design_for_dust
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User-touchable voltages must be non-
hazardous; such voltages are also called
safety extra-low voltage (SELV). SELV cir-
cuits often run at low power and logic lev-
els, such as ±3.3 to ±24 VDC. Examples of
user-touchable circuits are input/output
connectors and cables used to attach pe-
ripheral devices such as printers, keyboards,
monitors, and external drives.
You must insulate hazard-
ous voltages from user-touch-
able circuits. There are several
types of insulation:
● Functional insulation en-
sures correct product opera-
tion but does not provide
safety protection.
● Basic insulation provides a
single layer of insulation.
● Supplementary insulation is
the addition of an indepen-
dent layer of insulation to
basic insulation.
48 APRIL 2008
www.tmworld.com TEST & MEASUREMENT WORLD
Outdoors
Residential
Commercial
Industrial
4
2
2
3
FIGURE 2. Pollution degrees describe indoor and outdoor
environments. Artwork by Melinda Vaughan, National Instruments.
FIGURE 1. Dust can accumulate on
PCBs, which can cause electrical fail-
ures and safety hazards. Courtesy of www.computerservicesusa.com/dirty_computers.html.
Table 1. Pollution degrees and their descriptions
Pollution
degree level
Description
1
No pollution or only dry, nonconductive pollution, which has no influence on
safety. You can achieve pollution degree 1 through encapsulation or the use of
hermetically sealed components or through conformal coating of PCBs.
2
Nonconductive pollution where occasional temporary condensation can occur.
This is the most common environment and generally is required for products
used in homes, offices, and laboratories.
3
Conductive pollution or dry nonconductive pollution, which could become
conductive due to expected condensation. This generally applies to industrial
environments. You can use ingress protection (IP) enclosures to achieve pollu-
tion degree 3.
4
Pollution that generates persistent conductivity, such as by rain, snow, or con-
ductive dust. This category applies to outdoor environments and is not appli-
cable when the product standard specifies indoor use.
(continued)
● Double insulation combines basic and supplementary insulation.
● Reinforced insulation is a single system that provides the same
protection as double insulation.
Safety standards require that you protect users from hazards
during the normal operation of a product and under an abnormal
(single-fault) condition. Double or reinforced insulation provides
this protection; in the event of a failure of the basic insulation, a
second layer remains for protection.
Tables in the standards specify insulation spacing values be-
tween two circuits. To prevent electrical breakdown, you need
to increase the spacing distance as the pollution degree in-
creases (Table 2).