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keeping distance from AC mains.

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alphadog

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In my PCB, the AC_Neutral wire is the ground of the whole PCB (My power supply which is fed from the AC mains is non-isolating, therefore the AC_Neutral is the ground of the whole PCB).

I know that i need to keep the AC_Live away from any component in a 4mm radius (also from ground).
My question is, do I need to do that also for the AC_Neutral wire?
Cant keep components near him, which have less than 10V falling on them (related to ground = AC_Netural).

Thanks.
 

alphadog

Banned
And a second question if you allow me please.

Why do we place a MOV not just between LIVE and EARTH, but also between NEUTRAL and EARTH?
The NEATRAL and EARTH are connected to each other inside the wall, so how can be a potential difference between them?

Thank you.
 

ccurtis

Well-Known Member
Certified equipment cannot rely on the neutal to be tied to ground, nor can it rely on the neutral not being "live". Hot and neutral are all too often swapped, causing potential harm to the user.
 

dknguyen

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Most Helpful Member
No current is actually supposed to flow through earth when things are working properly. When we say "ground" in all our circuits we should actually be saying neutral. Neutral is where all the return current of the system flows through. Earth is a connection that is *ideally* at the same voltage as ground but carries no current (it's a dead end connection).

I'm not sure if this is entirely accurate, but I think about it this way: Neutral is at the same voltage as the earth at the POWER PLANT. And the live voltage is referenced to this voltage since it's generated at the power plant. After a long distance (ie. to your house) the voltage at the ground of your house and the power plant might be quite different...possibly dangerously different. Earth is the voltage of the ground at your house and so Earth (ground at your house) and neutral (ground at the power plant) might be quite different And the fact that current flows through neutral also makes it unsafe since, among other things, that can raise the voltage above what it's ideally supposed to be.

So, now that we've established earth is at the same voltage as the ground you are standing on when your near the plug, if you touch earth you should be fine. So what happens is earth is connected to the metal case of the equipment. Suppose a live wire (or neutral wire of high voltage) goes loose inside and touches the case. If you touched the case and it was not connected to earth the high voltage would flow through you to the ground. If the case is connected to earth the two of you form a parallel resistance connection, but since the connection of the case is a much lower resistance than you are, most of the current takes that path instead of through you (the lower resistance dominantes a parallel connection of resistors).

Think of the neutral wire as a really crappy high resistance ground wire. It should be the same voltage everywhere, but it's not so you stitch it to earth at different points. BUt for the safety reasons described above, you split it into two separate wires- one "local voltage reference" that carries no current, and one reference for the distant power supply that actually does carry current.
 
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Boncuk

New Member
No current is actually supposed to flow through earth when things are working properly. When we say "ground" in all our circuits we should actually be saying neutral. Neutral is where all the return current of the system flows through. Earth is a connection that is *ideally* at the same voltage as ground but carries no current (it's a dead end connection).
You might want to find out if it is really a dead end connection.

Just short life to PE (protective earth) and you'll see how "dead" the connection is. :D

If it were a dead connection it wouldn't make sense to use earth for safety reasons.

Boncuk
 

dknguyen

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Most Helpful Member
I was meaning more along the lines of the earth connects to the metal case, and that's it- it doesn't continue to anywhere else so that current from somewhere else is flowing through the case, into the earth under normal conditions.
 

alphadog

Banned
I really thank you all a lot!!
Thank you dknguyen for the detailed answer!

Could you help me out please with the clearance issue?
 
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dknguyen

Well-Known Member
Most Helpful Member
Why is neutral running all over your board? Does your board not run off of DC? Or use a AC in a significant number of parts?

If you look at a full bridge rectifier, you'll see that DC ground is not at the neutral voltage because of the way it works (it may be for a half-bridge rectifier).
 
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Sceadwian

Banned
I'm not sure where you're from Alphadog but neutral and ground should most definitely NOT be connected to each other in the wall.
 

tcmtech

Banned
Most Helpful Member
Nutral and ground are tied together at the fuse/breaker box.
That is the ONLY place they should be connected togther!
 

Sceadwian

Banned
Hmm, thought neutral/ground was supposed to be separate even there, shows what I know about power electronics =)
 

tcmtech

Banned
Most Helpful Member
Some are some are not. 2005 NEC code book I have says to do it. ;)

Years ago that was different. If the common line went bad and became shorted to the ground lead there was a strong posibility that the ground rod would not be able to pull the load down to a safe or stong enough level to trip a breaker or blow a fuse. :rolleyes:
Then all of your grounded appliances would actualy get voltage fed back to them from the bad ground system. :eek:
Now they connect them at the box to make sure that cant happen. :)
 
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