Measuring voltage on metal chassis - Page 1
Forum section: electronics / beginners

author: g.costanza
author_rank: Contributor
timestamp: September 25, 2017, 02:23:30 am
content: I’m in USA. Is it normal to measure 118v on the outer metal chassis of an audio amp, in reference to a wall outlet’s ground hole?

author: David Hess
author_rank: Super Contributor
timestamp: September 25, 2017, 03:16:41 am
content: No but make the same measurement with a 47k resistor across the multimeter to see if it is from capacitive coupling. If a 3 wire power connection is used, it should not matter because any capacitive coupling should be sunk to ground.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: September 25, 2017, 05:59:38 am
content: This is a common thing back in my country. We only had 2 prong plugs LIVE and NEUTRAL which is ground referenced and once we reverse the plugs because of sheer carelessness, we start to feel on appliance chassis just the slight 220V shock

author: macboy
author_rank: Super Contributor
timestamp: September 25, 2017, 01:00:24 pm
content: It is common in the USA/Canada to have a two prong plug on things like audio amplifiers. The metal chassis is also commonly connected to the neutral (should be ~= ground) with a high value resistor in the megaohms. If the plug is reversed then this will connect the chassis to live instead. In that case, the current would be limited to a nominally safe value but you might feel a buzz/tingle.Does your amp have a polarized plug (one wider prong)? If NOT then reverse the plug and try the test again. If so, then check whether the outlet is wired correctly. The narrower prong is the live, the wider one is neutral. You should measure around 120 V between live and ground, and around zero between neutral and ground. Please use a safe multimeter and safe technique when measuring mains, and avoid ever measuring anything at the panel.

author: g.costanza
author_rank: Contributor
timestamp: September 25, 2017, 01:31:29 pm
content: So place one lead on amp chassis, the other lead on the ground at the outlet and then wire a 47k ohm across (shorting) the leads? Watch voltage? Is there any chronological order to doing this? THX

author: g.costanza
author_rank: Contributor
timestamp: September 25, 2017, 01:32:34 pm
content: It is a 2 prong plug with 1 wide blade and 1 narrower blade. Ive verified outlet is wired correctly.

author: g.costanza
author_rank: Contributor
timestamp: September 25, 2017, 01:38:54 pm
content: The voltage drops to 14v with the test leads shorted with a 47k resistor

author: David Hess
author_rank: Super Contributor
timestamp: September 25, 2017, 05:06:34 pm
content: Quote from: g.costanza on September 25, 2017, 01:38:54 pmSo it is not directly connected and it represents 300 microamps of leakage. That could be from 350k of resistance between hot and the chassis or 7500 picofarads of capacitance which is about right for a Y class capacitor.Since you verified that the outlet is wired correctly, maybe the plug was wired wrong. I would reverse the plug if feasible and run the 47k test again.Quote from: macboy on September 25, 2017, 01:00:24 pmHasn't the standard for a long time been for 2 prong connection to have the chassis insulated from hot and neutral?

author: g.costanza
author_rank: Contributor
timestamp: September 25, 2017, 05:57:59 pm
content: So nothing will be harmed if I plug it in backwards?

author: David Hess
author_rank: Super Contributor
timestamp: September 25, 2017, 09:07:40 pm
content: Quote from: g.costanza on September 25, 2017, 05:57:59 pmNo, nothing will be harmed or if something is harmed, it indicates that there was a major problem. Your test with the 47k shunt resistor indicates that nothing major is wrong and at most there is a leakage issue.