Where should this earth grounding cable go?

[giosif]

Hi,

Sorry for the newbie question but I've opened up this power supply and now I'm not 100% sure which of the three connectors seen in the picture this earth grounding cable should connect to (actually, only two out of the three connectors, as L is out of the discussion, of course).
I mean, I am pretty certain it should go to N, but then is that a safe thing to do, as it is basically shorting neutral with earth ground?

This is a power supply taken from an oscilloscope I'm trying to repair.

Is that maybe related to what Dave was mentioning in his vblog titled “How NOT To Blow Up Your Oscilloscope”?

Thanks,
George

[malloc]

Hi,

the first on (L) - thats phase/your line wire; the second one (N) - thats your neutral wire. And the third one - thats your earth wire which is where your grounding cable should connect to.

That being said - you can short your N with earth - but I don't recommend it undless your power-outlet doesn't provide a connection to earth.

[Zero999]

The yellow & green wire should go to earth.

Live is brown and neutral blue, according to the UK/EU colour code.

[Monkeh]

That being said - you can short your N with earth - but I don't recommend it undless your power-outlet doesn't provide a connection to earth.

No, you really can't.

[Tandy]

The UK mains supply the neutral line is tied to earth at the distribution transformer, however an RCD will trip if you connect the Neutral to to earth in your home.

[giosif]

I should have mentioned this cable is on the same PCB trace as the ground connector.
So, I don't see the reason why you would connect the cable to a connector to which it already has a circuit path.

So, the only connector left is N but, then, why would you want to do that, especially since, on the back of the oscilloscope case, it is written:

Instrument must be grounded (earthed) via protective ground conductor of power cord.

Is that to make the power supply a mains earth reference power supply?

[Monkeh]

That wire is almost certainly meant to connect to something else. Probably a stud on the device chassis.

[malloc]

That being said - you can short your N with earth - but I don't recommend it undless your power-outlet doesn't provide a connection to earth.

No, you really can't.

Mhh - you're right (of course).

I agree - don't do it. (So - sorry for that suggestion - it IS dumb)

[giosif]

That wire is almost certainly meant to connect to something else. Probably a stud on the device chassis.

You are completely right.
I've just realised that the metal case in which the power supply PCB goes has a screw within reach of this cable.
And I now remember that cable was connected to that screw, rather than any of the three connectors.

Sorry for the misleading question in the first place!

Thank you all for the help!

[KM4FER]

I've seen this question come up quite often. 

Here's one short video that I think provides a pretty good explanation of the purpose of neutral and ground.

http://youtu.be/k3OHzKz0qNc


earl...

[giosif]

Thanks for the link, km4fer!

[lapm]

That being said - you can short your N with earth - but I don't recommend it undless your power-outlet doesn't provide a connection to earth.

Now this is bad advice... Depending on how you plug in your equipment you can have live wire voltages on chassis and that's just DANGEROUS FOR LIVES... 

[giosif]

i think i can see the PCB trace from E ... going to the chasis screw pad on the left side. if you screw down that, put the casing together you could check with DMM is the chasis is indeed shorted/connected to earth by default thru the PCB ... and that nothing else L/N have some kind of short to earth?

The cable was/is now connected to the screw going through that screw pad.
So, it is providing earth grounding to the case of the power supply.

[cybertronicify]

Here in United States, if you short the Neutral to Earth, it will trip the GFCI every time you draw current from the mains. If you have a house not fitted with GFCI in rooms, you may not have the problem but it is very dangerous to do so. current will go from HOT to EARTH and the case of the device will become live and you will get shock if you touch a metal part of the device.