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Function symbols
The Utilization Category should be suffixed ‘A’ or ‘B’ to indicate suitability for either frequent or infrequent use in service respectively, e.g. an AC21A device can be used to switch resistive type loads on frequent operations and an AC23B device, although suitable for high inductive loads, can only be used infrequently.
The standard does not give a definition of what is meant by frequent or infrequent use, but a reasonable definition for switching full load current would be as follows:
- Frequent (A) Up to 5 times a day for a small device (say up to 100A);
once a week for larger devices.
- Infrequent (B) Once a week for smaller devices; once a month for the larger devices.
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