Electronics > Beginners

Understand Ac Current and Grounding Chassi

(1/2) > >>

AngraMelo:
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)

bjbb:
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.

AngraMelo:

--- Quote from: bjbb on October 11, 2019, 03:59:27 pm ---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.

--- End quote ---

 I 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)
?

magic:
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.

AngraMelo:

--- Quote from: magic on October 11, 2019, 06:37:47 pm ---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.

--- End quote ---


Thank you! I understand it now!

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod