Quote:
Originally Posted by Lee
Thanks for your detailed reply Gary...
I was thinking that, with earthing the mount, that if it was grounded, and I was careful to ground myself prior to touching it (ESD shoes/mats/grounding point) this should stop ESD problems? I suppose that if problems persist then, I need to look elsewhere.... Would you agree?
|
Hi Lee,
You have to be careful when using the terms "earthing", "ground", "grounded",
"grounding", etc.
As you will be aware, electrostatic charge buildup can occur when two insulators with
different places on the triboelectric series are in close contact and then
quickly torn apart.
A classic example are the soles of your rubber boots coming into contact with
the carpet of the observatory floor and then being torn apart again as you
take steps. Your body acts as a capacitor and can build up charge with each
step. Some of that charge will leak back across stray capacitances on the same
rubber sole back to the carpet and some will dissipate with you coming into
contact with ionized air particles. However, despite these leakages,
you can quickly build yourself up to having several thousand volts potential and
when you touch or sometimes just get close to a metal object, a discharge occurs.
So one of the first things to consider is minimizing the chance of a static
charge buildup in the first place and one way of achieving that is to minimize
different insulators with different triboelectric scale rankings coming into close
contact and being torn apart again. In practical terms, that may simply mean
changing the carpet for different flooring, perhaps industrial rubber mats that
are a closer match for your rubber sole shoes.
In facilities manufacturing and handling electronics, we wear heel straps.
These tuck into the opening of the shoe where your foot goes at one end
and then wrap around to beneath the heel so they come into contact with the
flooring. If electrostatic charge is created as you walk across the floor, the
heel straps creates a path for the charge on your body to make its way back
to the floor.
In environments where antistatic flooring is in place, they are one
element to help control the problem of ESD. In the same environment, attempts
are often made to keep some critical metal surfaces equipotential by heavy
bonding between them and ESD table mats have cables that are connected to
the same reference point.
So a room where these surfaces are kept as equipotential as possible
is a good thing in creating an environment that is designed to minimize the
risk of ESD events.
However, and this is very important to appreciate, "grounding" conductive
objects in the observatory such as the mount is not in itself a cure for the adverse
effects of ESD.
For example, a visitor to the observatory wearing their new panda fur coat and
brandishing a rod made of amber which they insist on rubbing on the panda fur
will still create a charge and if they then touch the mount, whether it is
"grounded" or not, a discharge will still occur and they might still crash your computer.
If the mount were heavily bonded to ground using short, wide low-inductive braid,
it would still not prevent the discharge from occurring. However, it may divert
the discharge currents from flowing elsewhere, say into cables connected to
a camera on the mount.
The problem is, as I mentioned in my previous post, even if your mount were
connected to ground by a piece of wire of the types of diameters used in
household 240V wiring, given a choice of going down that wire or crossing an
air gap of a few picofarads capacitance, the initial part of the discharge will
choose the low capacitance airgap.
That is quite counterintuitive to many. Here we have a nice piece of low
resistance wire and over here we have an air gap of a few centimeters.
Which one will the current initially flow through? The problem is that the
initial part of the discharge has a rise time in the order of picoseconds.
When you do a Fourier analysis of that rising edge, that is going from the
time domain to the frequency domain, it becomes evident that much
of its energy is contained in high frequency regions. High frequency therefore
equates to capacitors having low capacitive reactance but inductors having
high inductive reactance. At high frequencies, currents want to follow the
path of least inductance. Our electrostatic discharge will happily sooner go across
the air gap rather than down our ground wire.
To bring this home further, you might have some piece of equipment, such as a
camera, sitting on a metal bench. Not even touching the metal bench but perhaps
a centimeter or so away there might be a metal filing cabinet. If you create
a discharge to the filing cabinet, it will readily traverse the air gap between it
and the metal bench where their capacitive coupling might only be a few picofarads.
From there, it might decide to go into the camera. The path such discharges take
is often dictated more by stray capacitances and apertures in equipment than
by the topology of where we might hope the current to go if we were to pull
out a multimeter and measure resistances at DC.
In practical terms, you don't necessarily want to go around having to wear a heal strap.
They are inconvenient to have to take on an off as you go back and forth to the
house and when they become dirty they become less effective. To those of us
who wear them in industrial and laboratory settings, it can also be socially awkward
if you forget to take it off and turn up to the restaurant still wearing one.
People stare.
However, they are low cost and if you are prepared to go through
the discipline of putting them on when working in the observatory and taking them
off when leaving, they can be one effective countermeasure in the arsenal.
Replacing the carpet with rubber or antistatic may end up being sufficient to
mitigate the problem.
Grounding of metal objects in itself will not prevent discharge though low
inductive bonding can help keep metal surfaces equipotential and help divert
currents from flowing into places where you would prefer they don't go.
Even barring panda fur wearing, amber rod wielding visitors to the observatory may
not prevent all sources of electrostatic build-up. A classic documented example
are some office chairs. Sit in the cushion and then stand up. As the cushion returns to
its original shape, for a period lasting as long as ten seconds afterwards, internal
ESD events can occur within it and radiate as strong electromagnetic fields from
the chair's metal legs, enough to upset some electronic equipment.
.
