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Input Protection Scheme For Msf


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Dear All,


MSF manual suggests on using normal fuses for short circuit protection of MSF.


There is no ground fault protection in MSF.


Question is , whether we can use MCCB , 3 pole with adjustable O/L, S.Ckt & ground fault protection or as O/L protection will be provided by MSF itself, wecan use normal HRC fuses along with ground fault relay/ monitoring relay.


What i know is that the earthing scheme used in the installation effects the choice of ground fault relay.


Except this what other protection is ideally required at the input of MSF for trouble free operation, as i recently faced a major failure & even the customer was not forthcoming on the events prior/during the fault.


Thanks & Regards,




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Hello Gaurav


It really depends on what you want to protect, and what conditions you want to protect against.


If you wish to protect the SCRs against an overcurrent situation (abnormal current such as a short circuit) then the best protection is offered by semiconductor fuses.

Bypass contactors also offer much higher security against a problem while the starter is running, but not during start.


Over voltage problems are usually an issue when the starter is OFF but has voltage applied. Under this case, the SCRs are blocking the full voltage and are subjected to transients which can damage the SCRs. The best protection against the transients is to use a line contactor which is opened when the starter is not running. Use the starter to control the contactor.


Standard HRC fuses are primarily to protect the cables used and should be sized accordingly. They are too slow to provide short circuit protection for the SCRs.

A circuit breaker can be even slower than the HRC fuses and allows more fault current through so provides a lower protection to the SCRs under short circuit conditions.


Apply earth fault and cable protection as you would without the soft starter. Then consider the addition of semiconductor fuses, line contactor and bypass contactor for added protection of the soft starter against major faults.


Best regards,

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