design_for_dust
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Two other factors to consider are creepage and clearance,
which define circuit spacings measured from the closest two
points across the insulation, such as between PCB traces or
between the edges of PCB pads around soldered connections
(Figure 3). Creepage is the shortest distance across a surface
between conductive circuits. Clearance is the shortest path
through air between the circuits. Insulating barriers or slots
can influence design decisions about spacing between cir-
cuits. Pollution, relative humidity, and condensation can affect
creepage distances. Clearance is most affected by air pressure
and temperature.
Figure 4
illustrates a 300-V digital multimeter (DMM) de-
signed for use in a pollution degree 2 environment. The DMM
needs reinforced spacing of 3.0 mm minimum between the
300-V input and the 5-V communication ports.
Component
(optoisolator)
Spacing
between
PCB circuits
Air gap (slot)
1.0 mm minimum
Insulated
barrier
Circuits
(PCB traces)
Creepage (on surface)
Clearance (through air)
Spacing
between
component
leads
PCB
PCB
FIGURE 3. Safety standards specify creepage and clearance
spacing. Artwork by Melinda Vaughan, National Instruments.
5-V
communication
ports, user
touchable
Reinforced
insulation: 3 mm
on PCB
300-V input
Optoisolator
Slots
in PCB
Hazardous
voltage section
Nonhazardous
voltage section
Note: Enclosure
top removed for
illustration
FIGURE 4. Insulation separates the hazardous voltages from
nonhazardous voltages on a PCB.
PRODUCT SAFETY
50 APRIL 2008
www.tmworld.com TEST & MEASUREMENT WORLD
(continued)
Testing for safety complianceThe IEC standards also specify an elec-
tric strength test that stresses the insula-
tion beyond what it should encounter in
normal operation. This test is called a di-
electr ic-withstand or high-potential
(hipot) test. Dielectric tests consist of
measuring the leakage current through
the product’s insulation under a high
voltage (overvoltage) for 1 min. Insula-
tion failure occurs when an arc-over oc-
curs, causing excessive current to flow.
The dielectric test voltage is based on
the working voltage and the required in-
sulation, such as 2500 V
RMS hipot from
the input-to-communication connections
in the DMM in Figure 4. To simulate
water pollution on PCBs and compo-
nents, you should perform dielectric tests
after 48 hr of humidity preconditioning.
Product designers should focus on insula-
tion spacings first, because dielectric test-
ing does not reduce or eliminate the re-
quired spacing distances. T&MW.
REFERENCES
1. “Air Quality in Offices,” Vermont Depart-
ment of Health, healthvermont.gov/enviro/
indoor_air/air_office.aspx.
2. “The Inside Story: A Guide to Indoor Air
Quality” US Environmental Protection Agency.
www.epa.gov/iaq/pubs/insidest.html.