Electrical Safety

Working voltage

Working Voltage is defined in IEC 60664 as “the highest rms 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.


SELV (safety extra-low voltage) circuits are 30Vrms <42.4 Vpk/60 VDC

Insulation Types

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.

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.

Overvoltage Category

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.

Overvoltage Category IV

refers to the primary supply level: overhead lines, cable systems, and so on.

Pollution Degree

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.

Insulation Material Group

Insulation Material Group is based upon the comparative tracking index (CTI)

  • I 600⇐CTI
  • II 400⇐CTI<600
  • IIIa 175⇐CTI<400
  • IIIb 100⇐CTI<175

Enclosure Protection (Water Resistance)

Engineering Documents

Prototype Processes