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T400 Ozone Analyzer Operator’s Manual A Primer on Electro-Static Discharge
307
Potentially damaging electro-static discharges can occur:
Any time a charged surface (including the human body) discharges to a device.
Even simple contact of a finger to the leads of a sensitive device or assembly can
allow enough discharge to cause damage. A similar discharge can occur from a
charged conductive object, such as a metallic tool or fixture.
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.
14.3. COMMON MYTHS ABOUT ESD DAMAGE
I didn’t feel a shock so there was no electro-static discharge: The human
nervous system isn’t 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 they
are 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.
06870C DCN6332