Type 2 Coordination for 3.3 kV Motors

I really don't know we are using HT fuse & contactor for HT motor protection.!!!Your vacuum contactor will open on fault wired through relay contacts.Latest relays used in motor protection such as RHO3 will help to do the coordination.

Relay is not suppose to protect motor from Short circuit. for Protection against Short Circuit, Fuse are must.

Reason is Short circuit capacity of Vacum contactor is ver y less compare to fault level of bus. Hence, contactors are not suppose to clear S/C fault.

Yea,...I agree for LT motors its protected by fuse,contactor.....but in case of HT motors we use VCB breakers for the range upto 11KV.thre we never use HT fuse.....relay will trip breaker on fault whatever the fault is .......vacuum contacotr in your case is nithing but a VCB I think.....pls clarify.

It is vacuum contactor. Vacuum Contactors are aslo avaialbe for MV Applications.

But why dont you use VCBs?What application is the motor for?

cost & low fault level of the system.

Hi, Good day.

Generally the HT fuse for HT motor is selected based on the 1.3 to 1.4 times of the full load current of the motor. Little more also depends on the fuse manufacturer. Following link of bussmann fuses will help you in identifying the fuse selection for HT transformer and motor. Hope i have clarified your query.

http://www.bussmann.com/pdf/abe14678-ac07-4a08-8813-faa12ccda82e.pdf

Though there are no ready-made Type-2 Co-ordination charts for HT/MV Motors as in the case of LV Motors, the concept to be followed is the same. You have to superimpose the curves of the Overload Relay (may be a motor protection relay's thermal overload element), the HT HRC Fuse and the Vacuum Contactor's Braking Capacity. As in LV Motors, the Over load Relay Curve operating time should be faster for all currents below the breaking capacity of the contactor and the HT HRC Fuse operating time should be faster very near to the breaking capacity of the contactor. That is the cross-over point of the OLR & the Fuse Curve should be well before the contactor breaking capacity.

Reputed Motor Protection Relay Manufacturers would either advise to disable or would disable at the factory itself, the overcurrent(short circuit) element in the motor protection relay if the motor is contactor-fed.

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There is an ANSI classification for MV motor starters, and Class E-2 is the standard for most applications. In order to claim a Class E-2 rating, the fuses and contactor must be tested / coordinated together, because unlike LV components, the Vacuum Contactors have a relatively lower Fault Current capacity. So as Electrical Expert said above, you MUST coordinate the breaking / withstand capability of the contactor with the let-through of the fuses, as well as with the OL protection device (Motor Protection relay or Thermal Overload relay). Basically, the OL protection and contactor must be able to interrupt anything up to the let-through of the fuses, the fuses must take care of everything else up to the Available Fualt Current of the system.