Medium Voltage Protection 1

[beghtesady]

Can anyone clarify why the NEC treats the medium voltage (> 600v < 38kv)protection differently as compared to low voltage (<600v)systems? It appears that the code is only requires short circuit protection for medium voltage system (with the exception of motors and transformers) while in low voltage system the code is also concerned with overloads in any feeder. Why such a difference? Is the code trying to tell us that we do not get an overload condition(s) (due to an increased load or a reduced fault current) in medium voltage systems!?

 

[peebee]

My guesses are as follows, in no particular order:

1. There's no goofy receptacle loads where you never know what's going to be plugged in. Specifically, very very few plug-connected motors.

2. There's usually a much higher level of engineering going on with MV systems (very few architects stamping drawings).

3. Loads are typically more constant/consistant.

4. Usually, you're feeding a transformer, which can handle an overload. If not a transformer, most likely a motor, which would require overload protection anyway.

5. MV systems often use fuses, and it's tough to build an MV fuse than can provide accuracy in both the OL & OC curves.

I could be wrong, wouldn't be the first time.

 

[pyramids]

In addition to the NEC, for MV short-circuit protection and design make sure that you consider IEEE color series particularly the Red book for application guidelines. While your questions specifically pertained to the NEC, the National Electric Safety Code (NESC) may also be germane to your MV short-circuit concerns.

Best regards,
Pyramids