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Switchgear Short time rating

Nick Parker
Typically, I see the MV Switchgear rating of 40kA, 3s  and for LV Switchgear 50kA, 1s.


Why 1 sec or 3 sec and why not other values?
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  • 0
    The time is all to do with the fault clearance times of the circuit breakers. With MV breakers the trip times will be set longer than for the LV to allow time for the LV circuit breakers to operate and clear the fault, to give protection discrimination and ensure that only the faulty circuit/section of the network is isolated. The values are selected to have a standard rating to compare or specify. It is possible to have other values but these are about twice the normal trip times and so are realistic values. However if you specified MV switchgear for 70kA, 1s then the manufacturer would be scratching their heads trying to decide whether their normal design/construction which meets 40kA, 3s would be OK.


    For the current it is again a standard value that is used in the testing of the switchgear assemblies. Other values could be used, but if you are comparing vendors it is easier to compare apples with apples rather than oranges.
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  • 0
    The time is all to do with the fault clearance times of the circuit breakers. With MV breakers the trip times will be set longer than for the LV to allow time for the LV circuit breakers to operate and clear the fault, to give protection discrimination and ensure that only the faulty circuit/section of the network is isolated. The values are selected to have a standard rating to compare or specify. It is possible to have other values but these are about twice the normal trip times and so are realistic values. However if you specified MV switchgear for 70kA, 1s then the manufacturer would be scratching their heads trying to decide whether their normal design/construction which meets 40kA, 3s would be OK.


    For the current it is again a standard value that is used in the testing of the switchgear assemblies. Other values could be used, but if you are comparing vendors it is easier to compare apples with apples rather than oranges.
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