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Teledyne API T703/T703U Calibrator Operation Manual A Primer on Electro-Static Discharge
221
When static charges accumulated on a sensitive device discharges from the device
to another surface such as packaging materials, work surfaces, machine surfaces
or other device. In some cases, charged device discharges can be the most
destructive.
A typical example of this is the simple act of installing an electronic assembly into
the connector or wiring harness of the equipment in which it is to function. If the
assembly is carrying a static charge, as it is connected to ground a discharge will
occur.
Whenever a sensitive device is moved into the field of an existing electro-static field,
a charge may be induced on the device in effect discharging the field onto the
device. If the device is then momentarily grounded while within the electrostatic
field or removed from the region of the electrostatic field and grounded somewhere
else, a second discharge will occur as the charge is transferred from the device to
ground.
10.3. COMMON MYTHS ABOUT ESD DAMAGE
I didn’t feel a shock so there was no electro-static discharge: The human
nervous system is not able to feel a static discharge of less than 3500 volts. Most
devices are damaged by discharge levels much lower than that.
I didn’t touch it so there was no electro-static discharge: Electro Static charges
are fields whose lines of force can extend several inches or sometimes even feet
away from the surface bearing the charge.
It still works so there was no damage: Sometimes the damaged caused by
electro-static discharge can completely sever a circuit trace causing the device to
fail immediately. More likely, the trace will be only partially occluded by the damage
causing degraded performance of the device or worse, weakening the trace. This
weakened circuit may seem to function fine for a short time, but even the very low
voltage and current levels of the device’s normal operating levels will eat away at
the defect over time causing the device to fail well before its designed lifetime is
reached.
These latent failures are often the most costly since the failure of the equipment in which
the damaged device is installed causes down time, lost data, lost productivity, as well as
possible failure and damage to other pieces of equipment or property.
Static Charges can’t build up on a conductive surface: There are two errors in
this statement.
Conductive devices can build static charges if they are not grounded. The charge will be
equalized across the entire device, but without access to earth ground, they are still
trapped and can still build to high enough levels to cause damage when discharged.
A charge can be induced onto the conductive surface and/or discharge triggered in the
presence of a charged field such as a large static charge clinging to the surface of a
nylon jacket of someone walking up to a workbench.
As long as my instrument is properly installed, it is safe from damage caused
by static discharges: It is true that when properly installed the chassis ground of
your instrument is tied to earth ground and its electronic components are prevented
from building static electric charges themselves. This does not prevent discharges
from static fields built up on other things, like you and your clothing, from
discharging through the instrument and damaging it.
07223C DCN6572