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Note that references are to IEC 60950 Third Edition 1999-04. Where the references in the Second Edition differ, they appear in parentheses.

In the majority of cases the primary and secondaries of a transformer are connected to circuits that are required by the standard to be separated. For instance, the power transformer separates the mains circuits from the secondary circuits of the product. The insulation and spacing requirements between these circuits also apply to the transformer. See circuit separation for a full discussion of this subject.

Transformers must comply with the requirements of the overload test. Secondary circuits are shorted or overloaded using the methods described in Annex C of IEC 60950. Temperatures in the transformer are checked to ensure that they do not rise above the specified limits.

Transformers which separate TNV circuits from each other and/or other circuits must comply with the separation requirements given in Clause 2.3, Table 2G and Figure 2F (Table 19, clause 6.2.1.2) of the standard.

The preferred option is to use a transformer which already has the relevant safety approvals. Make sure the transformer has the approvals you need for your target markets (see Submitting a product to a Test House for Approval).

If you plan to submit your product to a Test House, you will probably be required to submit a copy of the safety approval license of the transformer together with your product. The Test House needs the certificate to authenticate the approval and to ensure that you use the transformer in accordance with any Conditions of Acceptability which may be specified as part of the approval. The supplier of your transformer should be able to get you a copy of the license.

If you have a custom-designed transformer manufactured for you, you will have to have the transformer tested within the framework of your product's testing. You must ensure that it has been designed to comply with IEC 60950 and that the electrical diagram, mechanical drawings and component material details are available for submission to the test house. You will also have to have in place a mechanism to ensure that the manufacturer cannot change any detail of the transformer without your approval.