The Evaluation of Spacings in Electronic Product Design
M. A. Lamothe
Creepage distance and
clearance requirements are tied to a variety of electrical and
environmental factors addressed by safety standards.
 Equipment
intended to be approved to IEC standards must be designed with spacings
between conductive parts that are sufficient to ensure user safety in
the presence of hazardous voltages. Designers need to be familiar with
the effects of pollution degree, overvoltage category, and working voltage
on their decisions regarding spacing distances. The role of creepage distance
and clearance in the designing of an electrical product with hazardous
voltages present is often imperfectly understood, as is the fact that
these influences represent a three-dimensional problem.
Clearance is the shortest distance between two conductive parts, or between
a conductive part and the bounding surface of the equipment, measured
through air. Components that are mounted above the printed circuit board
(PCB) must also be considered in the evaluation of clearance. Creepage
distance is the shortest path between two conductive parts, or between
a conductive part and the bounding surface of the equipment, measured
along the surface of the insulation. All conductive parts are considered
in evaluating creepage distance, including the pads around soldered connections.
The typical solder resist does not reduce the creepage distance required
on a PCB.
These requirements for spacings—creepage distance and clearance—can
best be understood by looking at the electrical and environmental factors
that affect them. Such factors are the pollution degree of the environment
that the equipment will be installed in, the overvoltage category of the
equipment's power source, the working voltage, the comparative tracking
index of the substrate material, and the specified maximum installation
altitude. This article examines the relationship of these elements to
spacing requirements and illustrates the practical application of the
concepts. Figure 1 depicts the general
influence of each factor on necessary clearance and creepage distance.
All dimensions cited come from the third edition of "Safety of Information
Technology Equipment," CSA CAN/CSA-C22.2 No. 60950-00/UL 60950.1
Pollution Degree
Four levels of pollution degree signify increasing ambient influence
on the internal equipment environment and have different effects on product
design. Definitions are based on IEC 60664.2
Pollution Degree 1 refers to a condition of no pollution or only
dry, nonconductive pollution. Likely to be characteristic of cleanroom
equipment, this type of pollution has no influence. Only components or
subassemblies that are adequately enclosed by enveloping or hermetic sealing
to prevent ingress of dirt and moisture qualify to use Pollution Degree
1 spacings.
Pollution Degree 2 is nonconductive pollution of the sort where
occasionally a temporary conductivity caused by condensation must be expected.
This is the usual pollution degree used for equipment being evaluated
to 60950 and is suitable for equipment employed in an office environment.
Pollution Degree 3 covers conductive pollution and dry, nonconductive
pollution that becomes conductive owing to condensation that can be expected.
The local internal environment of the equipment is subject to conductive
pollution because the device is permanently or temporarily exposed to
the outdoors.
Pollution Degree 4 refers to pollution that generates persistent
conductivity caused, for instance, by conductive dust or by rain or snow.
This category is not applicable to products covered in 60950.
Again, the higher the pollution degree, the worse the environment. Greater
spacings are required in response to higher degrees of pollution in order
to prevent breakdown between parts of the circuit or equipment (see
Figure 1a).
Overvoltage Category
The overvoltage category of the power source used to run the equipment
also has an effect on product design. The definitions presented here are
based on IEC 60664.
Overvoltage Category I refers to the signal level and encompasses
secondary circuits, special equipment or parts of equipment, telecommunications
devices, and the like, which experience smaller transient overvoltages
than normal in Overvoltage Category II. Category I spacings are usually
employed for battery-powered or safety extra-low-voltage (SELV)—powered
equipment where there are not likely to be power-source transients.
Overvoltage Category II is local level, covering appliances, portable
equipment, etc., with smaller transient overvoltages than those characteristic
of Overvoltage Category III. This category applies from the wall plug
to the power-supply isolation barrier (transformer). The typical office
and small plant environment is Overvoltage Category II, so most equipment
evaluated to the requirements of 60950 are considered to belong in that
classification.
Overvoltage Category III refers to the distribution level, that
is, building wiring and fixed installations. This level experiences smaller
transient overvoltages than occur in Overvoltage Category IV. A large
industrial plant would be considered Overvoltage Category III. Equipment
for use in this environment must receive special consideration for both
pollution degree and overvoltage category. Many standards require that
the environment be specified in the product manual.
Overvoltage Category IV refers to the primary supply level: overhead
lines, cable systems, and so on. This category is not relevant to most
product standards.
Just as higher pollution degree levels require greater spacing in product
designs, so do higher overvoltage category levels (see
Figure 1b).
Working Voltage
Working voltage is defined in iec 60664 as “the highest rms [root-mean-square]
value of the ac or dc voltage that may occur locally across any insulation
at rated supply voltage, transients being disregarded,” in open-circuit
conditions or in normal use. All voltages must be measured using a true
rms meter or scope. A scope has to be used to determine repetitive peak
voltages because the spikes may be very narrow (and thus have a low rms
value) but also high in voltage, which could contribute to the occurrence
of breakdown.
All of the following requirements apply in determining the working voltages.
- The value of the rated voltage or the upper voltage of the rated voltage
range is used for working voltage between a primary circuit and ground,
and it is taken into account in determining the working voltage between
a primary circuit and a secondary circuit.
- Ungrounded accessible conductive parts are assumed to be grounded.
- A transformer winding or other part that is floating (i.e., that is
not connected to a circuit that establishes its potential relative to
earth) is assumed to be grounded at the point by which the highest working
voltage is obtained.
- Where double insulation is used, the working voltage across the basic
insulation is determined by imagining a short circuit across the supplementary
insulation, and vice versa. For double insulation between transformer
windings, the short circuit is assumed to take place at the point where
the highest working voltage is produced in the other insulation. (See
the section "Types of Insulation," below, for definitions.)
