Re: Static Discharges from PE etc... (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Sat, 17 Jan 2004 14:45:51 -0500
From: David Sharpe <sccr4us@xxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Static Discharges from PE etc... (fwd)

Hi Dan!

High Voltage list wrote:

> Original poster: Steven Roys <sroys@xxxxxxxx>
>
> ---------- Forwarded message ----------
> Date: Fri, 16 Jan 2004 11:04:29 -0500
> From: "Mccauley, Daniel H" <daniel.h.mccauley@xxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: RE: Static Discharges from PE etc...
>
> Dave,
>
> My first job out of school was with Anheuser-Busch working on their
> bottling and packaging conveyor systems.
> Typical practice is using static-dissipative polyethylene or similar
> plastic which quickly dissipates any
> charge built up on the material due to passing packages etc...  Although
> the static-dissipative PE is much more
> expensive than standard PE, it works extremely well.
>
> Dan
> ----------------------------

The PE material used was virgin grade, and used as a "wear surface".  Since
PE has similar self lubrication characteristics as PTFE, improved conveyor
material wear life was assumed.  Another reason why this situation escalated

into a major issue, the conveyor belt had metallic fibers in it to
"dissappate"
(read equalize) the charge.  Lets see, metal over metal with a PE insulator
the width of the converyor, 15 feet long (180") conveyor, with belt
(electrode)
width of 6" .  Assuming wear surface was 20 mil thick PE (phi ~2.0) and
actual active capacitor area is 1/3 total would approach

Cest(pf) = 0.224 * k * A / d
C          =  0.224 * 2.0 * (6 * 180/3) / 0.02
C           ~ 8000pF

If you assume 15kVDC / inch (high humidity) environment, and 8" discharges
(maximum);

V = 8* 15  = 120kV.

E = 0.5 * C * V^2;  with these values stored E is >50J...  < OUCH >

Even if the actual capacitance is 1/4 what is represented here; clearly a
dangerous and potentially (NO PUN) lethal condition exists.

With the attractive force being presented to cartons and actual jamming
(and damage) to upstream machines, I have no doubt that these values are
close to what was seen in field...

Regards
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA

>
> > HV List
> >
> > I ran into a very interesting "Van de Graff" type problem while
> > working on package conveying systems about 12 years ago.

<<SNIPP>>