from line-neutral, one from neutral-ground, and one from line-ground. Here, "ground" means the metal case of the filter which connects to the chassis. Therefore, with the ground wire open, and no external connections to ground, the chassis will have an AC voltage roughly 1/2 the line voltage, in series with an effective capacitance equal to the series combination of the two capacitors to ground. Since that is only a few nF, the reactance (> 100 k ohm) limits the current through your body to an uncomfortable yet probably non-lethal value.

author: budhafoo
author_rank: Newbie
timestamp: June 15, 2015, 02:33:43 am
content: Hi everyone *my first post*!I am too trying to understand a "leakage path to earth".I am currently troubleshooting an LCD monitor that has a "leak path to earth" - nasty shocks 120V AC when touching the screen (glass optical touchscreen) or metal frame. I have made sure the earth ground is present, but still the "leak" is there.Facts that needs explanation:1. Why is the monitor still working, it displays with some picture (missing one of the RGB channel) eventhough there;s AC leakage on the DC circuit2. Took the powersupply board, and it works fine other than the 120v AC on the DC / secondary power supply unit.Is there an eevblog video on this ? ground-earth talks ..thanks!

author: vk6zgo
author_rank: Super Contributor
timestamp: June 15, 2015, 03:27:05 am
content: Quote from: new299 on April 21, 2015, 01:13:09 pmFor the HF noise,Neutral doesn't look like ground,so it can carry interference just as well as the Live wire.