Requirements Design Tips Reference Note that references are to IEC 60950 Third Edition 1999-04. Where the references in the Second Edition differ, they appear in parentheses.
There are two principle methods by which an electrical
device is provided with safety protection from electrical hazards. These methods are classified as Class I
, which depends of the use of earthing or grounding, and Class II, which uses the concept of double or reinforced insulation.For Class I devices, accessible conductive parts must be
reliably connected to the earth conductor of the electricity supply. If a fault in the product causes accessible conductive parts to become live, the electricity system
protective devices (fuse, residual current device or earth leakage trip) will cut off the power to the device.
REQUIREMENTS Earthing conductors must not be fused and must not include switches. Any power supply connections between user detachable
parts of a product must be of a type that the earth connections make earlier and break later than the live and neutral connections. Internal protective earth wiring can be uninsulated or
insulated. Insulation, if used, must be green/yellow, except for insulated earthing braids, where the insulation can also be transparent. Any color insulation may be used in ribbon
cable, PCB's, etc. as long as this does not misrepresent the use of the conductor. The effectiveness of earthing is checked using the earth continuity test. For Canada, the capacity of connection
between earthing terminal and parts required are to be earthed evaluated according to CSA C22.2 No.0.4, Bonding and Grounding of Electrical Equipment.
For non-detachable power cords with a protective earth conductor, the construction of the connections of the conductors should be such that, if the cord were to slip in its
anchorage, the protective earth conductor would be the last to take the strain.
DESIGN TIPS In many situations the earth conductor from the power supply cord or the appliance inlet would be connected to an earthing stud.
The wire is terminated with a closed loop connector which is fixed to the earthing stud or screw with a star or lock washer and a nut. Other parts of the product that need to be
earthed are connected by closed loop connectors to the same stud and locked with an additional nut. It is important that the earth wire from the power supply cord is located at
the bottom of the stud and locked with its own nut. The earthing stud must not be used for any purpose other than earthing. It cannot be used, for example, for the mechanical
fixing of parts other than the earth conductors. Its mechanical structure must also be such that it cannot be loosened from outside the device. For example, it cannot
be a post fixed with a screw from outside the product. The earth terminal marking
is placed adjacent to the earthing terminal.
REFERENCE Complete requirements are given in IEC 60950, Clauses 2.6 and 3.2.6 (2.5 and 3.2.5)
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