UL 60950 3rd Edition (formerly UL 1950, 3rd edition)
UL 60950 Terminology
The following definitions assist in understanding UL 60950:
SELV
Secondary circuit whose voltage values do not exceed a safe value
(voltage less than hazardous levels of 42.4 V peak or 60 V dc)
TNV
Telecommunication Network Voltage (a secondary circuit)
O SELV but with exposure to surges
TNV3
TNV2
TNV1
O SELV but without exposure to surges
? SELV with exposure to surges
Creepage distance is the shortest distance between two conductors, measured along the
surface of the insulation. DC voltages shall be included in determining the working voltage
for creepage distances. (The peak value of any superimposed ripple or short disturbances,
such as cadenced ringing signals, shall be ignored.)
Clearance distance is the shortest distance between two conductive parts or between a
conductive part and the outer surface of the enclosure measured through air. DC voltages
and the peak value of any superimposed ripple shall be included in determining the working
voltage for clearance distances.
Creepage and clearance distances are also subject to the pollution degree of the
equipment:
•
Pollution degree 1 — components and assemblies that are sealed to prevent ingress of
dust and moisture
Pollution degree 2 — generally applicable to equipment covered by UL 60950
Pollution degree 3 — equipment is subject to conductive pollution or to dry non-
conductive pollution, which could become conductive due to expected condensation.
•
•
To ensure safe operating conditions of the equipment, UL 60950 focuses on the insulation
rating of the circuit(s) under consideration. Table 4.15 and Table 4.16 indicate the required
creepage and clearance distances depending on material group, pollution degree, working
voltage, and maximum transient voltage in the secondary circuit. For a typical
telecommunication application with a working voltage of 200 V, pollution degree 2, material
group IIIb, the creepage distance is 2 mm. The clearance distance is 2 mm for reinforced
insulation.
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