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Glossary
SWITCHING DEVICES
Definitions:
Switching
Switching is the ability to make and break defined load and overload currents at a rated operational voltage, for the useful life of the device. Switching devices come within the scope of EN 60947-3: Specification for low-voltage switchgear and controlgear. Switches, disconnectors, switch-disconnectors and fuse-combination units, which details the following definitions.
Switch
A mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions which may include specified operating overload conditions and also carrying, for a specified time, currents under specified abnormal circuit conditions such as those of short-circuit.
NOTE: A switch may be capable of making, but not breaking, short--circuit currents.
Disconnector
A mechanical switching device which, in the open position, complies with the requirements specified for the isolating function.
NOTE: A disconnector is capable of opening and closing a circuit when either a negligible current is broken or made, or when no significant change in the voltage across the terminals of each of the poles of the disconnector occurs. It is also capable of carrying currents under normal circuit con-ditions and carrying, for a specified time, currents under abnormal conditions such as those of short circuit.
Switch-disconnector
A switch which, in the open position, satisfies the isolating requirements specified for a disconnector.
Fuse-combination unit
A combination of a mechanical switching device and one or more fuses in a composite unit assembled by the manufacturer or in accord-ance with his instructions.
Switch-fuse
A switch in which one or more poles have a fuse in series in a composite unit
Fuse-switch
A switch in which a fuse link, or a fuse carrier with fuse link, forms the moving contact
Disconnector-fuse
A disconnector in which one or more poles have a fuse in series in a composite unit.
Fuse-disconnector
A disconnector in which a fuse link or fuse-carrier with fuse link forms the moving contact
Switch-disconnector-fuse
A switch-disconnector in which one or more poles have a fuse in series in a composite unit
Fuse-switch-disconnector
A switch-disconnector in which a fuse link, or a fuse-carrier with fuse link, forms the moving contact.
Whilst at first sight these definitions may appear complex and confusing there is a logic to the terminology with each device having its own specific features and functions. The key to identifying the functions of a device and specifying it for a particular application rests in the recognition of terms and the order in which these terms are used in the definition.
The position of the word ‘fuse’ at the beginning of the description indicates that the fuse link forms part of the moving contact system whereas the word ‘fuse’ at the end indicates a static fuse link.
The symbols used to identify these functions are shown in the following table from B EN 60947-3 and must be placed on the front of the device in such a position that they are visible when the device is installed.
It is important to note that the definitions do not indicate whether the device is capable of being isolated at both ends of the fuse. Where this is not the case, adequate marking of the line/load terminals is essential for safe use.
Category of Duty:
Switching devices are classified according to Utilization Category, or Category of Duty, thus:-
AC20/DC20 |
Connecting and disconnecting under no-load
Assumes all switching operations are carried out by other capable devices before this device is operated.
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| AC21/DC21 |
Switching of resistive loads including moderate overloads.
Suitable for purely resistive type loads. Device can switch 150% of its rated current under fault conditions. |
| AC22/DC22 |
Switching of mixed resistive / inductive loads, including moderate overloads.
Suitable for mixed resistive / inductive loads. Device can switch 300% of its rated current under fault conditions. |
| AC23/DC23 |
Switching of highly inductive loads.
Devices complying with AC23/DC23 are provided mainly as back-up to other means of switching, e.g. Contactors. In the event of failure of the functional device, an AC23/DC23 type device can safely interrupt a stalled motor current. Where devices are the only means of controlling individual motors, they should comply with the requirements of appendix A of the EN60947-3 |
Switch Disconnectors for AC21/DC21, AC22/DC22 and AC23/DC23 also have to meet the requirements of AC20/DC20.
For specific and special applications such as switching of capacitors and tungsten lamps, the manufacturers' advice should be sought.
Frequent and Infrequent Use
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 EN60947-3 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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