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Erath and power-down.

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alphadog

Banned
As i wrote in earlier post, I have a PCB which its 0V (its GND) is the Neutral wire, and the PCB is floating, meaning its ground (the Neutral wire) is not connected to earth wire on PCB (actually they are only connected through MOV, but it's still considered floating, right?).

I have a scope which is connected to Earth, and I wanted to connect a prob from the scope to the PCB.
I connected the GND pin of the Probe to the 0V (Neutral wire) of the PCB, and the (+) pin of the Probe to some resistor.

When i turned on the PCB, the power has failed.

So i disconnected the Earth wire of the scope, and then the power didnt fail.

Why did it happen?

Thank you very much.
 

dknguyen

Well-Known Member
Most Helpful Member
You have to be careful when hooking things that are unisolated wall-powered devices to unisolated scope probes because they are using the same reference voltages.

For example, something AC will work whether or not you mixed up the Live and neutral. You might end up accidentally calling the live wire neutral since it doesn't really matter for circuit functionality. BUt when you hook this so called "neutral" to the probe's neutral (which is actual neutral) bad things can happen. This can also be dangerous when working with devices that are supposed to be isolated for safety since connecting them to the scope can remove the safety effects of isolation (by connecting the isolates side to the actual neutral/earth from the wall).

Although, at least for me, without your circuit, I don't know why that happened. What can happene is something that you called neutral on the board wasn't actually neutral and is actually required to float relative to neutral to work and when you connected it to earth through the scope's earth connection it changed the functionality and made it stop working.

Did you say you connected the ground/neutral of the PCB to the neutral of the scope? or the earth of the scope? A scope can seem to measure a circuit whose reference is neutral to the wall with just the positive probe because the neutral is connected in the wall.
 
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alphadog

Banned
Thanks for the concern, i take my precautions before each experiment.

As you know, a probe has two pins, one is the (-) and one is the (+).
When the Scope was connected to earth (meaning its plug to wall had 3 pins), And when I connected Pin (-) to PCB neutral and Pin (+) to PCB resistor, The power Failed.

When the scope was not connected to earth (meaning its plug to wall had 2 pins), And when i connected again Pin (-) to PCB Neutral, and Pin (+) to PCB resistor, the Power did not fail.
 
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ccurtis

Well-Known Member
What do you mean when you say the power failed? Did you see sparks? Did breakers trip? Did a GFI outlet go active?
 

alphadog

Banned
I mean that the switch in the power box has went down, and i needed to bring it up in order to have the power back in the room.
 

ccurtis

Well-Known Member
The only explaination I have is that the N and L to your circuit is swapped. With the scope ground connected, the - probe is also connected to ground and then when you connect the -probe to your circuit's N you are really connecting it to L, thus shorting the mains.
 

alphadog

Banned
Are you sure?
Because the only difference between the two tries was 'scope with Earth' and 'scope without Earth', there was no polarity difference.

What causes the power to fall down?
is that:
- Current flows through Earh wire?
- too much current flows out of L wire?
- too much current flows to N wire?
 
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ccurtis

Well-Known Member
Are you sure?
Because the only difference between the two tries was 'scope with Earth' and 'scope without Earth', there was no polarity difference.

What causes the power to fall down?
is that:
- Current flows through Earh wire?
- too much current flows out of L wire?
- too much current flows to N wire?
Of course, I am not sure. I can only judge from the input you give. My explaination takes into account the difference you describe between when the scope ground is connected and when it is not connected. My explaination concludes that the current is flowing from L to Earth ground. Remember too, that Earth ground and N are tied together at the breaker panel.
 

alphadog

Banned
Of course, I am not sure. I can only judge from the input you give. My explaination takes into account the difference you describe between when the scope ground is connected and when it is not connected. My explaination concludes that the current is flowing from L to Earth ground. Remember too, that Earth ground and N are tied together at the breaker panel.
Thank you.

So you say that the reason that the power falil down in our homes is that current flows from L to Earth?

If it is, then why the power also falls down when you have no Earth connected to circuit?
Is it just because that in some places the Earth and N are connected inside the wall as you said?
 

Hero999

Banned
I power my scope from an isolation transformer so it doesn't need earthing.

I also power the device I'm testing from a different isolation transformer.

This makes it impossible to get a shock by touching the scope's case.

I've not modified the scope's mains lead, it still has the earth connected, the socket on the isolation transformer doesn't have an earth connection.
 

dknguyen

Well-Known Member
Most Helpful Member
I power my scope from an isolation transformer so it doesn't need earthing.
You would think that would be standard on scopes. But instead they have isolation probes that cost a fortune...though they also have their uses like if you want to measure two different signals with two different references.
 

ccurtis

Well-Known Member
So you say that the reason that the power falil down in our homes is that current flows from L to Earth?

No. The soil has an appreciable amount of resistance so I doubt that plugging a live wire into the soil is going to result in much current flow (but it could easily result in enough current to kill). If the N and Earth are not connected inside the breaker panel, it is unlikely you can power-up much of anything using the Earth ground, alone, as a conductor. Otherwise, the N wire wouldn't be needed. However, the Earth ground runs as a low resistance copper wire connected to N and to the Earth at/near the service panel.

The Earth, itself, is not a reliable low resistance, zero potential point for powering applicances, but it is a zero potential reference point relative to any non-zero voltage node anywhere on the planet (or so I was told).
 
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dknguyen

Well-Known Member
Most Helpful Member
I think the earth is a zero voltage reference only if no current is flowing (because it's too high resistance and a voltage drop would form). But a resistor, no matter how high has the same voltage at both ends (zero voltage drop) if the current in it is zero.
 

ccurtis

Well-Known Member
The notion of the earth as a zero potential reference is something that I never really considered independently. Out of curiosity, I found that the earth, as a whole, including the atmosphere has a neutral charge. So, in that sense, the earth as a whole is not a reference point at all since there can be no current flow from a charged body (a voltage node) to a neutrally charge body, thus no potential difference between them.

The solid earth, alone, is negatively charged (electrified). This is balanced out by the positively charged atmosphere concentrated mostly in the ionosphere, so the two "plates", combined with the dielectric air between the two, can be considered a poor capacitor across which electrons from the solid earth continually leak back into the ionosphere. Lightning strikes are the electrical generators that keep pumping the leaked electrons back to the solid earth, maintaining a huge charged capacitor with about 300,000 volts across the "plates".

So, I would think, the solid earth, by itself, having a huge accumulation of electrons (negative charge of about 500,000 coulombs) should exhibit a potential difference (voltage) against a man made body with a lesser negative charge, or positive charge. I would think. Theoretically. I cannot say that it is a zero voltage reference, though. Hmmmm.
 
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