Additional Grounding for ESD reduction?

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jay80424

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Hi there,

We have a metal equipment rack in a project, full of AV gear. It's a Middle Atlantic for anyone familiar. This house producs a tremendous amount of static electricity (dry air, thick carpet), and we are starting to see problems with the operation of the equipment when someone walks across the carpet and touches the equipment, as a static shock is discharged.

Is there any way to bolster the ground and direct this static energy away from the equipment with the exiating wiring?

Right now, all the equipment plugs into a surge supressor, that is plugged into a dedicated 15amp duplex.

There is no way to add a ground rod, or tap a cold water pipe.

Thanks for any input. If I know it can be done, i'll call an eletcrician.
 
Adding additional grounding to the equipment or the equipment rack won't help at all, and could potentially make the problem worse.

The equipment is already grounded. When someone walks across the carpet the they get charged up, and this charge then dissipates from the person _through_ the equipment and then to ground.* If you improve the grounding of the equipment all you do is reduce the resistance of the path from the equipment to ground; the charge is still going _through_ the equipment.

What you need to do is provide some path to ground that circumvents the equipment, and you need to also make sure that the current flowing to ground is not going via signal wires but instead via the intentional grounding conductors.

You should also attack the source of the problem: humidifier for the air and carpet treatment to reduce static charging.

-Jon

*(Note on 'dissipating to ground'. Many here, myself included, try to fight the misconception that electricity is trying to 'seek ground', and replace it with the reality that electricity is trying to get back to its source. This is essentially true for any power distribution systems in use; the large bulk of the current is returning to the source via the resistance of the Earth and the Earth electrodes. However a very small amount of current will flow to the Earth itself because the Earth has capacitance. When you are talking about _static_ electricity, the capacitance of the Earth is so great relative to the capacitance of everything else involved, the voltages are so high and the resistances of the ground electrodes so low, that it is reasonable to talk about charge dissipating to the Earth itself. Over time and on average this charge recombines with charges separated at the carpet (thus returning to the source).
 
Thanks Winnie,

So I guess the question is, is a wire was run from the metal rack, to the ground on the duplex, would the electricity take that path to ground, instead of through the equipment? And would that be safe and to code?
 
You might try adding a static dissipative material near the gear and instruct people to discharge themselves through that before touching the equipment.
Static dissipative material is actually of a higher resistance and thus limits the current level in the discharge. Winnies suggestions are good ones and easy to implement.

Winnie,
I think I disagree with comparing a static charge to a power distribution system.
I do 100% agree with power systems wanting to return to their source and not seeking ground.
However,
with Static charges my understanding is that the charges want to neutralize or redistribute and thus will "seek ground" as a large reservoir of free electrons/charges. I do not think a static charge necessarily wants to seek its source.

As an example think of the charge you get on your pants when you slide across the car seat when exiting the vehicle. Your pants and the car seat are both insulators. You walk away from the car with a large charge on your body and the car seat has an equal but opposite charge. You now touch a grounded surface away from the car and discharge your body. The car seat does not exchange or equalize charge in this process. In fact if you measure the charge on the seat it is still what it was just after you exited the car -minus a small amount that dissipates over time. Your body however now reads zero volts.

I do not pretend to understand all there is to know about static charges. It is a field of study in itself. I do own a electrostatic charge meter and have done a lot of static charge measurements which confirm the example I presented.
 
Jay,

The metal rack should already be grounded via a wire to the duplex. The wire is in the power cord (the EGC), and should be bonded to the chassis of the power distribution unit which should be bonded to the metal rack, etc. It is possible that this bonding is not correct, and that there is unbonded metal on the rack, but it should all be bonded already. This may be worth double checking.

ELA,

I believe that we are in agreement. I brought up power distribution systems as a contrast; in a power distribution system the vast bulk of the current is seeking to return to the source, and only a very minute fraction is 'dissipating' into the capacitance of the Earth. For static electricity the vast bulk of the charge can be spread out into the capacitance of the Earth.

In the example that you gave, charged pants and charged seat, the closed circuit path is still from one object to the other, and there is still electrostatic force causing the charges to want to complete the circuit, but you can spread most of the charge from a single object to the Earth (discharge one object) and leave the other object charged.

-Jon
 
Electrically Conductive mats

Electrically Conductive mats

You can take the thick carpet out, or Add Electrically Conductive mats and ground the mats per manufactures recommendation. (Place the mats around the A.V equipment and upon entering the room, This should discharge the static from person threw the mat to ground.)
This depends on your budget also.

http://www.allmats.com/site/439205/page/505878

Good Luck.
 
I would not recommend a conductive mat alone.

Many shoe soles are insulators. They are one of the insulators involved in the electrostatic charge generation, along with the carpet when you walk across it.

If a person is already charged, via walking across carpet, they will not discharge when stepping onto the conductive mat (unless they are wearing shoe straps).

Your shoes would need to be conductive (or shoe straps used, which is a requirement in ESD protection for Semiconductor handling environments).
 
Static electricity is different than power electricity, as has been touch upon by a couple of other posters.

Static electricity in it's simple form is one charge flowing to another charge.

For the OP, there are many ways his situation can be treated, it just depends on how he/they may want to treat it.
Mention has been made to the mats, there are also "overshoe" protectors that can be worn to help reduce this issue.

The humdifier may help reduce the static, but may also create too much humidity for the equipment.
 
Can you get people to touch bare metal on the rack before touching a knob or switch on the AV equipment? This should suck the charge out of the person before it harms your electronics. A metal rack door would work too, but I'm not sure if the painted surfaces of racks (and their doors) would hamper that static discharge (it may, but I'm not sure -- a 10K resistance is usually enough to bleed off static charge, but I don't know what resistance paint presents to static).
 
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