LV vs MV short circuit ratigns

[philly]

Why are there differences between LV short circuit ratings and MV short circuit ratings?

LV breakers only have interrupting ratings and MV breakers have withstand and interrupting ratings. Is there a reason why two ratings are needed for MV equipment? Does it have something to do with the delayed opening of MV breakers where LV breakers open faster?

 

[topgone]

Interrupting ratings -->the fault current level that the equipment can withstand without getting damaged when the equipment contacts part or break. a.k.a. breaking current rating
Withstand ratings--> the fault current level that the equipment can withstand/ let-through or carry without melting/getting damaged. a.k.a. let-through rating

 

[SG-1]

Why are there differences between LV short circuit ratings and MV short circuit ratings?

LV breakers only have interrupting ratings and MV breakers have withstand and interrupting ratings. Is there a reason why two ratings are needed for MV equipment? Does it have something to do with the delayed opening of MV breakers where LV breakers open faster?

I think that information is used to design selective coordination into a MV system.

 

[GoldDigger]

I think that information is used to design selective coordination into a MV system.

+1
I think that the assumption related to the withstand rating is that some other device will be interrupting that level of current first or that the short will be momentary by its nature.

 

[Tony S]

If you look at the manufacturer’s full literature for both LV and MV breakers you will find the same information.

It is basically for the design of protection and discrimination.

Schneider have a very good network design guide you can download for free.

 

[lielec11]

Interrupting ratings -->the fault current level that the equipment can withstand without getting damaged when the equipment contacts part or break. a.k.a. breaking current rating
Withstand ratings--> the fault current level that the equipment can withstand/ let-through or carry without melting/getting damaged. a.k.a. let-through rating

If you take out a few words, you're basically saying the same exact thing...

Interrupting ratings -->the fault current level that the equipment can withstand without getting damaged when the equipment contacts part or break. a.k.a. breaking current rating
Withstand ratings--> the fault current level that the equipment can withstand/ let-through or carry without melting/getting damaged. a.k.a. let-through rating

I too am interested in a more detailed answer. I always took it to assume the withstand was for a momentary fault, which is why some manufacturers list it as momentary interrupting rating sometimes as well.

 

[GoldDigger]

If you take out a few words, you're basically saying the same exact thing...

Interrupting ratings -->the fault current level that the equipment can withstand without getting damaged when the equipment contacts part or break. a.k.a. breaking current rating
Withstand ratings--> the fault current level that the equipment can withstand/ let-through or carry without melting/getting damaged. a.k.a. let-through rating

I too am interested in a more detailed answer. I always took it to assume the withstand was for a momentary fault, which is why some manufacturers list it as momentary interrupting rating sometimes as well.

The "few words" difference is "when the contacts part or break" in the first case.
That is no small distinction since the energy released inside the breaker is far higher when an arc is present than when there is a totally resistive path.
The mechanical (magnetic) forces are similar though.