Electronics & Appliances Electrical grounding

Isn't grounding primarily done for safety reasons, since Electricity always takes the path of least resistance. Isn't the hope that if there is short it goes to ground through the neutral line, instead of through your body? Lots of appliances that don't draw much current seem to use 2 prongs instead of 3, but not sure the reason for that. How does having a ground reduce noise/static/feedback in things like stereos and speakers? Is it because it helps to eliminate electric fields which can be picked up. I'm a mechanical engineer so the electrical engineer force is weak in me. Just curious.

 

Grounding is primarily for safety, but to say electricity takes the path of least resistance isn't exactly correct. It would be better to say that more current will flow on the path of least resistance but there is still current on parallel paths of resistance as well. Look at a receptacle circuit as an example, from panel box to receptacle all metal boxes and EMT conduit. If you had a direct short from hot to the metal box at the receptacle, current will flow between the metal yoke of the receptacle to the grounding conductor and trip the breaker. Copper is a better conductor than steel so you would assume all the fault current would travel back on the grounding conductor. But now look at the conduit itself, it is also a relatively low resistance path due to the surface area of the conduit (taking in account both inner and outer) so fault current will also flow on the conduit system. Another example, it I took a piece of romex and connected the hot wire straight to the bare copper wire and turned the breaker on it would trip the breaker; but would you be comfortable holding onto that bare copper ground wire for the short period of time it would take for the breaker to trip...if you had on a pair of insulated shoes perhaps, if you were barefoot in a puddle of water on a concrete floor not so much.

If a short is between hot and neutral then fault current would travel back on the neutral, if the short is between hot and ground then the fault current travels back on the grounding conductor (and/or the conduit system in the above scenario). Many hand tools for example are double insulated so they do not need a grounding conductor to operate safely. Other appliances do not require a ground wire by manufacturer (UL listed for us in the US) because of the construction of the device the chance of getting shocked by touching has been tested and approved. As many already know, most homes in the Philippines are not properly grounded and their appliances usually are just 2 prong. My refrigerators and microwave for example have a 2 prong plug with a grounding conductor just hanging there (goes to the case of the appliance). Where this grounding conductor is supposed to go on an improperly grounded system is a guess, but I jam the wire into the ground pin on the receptacle as my house is properly grounded. If this wire falls out I will feel a "tingle" if I touch my microwave (I still need to replace the plugs with grounding types).

I'm not in anyway an electronics person but any device that switches AC to DC you will introduce harmonics onto the system which is basically a different frequency induced onto your neutral, this may cause noise. Many electronic devices now a days have filters built into the electronics to minimize these impacts. Other issues arise when your ground and neutral are bonded other than at your service entrance panel (receptacles, subpanels, etc).

Static can also be present. For the OP I would guess static could be a cause; touching the case of the computer you could bleed off the static. I would try that to see if the noise goes away. If it does there are a few things to do to bleed this off. You could chip out to find some rebar in the wall (rebar in concrete is a very good ground, ufer ground), the long nail in concrete could work and a driven 1 meter ground rod bonded to the case would also work for static.

The best set-up would be to properly ground your panel as I stated previously and run a grounding conductor to your receptacles and change out with 3 prong receptacles. It is not something that is too terribly expensive to do, but if you are renting it may not be an option unless you go out of pocket.

Shawn

 

ShawnM's drawings are a great example of how the GND is common to the Neutral when it comes to house wiring

 

That's is why you need that third wire. to bleed off stray voltage from the chassis. My fix was to attach a wire from the computer chassis to a 3" concrete nail driven into the concrete wall next to the outlet. It worked for my computer which did the same thing you described. I also did the same for my Refrigerator as the new Inverter style Ref's use a controller circuit board to manage the Inverter. As a cautionary note. I learned after I purchased my Ref that Inverter style A/C and Refs are easily damaged by the Power outages we suffer from here. The old style Refs don't seem to mind the power surges and fluctuating voltages as much.

 

Pat, Try unplugging, rotating the cord cap 180 degrees and replugging. Even though we are dealing with AC, there could be a polarity issue.

 

I have seen this many times in stereo and home theater installations when components are plugged into different outlets.