Motor Terminal Box 1

[natee]

My 6.0 kV distribution system have been designed the short circuit level at 25kA for 1 second. In case I purchase a MV motor, I always ask supplier to confirm stator terminal box shall be designed to withstand 25kA for 1 second.

Unless, the case motor is controlled via MV-VSD, most suppliers deviate this requirement. They defence that the motor is not directly connect to distribution system.

My question is how can I evaluate the offered stator terminal box? How many short circuit level can it be withstood??

MV-VSD is connected to MV power supply via a step down transformer.

Thanks for any valuable response.

Natee.

 

[jbartos]

Comments marked ///\\My 6.0 kV distribution system have been designed the short circuit level at 25kA for 1 second. In case I purchase a MV motor, I always ask supplier to confirm stator terminal box shall be designed to withstand 25kA for 1 second.
///This is a correct intent in general.\\Unless, the case motor is controlled via MV-VSD, most suppliers deviate this requirement. They defense that the motor is not directly connect to distribution system.
///They are partially right since the motor feeder will reduce the 25kA fault level to some lower number of kA that may stay longer than 1 second. Try to model the motor application by using suitable commercial software that has motor starting module and short circuit voltage drop module.\\My question is how can I evaluate the offered stator terminal box?
///This may be difficult without the motor manufacturer input. The type of conductors, conductor size, and insulation type will be required.\\ How many short circuit level can it be withstood??
///If properly engineered and designed, it can withstand many short circuits.\\MV-VSD is connected to MV power supply via a step down transformer.
///This transformer will reduce the short circuit current level. It is a good idea to model this on computer.\\

 

[aolalde]

In my opinion requiring a high short-circuit current withstand for a motor terminal box increases the cost and delivery time of a motor unnecessarily. A criteria similar to NEC 430-124 or 126 could be followed for the terminal box; Motor supplying conductors,controllers and motor branch-circuit disconnecting means shall have a continuous ampere rating not less than the current at which the overload protective device is selected to trip.

 

[jraef]

To further aolaldes comment, the fault level that will be seen at the motor terminal box will be the let-through of the short circuit protective device, which by code MUST be in the motor feeder circuit. I think that this may be what the motor manufacturers may be menaing in their explaination.

For instance, your switchgear must be rated for 25kA because that is the available fault level at the input terminals / busbar. But to feed a motor, you must have fuses or a circuit breaker. The energy that gets to the motor terminals is limited by what gets through the fuse or circuit breaker berore it clears the fault. In the case of current limiting fuses that may be as low as 5kA.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jbartos]

Comment: The motor manufacturer may decide to install current limitting fuses on the motor terminal box input side. Then, the motor terminal box may be rated for 200kA, for example, and the motor side will see 9kA, for example for 30A current limiting fuses.

 

[uno]

The short circuit withstand levels are very important in cases where manufacturer's provide motor terminal boxes which are fabricated instead of being cast , due to its large size to accomodate larger sizes of cables.

Also some of the manufacturers confirm the that the box can withstand fault up to 25 kA for 1 second although it will be damaged. The withstand levels may be applicable only when the motor is fuse protected and may not be applicable when it is breaker protected.

Check whether the boxes have explosion relief vents.