|Pollution Degree||X (mm)|
The Evaluation of Spacings in Electronic Product DesignM. 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
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).
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 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.
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.
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.
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.
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).