- For insulation between two transformer windings, the highest voltage
between any two points in the two windings is used, taking into account
external voltages to which the windings will be connected.
- For insulation between a transformer winding and another part, the
highest voltage between any point on the winding and the other part
is used. For a working voltage to be used in determining clearances
for primary circuits:
- The peak value of any superimposed ripple is included for dc voltages.
- Nonrepetitive transients, such as those due to atmospheric disturbances,
are disregarded. (It is assumed that any such transient in a secondary
circuit will not exceed the transient rating of the primary circuit.)
- The voltage of any extra-low-voltage (ELV), SELV, or telecommunication-network-voltage
(TNV) circuit is regarded as zero.
- The maximum repetitive peak value is used for repetitive peak voltages
exceeding the peak values of the mains supply voltage. For a working
voltage to be used in determining clearances for secondary circuits:
- The peak value of any superimposed ripple is included for dc voltages.
- The peak value is used for nonsinusoidal waveforms. For a working
voltage to be used in determining creepage distance:
- The actual rms or dc value is used.
- If the dc value is used, any superimposed ripple is ignored.
Note that rms voltages are used in calculating creepage distance and
that peak or dc voltages are employed to calculate clearances. The higher
the working voltage, the greater the spacings required (see
Figure 1c). Figure 1d shows the cumulative effect on product design
of pollution degree, overvoltage category, and working voltage.
Comparative Tracking Index
The comparative tracking index (CTI) of the material affects the creepage
distance. The CTI value is a measure of the resistance to surface tracking
that a particular material exhibits under specific test conditions. The
lower the CTI for that material, the greater the creepage distance required
(see Figure 1e). Materials fall into
four material groups: I (CTI > 600), II (CTI > 400
and < 600), IIIa (CTI > 175 and < 400), and IIIb (CTI >
100 and < 175). Most pcbs have a value of 175.
Installation Altitude
Altitude with reference to sea level affects the required clearance.
Most standards use 2000 m as the baseline. Higher installation elevations
require the addition of a correction factor because the lower atmospheric
pressure at high altitude has less resistance to breakdown. For elevations
above 2000 m, the required clearance is increased by the following factors:
to 3000 m, 1.14; from 3000 to 4000 m, 1.29; and from 4000 to 5000 m, 1.48
(see Figure 1f).
Types of Insulation
Wherever the operator, under all normal conditions and under any single-fault
condition, can contact a part, there must be no hazard. As a consequence,
the product designer must define the type of insulation required as a
minimum between various areas of the device. For example, primary-to-SELV
will require double or reinforced insulation.
Also, if an accessible conductive component that is not grounded could
become energized by the failure of basic insulation of another component,
then supplementary insulation must be applied to protect the accessible
ungrounded component. An example might be a metal handle on a power switch.
Basic insulation provides basic protection against electric shock.
This insulation is used between parts at hazardous voltages and a grounded
conductive part or SELV part, between primary and the grounded screen
or core of a primary power transformer, and as an element of double insulation.
Supplementary insulation is independent insulation applied in
addition to the basic insulation in order to reduce the risk of electric
shock in the event of failure of the latter. Supplementary insulation
is generally used between an accessible conductive part and a part that
could become energized if the basic insulation failed or else as an element
of double insulation. This insulation is required to ensure protection
of the operator should basic insulation fail.
Double insulation is composed of basic and supplementary insulation.
It is used between an ungrounded conductive part or floating SELV circuit
and a primary circuit.
Reinforced insulation is a single-insulation system that provides
the same protection against electric shock as double insulation. Unlike
basic or supplementary insulation materials, reinforced insulation may
consist of layers of material that cannot themselves be tested singly.
Functional insulation is insulation needed for correct equipment
operation. It does not protect against electric shock. Functional insulation
would be used between parts having different potentials or between ELV
or SELV circuits and grounded conductive parts. This type of insulation
replaces operational insulation in the third edition of IEC 60950.
In summary, basic and supplementary types of insulation each consist
of a single layer, double insulation involves two layers, and reinforced
insulation is a single layer that is equivalent to two layers of insulation.
Table I charts the different types of
insulation required between pairs of device components.
The Concepts in Application
To determine the required creepage distance and clearance spacings for
an electronic product, the best method is to first draw a block diagram
of the design (see Figure 2) from which a table of the required spacings
can be prepared (see Table II).
For example, the primary section of a power supply is treated as a block.
Spacings are examined between hot and neutral to ground and between all
primary and secondary parts. Each circuit should be regarded as a block.
 |
Figure 2. A block diagram of the electronic device
for which the spacing requirements in Table II have been calculated. |
It is important to note that the voltages of each block are not added
together if they are powered from the same source (i.e., across a transformer).
In the case of a relay contact where the contacts are connected to a diferent
source, the two voltages are added together to determine the working voltage.
Conclusion
The evaluation of electronic device spacings is essential in the designing
of products that are to be approved to any of the IEC product safety standards.
The standard should always be consulted. The information provided in this
article is intended to be illustrative and to provide a basic understanding
of the principles involved, and should not be used in actual product design
in lieu of authoritative reference documents.
References
1. Safety of Information Technology Equipment, IEC 60950 (Geneva:
International Electrotechnical Commission, 1999).
2. Insulation Coordination for Equipment within Low-Voltage Systems,
IEC 60664 (Geneva: International Electrotechnical Commission, 2000).
M. A. Lamothe, PE, is the president of M. A. Lamothe & Associates
(Georgetown, ON, Canada). He can be reached at moe@lamothe-approvals.com.
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