OT: ESD prevention (no ground situation)?


Let's say you want to build a computer(*) into the
interior of some wooden wasted space in a travel
trailer. There's no ground in the conventionl sense,
so how do you go about preventing static arcs from
finger to computer when, for instance un/plugging
something to/from a USB port, or prevent such
ESDischarges from causing ESDamage?
* For those interested, the parts list:
picoPSU-120, 120w output, 12v input DC-DC Power Supply, 96% eff.
formatting link

mini-itx EPIA-CN motherboard/CPU, 16 watts
formatting link

Seagate 250GB SATA Hard Drive, ~6 watts
formatting link

LG 22X DVD RW Burner IDE GH22LP20, max ~24 watts?
(power consumption is pure guestimate)
formatting link

Wireless Network Kit USB Booster & Yagi Antenna, ~1 watt?
formatting link

Vizio 19" Razor 1080i (1368x760), 23 watts
(so normally about 4 amp draw from the RV battery,
except jumps to ~6 during CD/DVD usage)
OS - some kind of 'nix, probably a Linux... haven't
decided what flavor yet. Mepis detects the wireless
and allows effortless setup as do the Ubuntus and some
others I don't recall at the moment (not that iwconfig
is all that tough, but networkmanager does simplify
things). So far, I've only run live CDs on it.
Reply to
Steve Ackman
Loading thread data ...
...
My computers are on a recycled laboratory static mat, which I touch while installing memory and boards. Could you cover the table with sheetmetal and ground it to the inverter?
jsw
Reply to
Jim Wilkins
Jim, AC return (neutral) is never connected to safety ground. Safety ground is not DC return, it also must be separate. Trailer chassis should connect to safety earth. The AC power supply safety earth connection should connect to chassis. It is not relevant that the chassis be connected to external earth, if no external connection is made. Earth is relative to the trailer. If, for instance, you should connect to an external AC power source, then by connecting the safety earth of the supply to the trailer all is common. No problem. (Ground is not ground the world around) Steve
...
My computers are on a recycled laboratory static mat, which I touch while installing memory and boards. Could you cover the table with sheetmetal and ground it to the inverter?
jsw
Reply to
Steve Lusardi
First, the USB connector pins connect the 5 volt power and ground before connecting the data lines, so any potential difference is eliminated before it can get to any components that can be damaged by ESD.
Second, an ESD mat is just one plate of a capacitor unless the drain wire for the mat is connected to a grounding point. Same with a metal covered table, etc. The static charge, positive or negative, must have a path to be dissapated.
Third, your trailer probably has some type of battery charging system connected to AC power. The chassis is connected to ground when the AC is connected.
Finally, what you are contemplating is no different than a lap top computer, as far as power and USB, and other external connections. Millions are in use an have never been damaged by an ESD when not connected to an external ground.
I think you are looking for problems where non exist.
Paul
Reply to
co_farmer
In ,
Well... first, there's no table. The computer will be in a wooden "enclosure". The "inverter" is DC to DC which is plugged into the power plug on the motherboard itself.
Reply to
Steve Ackman
In ,
I didn't know that. But there's no ground, so... ?
It does, but the primary reason for this computer is to run it off the RV battery... when there's no AC available.
But the DC isn't connected to the chasis.
I hope you're right. However... If you have a motherboard on a table, and install RAM, do they not advise using a wrist strap, and taking other anti-static precautions? Is that all just overly cautious nonsense?
Reply to
Steve Ackman
The computer's sensitive bits are inside a metal enclosure, which forms a Faraday shield; all static charge migrates to the enclosure's exterior (you can ground that to any convenient point). If your plan doesn't include a metal enclosure, the machine is likely to create unacceptable radio interference, and to be sensitive to external charges and currents.
All your shielded cables are intended to extend that Faraday cage shield... the metal box is NOT just a box, it's a shield component.
Reply to
whit3rd
There is a ground in the conventional sense. Anything metal that attaches to the ground wire of the system. Just touch the metal frame.
Reply to
Califbill
If your truly in an un-grounded system then create your own dis-charge ground point. A length of chain under the chassis making contact with earth ground. Wire from chain to metal touch point (flashing around cabinet opening). The idea is to bleed off the charge slowly.
If your using this system on the road (moving) the chain system will provide minimal grounding (chain bounce). If parked, push a short rod with a wire connected to the same point on the chassis into the ground.
Happy putering
Jim Vrzal
Reply to
mawdeeb
The idea is to minimize potential difference between your body and the components. Just have your various parts on a conductive mat or surface and connect your wrist strap to that mat.
With your trailer isolated from powerlines and on rubber tires, it could be a godzillion volts above earth ground, wouldn't matter to sensitive bits within. However, you'd better discharge the trailer to ground before connecting a telco line to the internal modem!
Reply to
Don Foreman
Circumstances have developed that may delay the implementation of this plan, but in the meantime, I have a couple more things to clusty. Thanks to all for the input.
Reply to
Steve Ackman
Everything has a ground! How do they get ground in a satellite? Little pile of dirt in the corner? Ground is what ever the unit is referenced to. So as long as the frame of the unit is the same as the reference point, just touch the frame. Or any ground lug on the equipment. Caveat: Do not touch equipment ground if it is referenced a lot a volts away from earth ground and you are contacting earth ground.
Reply to
Califbill
I don't know how mant times I've walked into a shop where the tech was using a wrist strap & pad carefully hooked up to a ground point while working on a piece of equipment mounted on rubber feet. :)
H.
Reply to
Howard Eisenhauer
It is called a floating ground. There is always a logical and physical.
Some things have 'safety' earth ground only - and 'return' apposing the source of current/voltage signal / power.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net "Our Republic and the Press will Rise or Fall Together": Joseph Pulitzer TSRA: Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal. NRA Second Amendment Task Force Charter Charter Founder IHMSA and NRA Metallic Silhouette maker & member. http://lufk> >
Reply to
Martin H. Eastburn

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.