Understand Ac Current and Grounding Chassi  - Page 1
Forum section: electronics / beginners

author: AngraMelo
author_rank: Regular Contributor
timestamp: October 11, 2019, 03:22:43 pm
content: Hello everyone!Ive been watching a lot of videos by Mr Carlson's lab and when he is repairing old radios, specially those with metal chassis, he talks about the importance of having a polarized plug so the neutral gets tied to the chassi and not the live.Im struggling to understand that. Even though the neutral is at ground potential (usually tied to earth on the house's electrical board) given that we are talking about Ac, wouldnt the neutral also present mains voltage potential every semi cycle of the Ac line? The thing is, I dont understand the difference of tying the neutral to earth or the live to earth given that the voltage potential and current direction keeps changing.Well, I live in Brazil, specifically in my city we have 220Vac and we dont have a neutral. We have 2 lives. Im sure of that because I have a LowZ meter and it measures around 127Vac on each wire relative to ground (earth)

author: bjbb
author_rank: Contributor
timestamp: October 11, 2019, 03:59:27 pm
content: Problems await those that make assumptions about equipment safety grounds and building wiring. Per your local code and electrical equipment requirements (see INMETRO), most residential appliances do not require certification - it is voluntary. But there are many components and materials found in residential construction and appliances that do require mandatory certification. Also, look at NR10 and note that this regulation scopes wiring and operations for commercial buildings and grid interface; so no references for residential stuff. And there are some regional building codes that are similar to the (U.S.) NEC article 250. So there can be significant variance in the implementation and interface to the power grid in Brazil.The terminal marked for 'safety' earthing in an appliance cannot necessarily be considered the be the neutral equipotential. The requirement to join 'neutral' to the building's ground bond is not universal in Brazil.As for half of the mains voltage being posed on the chassis, this could happen in areas where the 127V comes from an improperly referenced center-tapped 220V system.Brazil has three AC plug forms - Types A, B, and C. Types A and C are not polarized, so should be used only with Class II construction. Class II equipment has double or reinforced insulation between hazardous voltage and stuff that can be touched, so grounding is not a safety issue. Type B is a three-prong plug having a separate earthing pin and is intended for Class I construction. The safety of Class I equipment is dependent on the building wiring - it requires that the neutral wire is properly referenced to the building ground for the secondary distribution system.

author: AngraMelo
author_rank: Regular Contributor
timestamp: October 11, 2019, 06:07:34 pm
content: Quote from: bjbb on October 11, 2019, 03:59:27 pmI appreciate the complete answer but I think my question was missing some information.Lets forget that I live in Brazil, why is it safe to tie the neutral to the chassi but not the live if we are talking about Ac current? wouldnt the neutral have full 110Vac voltage potential for each half cycle of the wave? So why is it considered important to have a polarized plug or discern between live and neutral when tying one of those to the chassi?Also, when I referenced in my questions that I live in brazil (I should have kept writing and explained why I wrote that) I meant that if I buy a radio that has a polarized plug, and was meant to be used in the US, also having the neutral tied to the chassi, wouldnt the chassi become live in any place (country, city, region...) where the neutral is not actually a neutral but another live (for example my city, where we have a earth connection and 2 live with no neutral)?

author: magic
author_rank: Super Contributor
timestamp: October 11, 2019, 06:37:47 pm
content: If you have a true neutral, buried in ground, there is obviously no voltage on it because all current that finds its way there flows straight into ground. Live goes positive for half of the time and negative for the other half, neutral stays zero.Connecting equipment chassis to neutral was sometimes done in old times but it isn't entirely safe and is prohibited by safety regulations in most of the world these days. Problem is, if the neutral cable breaks then current from all the appliances has nowhere to go and the disconnected neutral becomes live.The proper way to ground chassis is through the third "protective earth" conductor.

author: AngraMelo
author_rank: Regular Contributor
timestamp: October 11, 2019, 06:56:42 pm
content: Quote from: magic on October 11, 2019, 06:37:47 pmThank you! I understand it now!

author: rstofer
author_rank: Super Contributor
timestamp: October 11, 2019, 09:09:51 pm
content: Quote from: AngraMelo on October 11, 2019, 06:07:34 pmIn the bad old days, it was common to tie the neutral to the chassis but there was also a time before polarized receptacles. I suspect they became common in the '50s. There are a lot of homes in the US that predate polarized receptacles.So, given that one wire is tied to the chassis, you need to deal with the situation that it is the hot wire. That works safely as long as the knobs are on the controls and the back cover is in place. However, when you work on the device, you probably open the cover. I got knocked on my butt touching the shaft of a radio control back in the mid '50s. The plug wasn't polarized (the receptacle may have been) and the chassis got 'hot'. That's why they talk about 'hot chassis' appliances.These days, the receptacles are likely grounding (3 pin) and older houses probably have at least polarized receptacles. Here's what you need to know about 2 wire cord. One of the conductors will have a couple of 'ridges' that identify it. The 'identified conductor' is the neutral - always. For a table lamp, this identified conductor would connect to the lamp holder shell and the wide blade on the plug.Now that we use mostly low voltage electronics, there is almost always a low voltage DC supply involved - maybe something like a wall wart. No issues with 'hot chassis' with this kind of equipment.Remember, the neutral is tied to earth ground at the pole and very probably at the home service entrance. The neutral should always be very close to zero volts. Only the phase conductor is 'hot' to ground.

author: magic
author_rank: Super Contributor
timestamp: October 12, 2019, 08:01:02 am
content: Quote from: rstofer on October 11, 2019, 09:09:51 pmHmm, I thought that "live chassis" concerned vacuum tube devices which simply rectified mains to get a 300V supply and grounded themselves to the negative output of the rectifier, which is "hot" for half of the time. Kinda like the primary-side heatsink in a computer or laptop PSU; been there, touched that

author: rstofer
author_rank: Super Contributor
timestamp: October 12, 2019, 03:33:04 pm
content: Quote from: magic on October 12, 2019, 08:01:02 amCorrect! We haven't seen this kind of thing since a) tube type audio equipment started using transformers and b) semiconductors took over the world. Things were pretty lax back in the '50s and I guess they figured Darwin would separate out those who understood the problem from those who didn't.