Computer PSU and earthing

[Styx]

right someone said something that got me a bit worried so just posting here

bear in mind that this is for a USA based computer


He said that in his flat there is no EARTH connection so he has taken the NEUTRAL and used that on his PC case.

Now this gets me a bit concerned.

this is what I think:

For starters that is bypassing the isolation from the PSU

Also what is to stop the case (now at NEUTRAL) to floating to a potential above EARTH so if you touched it and also something at EARTH you would get a shock! (could be wrong since it is US - clarify anyone??)

The point of connecting CASE to EARTH is to protect if there is a fault in the PSU such that the case goes LIVE. having an earth leakage breaker would mean that as the case was going live (not fully yet) it would trip out the breaker. This method it would have to reach a potential to allow enough current to flow to blow the fuse??

I dunno it just sounds wrong?


--EDIT--

nvm got him to see sense, Pointed out the isolation aspect. I was really starting to doubt myself on this one,

posted here I guess for some re-assurances...

 

[stevez]

I think you are correct in being concerned. The neutral is a current carrying conductor and in a normal state it will be at some potential above/below "earth" though it should not be much. If for some reason the neutral line broke "downstream" of the computer the case would then take on the potential of the "hot". I'd urge your pal to consult with someone with a better knowledge of power systems for a better understanding. I suspect the "earth" connection might also help with transients, noise, RFI, etc.

With the help of some electricians and electrical engineers I've learned that the rationale for the electric codes (rules, laws, whatever) takes into account the normal operating mode as well as potential failure modes. We/they learned about many of these failure modes the hard way - it actually happened and killed or injured someone. Other codes and rules have a similar basis - and it's not always obvious even to someone with a fair degree of technical knowledge.

 

[Dngrsone]

All of the above is correct. In most instances, his method would work fine, however, as noted, certain failure modes (in wiring or misbehaving appliances) could cause a live case condition-- not happy.

Your friend can make a ground for his triple-prong outlet by running a seperate ground wire to a pipe (if the household plumbing is galvanised) or to a ground stake driven into the ground at least two feet deep. Local codes would specify exactly how deep for your area.

 

[fsahmed]

Er..... isnt ground usually shorted with earth?

 

[Dean Huster]

Styx, here in the U.S., connecting neutral to a case is a severe NEC violation. If you look at any 2-wire appliance in your home (mixer, toaster, fan, light, sewing machine, etc.), you'll find both hot and neutral are isolated from any (ANY and ALL) externally-available parts. They've even started polarizing 2-wire line cords just to make it even better (the wide blade - neutral - on a table lamp is connected to the shell of the lamp socket, which is more prone to being touched first in most situations).

Newer appliances are now grounded with a 3-wire cord, such as washing machines and refrigerators. Even our 1978 model Bernina (Swiss) sewing machine has a 3-wire grounded cord. If it isn't double-insulated, there should be a ground connected to the case of the appliance.

Now, Nigel doesn't ground anything (I don't think -- well, maybe if he has a disobediant child ...) and his shop is an orchestra of all kinds of leakage voltages on the cases of his various instruments and appliances. But I'll bet he doesn't connect the cases to neutral, either! Not grounding a case or cabinet is one thing. Connecting it to one side of the line is disaster. With most 2-wire cords, simply reversing the plug in the receptacle will instantly make such an appliance a killer.

The only time that ground is a killer is when you're 1500 feet above it with a flameout.

Dean

 

[Nigel Goodwin]

Now, Nigel doesn't ground anything (I don't think -- well, maybe if he has a disobediant child ...) and his shop is an orchestra of all kinds of leakage voltages on the cases of his various instruments and appliances. But I'll bet he doesn't connect the cases to neutral, either! Not grounding a case or cabinet is one thing.

Somewhat of an exaggeration :lol:

EVERYTHING domestic is grounded correctly, and in the UK of course we don't have two pin plugs, so every socket is grounded.

The only things NOT grounded are test equipment, in a workshop environment, where grounding them would be more dangerous than floating them.

It's CERTAINLY a VERY BAD idea to connect a PC case to neutral, it MUST have a three core lead and plug and be properly grounded.

The only time that ground is a killer is when you're 1500 feet above it with a flameout.

Or when you're connected both to ground and a live wire :lol: