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Note that references are to IEC 60950 Third Edition 1994-04. Where the references in the Second Edition differ, they appear in parentheses.

The separation of circuits is defined in IEC 60950 both in constructional terms (physical separation) and electrical terms (compliance with electric strength tests).

Physical separation is defined in terms of creepage (distance over the surface of insulating material), clearance (distance through air), as well as distance through insulating material. Actual distances required depend on the working voltage of the circuits, the class of insulation needed (see below) as well as other factors. Clearance distances are given in Tables 2H, 2J and 2K (Tables 3, 4 and 5) of the standard. Creepage distances are given in Table 2L (Table 6).

When looking up these tables, you have to decide the pollution degree (1, 2 or 3) of the environment to which the insulation is exposed. In the majority of situations pollution degree 2 would be used.

As examples of clearance distances; for pollution degree 2, a mains supply of 230Vac and a secondary working voltage of up to 300V rms or 420V dc, an air clearance of 4 mm is required for double or reinforced insulation, or 2 mm for basic insulation. Examples of creepage distances for 230 Vac working voltages are 5 mm for double or reinforced insulation and 2.5 mm for basic insulation. Please note that the Tables in IEC 60950 mentioned above take several factors into account. You should consult these tables before deciding the relevant creepage and clearance distances in each situation.

Note that these distances must be maintained within components that link circuits, e.g. transformers, optocouplers, etc. During testing, transformers may be dissected in order to measure the creepage and clearance distances between the primary and secondary windings. Creepage distances apply not only over the surface of printed circuit boards, but also over the surface of components such as optocouplers.

The voltage used for electric strength tests depends on the working voltage and the class of insulation required. Test voltages are given in Table 5B (Table 18) of the standard.

Examples of test voltages  are:

Basic Insulation - (e.g. between phase and neutral supply leads connected together and the earthed body) - 1000 Vac or 1414 Vdc for 120 Vac working voltage; 1500 Vac or 2121 Vdc for 230 Vac working voltage.

Double Insulation (e.g. between live circuitry and SELV circuitry ) - 2000 Vac or 2828 Vdc for 120 Vac working voltage; 3000 Vac or 4243 Vdc for 230V working voltage.

Table 2G and Figure 2F (Figure 5A) of IEC 60950 gives the circuit types and the insulation classes required between them. There are four types of insulation recognized by the standard, basic insulation, double or reinforced insulation, supplementary insulation and functional insulation.

The classes of insulation link in with the concept of single fault conditions. Basic insulation is insulation which is considered as shorted under single fault conditions. Double or reinforced insulation is of sufficient effectiveness, so that under single fault conditions it is not considered as shorted. Supplementary insulation is the insulation which must be added to basic insulation to make the combination double insulation , while functional insulation, if shorted, would have no implications for electrical safety. Reinforced insulation is equivalent to double insulation, except that insulation is provided by a single insulating layer or component, rather than two or more layers.

As an example, consider a circuit which may be touched by the operator. This circuit must not cause a hazard, even under single fault conditions. A circuit of this type is considered SELV (safety extra low voltage), and if not earthed, must be separated from primary circuits by double or reinforced insulation.

If, however, the SELV circuit is earthed, and complies with the earth continuity test, it only needs to be separated from primary circuits by basic insulation. In a single fault condition, the primary circuit would short to earth via the SELV circuit, causing the supply wiring protection to operate.

Under normal conditions, the voltage between any two points in SELV circuitry, or between SELV circuitry and earth in Class I equipment, must not exceed 42.4 V peak or 60V dc. For fault conditions, the voltage must not exceed these limits for more than 0.2 seconds and must not exceed a limit of 71 V peak or 120 V dc.