what does resistance have to do with grounding?

[Caroline]

Hey guys,

I'm a complete moron when it comes to wiring, but I am trying to learn. So, I thought someone here could help me. Here's the situation. I've built a metal rack with an electronic control system. I've grounded the power supply to a painted metal housing that sits on the rack. I occasionally get a little shock, but I've been having trouble telling if the rack (which is on rubber wheels) is picking up a charge somewhere, or if it's just static electricity.

So, my boss pulls out the ohmmeter and we go about trying to figure out if the rack is grounded. For the most part we had a resistance of 0, and somehow this means the ground is working. We tried with one wire on the rack and the other on copper pipe, and also between two different racks.

I really don't know what all of the means. You either get a resistance, or OL (overload?) or 0. Can someone explain this to mean? Is there any info out there for teaching a dummy like me the real world basics of wiring? I can find plenty of info on current and resistance, but I have problems translating that into real world situations.

Thanks,

Caroline

[mikebw]

What kind of voltages are you dealing with here? This is potentially dangerous and you may need to consult a professional, but let me try and help.

The ground wire should provide the path of least resistance to earth ground. Paint is an insulator so you don't want to connect a grounding wire to a painted surface.

Zero ohms means no resistance, and on many digital multimeters "OL" or overload also means zero resistance.

If the shock you are getting is from static electricity that would mean you have a difference of potential between your body and the metal rack.

I would make sure that the power supply is grounded properly first, then the metal frame of your rack is also connected to ground. There should be zero (or very little) resistance between the metal case and your grounding wires.

[davester]

The resistance only gives an indication of how good an electrical conductor lies between the two ohmmeter probes. If the resistance is zero, then there is a good conductor between the probes. If it is infinity, then no electrical current can flow between the two probes. All the resistance meter did was verify that your "ground" wire was electrically attached to the rack. However, it doesn't sound to me like your device is grounded at all. If the rack is on rubber wheels and there is no metal conductor (i.e. wire) going from an unpainted part of the frame to the grounding pin of a grounded outlet (which will be grounded to earth if installed properly, then your gizmo is not grounded. That may be why you are getting zapped. You are the path of least resistance to ground. In any case, a ground is generally a safety feature that doesn't get used unless there is a fault in the device being grounded. Sounds to me like you have some issues. This could be serious if you are playing with line voltage.

[Dick Moore]

Your power supply has a 3-prong power cord connector, yes? If not, fix that, stat. If so, the power supply and anything connected by wire to its chassis is grounded to the wall socket ground and is grounded IF the wall socket is, in fact, grounded (most are, but it never hurts to check).

Static charge build-up will give you a shock, especially if you are wearing rubber-soled shoes. Occasionally, electronic gear will have bypass capacitors from the AC line input connections inside the gear to its chassis ground, and the low level of leakage current through these caps can often give you a "tickle" but is not usually dangerous.

[freeradical]

[quote Caroline]Hey guys,

I've grounded the power supply to a painted metal housing that sits on the rack.

Caroline
It should be grounded to bare metal. I'd ground the chassis of each piece of equipment to the same common ground.

Something like this will tell you if your wall outlet is correctly wired.

http://www.tripplite.com/products/static/ct120.cfm

[RAMd®d]

"Grounding" an item to a rack (painted or not) on rubber wheels, generally is *not* grounding.

First, as previously asked, do you have "three-prong outlets", that is electrical outlets for three-prong plugs?

[cbelt3]

Caroline, first let me say from all of us that we admire your bravery at attempting this without detailed training. We were all there once. fear not.

Some basic terms:

Grounding- (also known as Earthing in the UK) Is when you connect the outside surfaces of your metal electrical gizmo to the planet we live on.

Bonding- When you connect the outside surfaces of your metal electrical gizmo to each other

Double Insulated: The electricity on the inside of the gizmo cannot possibly connect to the outside of the gizmo.

UL and CSA certified electrical gizmos are either double insulated or well bonded and grounded. Most gizmos are grounded through the 'ground pin' on the power plug. Some gizmos may actually have a separate 'grounding terminal' which is typically a bare copper terminal where you attach a bare copper grounding wire. If the electrical rack is UL or CSA certified, the entire rack should be bonded to itself and grounded, usually through the main rack power strip, or through a separate grounding wire.

When you install a gizmo into a metal rack, you need to bond the gizmo to the other gizmos, and to the rack. Some racks do this with a rack power strip (everything in the rack plugs in there, and the ground plugs connect each component to the rack) and/or through bare metal mounting screws and screw holes.

A wheeled rack that is properly grounded and bonded won't be able to keep a static electricity charge. YOU, on the other hand, have the ability to store tens of thousands of volts of static electricity as you walk around your heated, dry office. You may be experiencing a static discharge, just like touching the metal lightswitch plate gives you a shock on a winter day.

[Caroline]

Hi Guys,

Thanks for the explanation. I've neglected to mention that the ground wire is also connected to the ground of a three prong plug. So the ground runs from various electrical components, to the painted housing as well as the plug going to the wall outlet. I know some three pronged outlets are not truly grounded, but the ohmmeter indicates that the ground is active (at least according to my boss).

The issue of grounding to a painted housing has come up before. That's why we checked the rack with the ohmmeter. However, the results would indicate that the painted metal does in fact work.

Ok, here is another question: should I be concerned about mixed metal on the rack? So, the rack is stainless steel, but there are several aluminum items with stainless steel parts (some of which are wired) sitting on the rack. There is a small resistance being picked up from the stainless steel parts of these items. It's barely measurable, but seems to be real. What does this mean?

Caroline

[OWC Jamie]

Next time you get a shock, instead of letting go, hold on.

If you feel 50/60 hertz (and you will know it) you are getting a voltage shock rather than a static shock.


:-)

[BigGuynRusty]

[quote mikebw]Zero ohms means no resistance, and on many digital multimeters "OL" or overload also means zero resistance. You just failed my "Basic MutiMeter Class" mikebw!!
Zero Ohms is Zero Ohms.
~BUT~
OL means INFINITE Resistance, NOT Zero!!
To put it in other words.:
Zero Ohms is a "Short".
Infinite Ohms is an "Open".
I don't know of one Digital or Analog meter where "OL" means Zero Ohms while reading resistance!

BGnR