How to calculate sizing of vacuum circuit breaker

[mooyong]

Hi- I am looking for some help/ advice please. I'm confused about how How to calculate sizing of vacuum circuit breaker.

This project I work on it use 40m0A TP Vacuum circuit breaker to support pump 800 kW 11kV

I calculate the current just 49.4 A (800*1000/1.732/11000/0.85 ) I don't know why it use 400 A Someone know please explain me

Sorry for bad english i'm not native language

 

[Knobhead]

It’s the smallest size made by most manufacturers. For most the smallest size is now 630A

 

[mooyong]

It’s the smallest size made by most manufacturers. For most the smallest size is now 630A

Thank you for answer but I would like to know I calculate that (49.4A) correct ot not. There have any factor I must consider or not.

 

[Knobhead]

Other than efficiency your calculation is correct.

But you asked why a 400A VCB.

 

[caddy]

Other than efficiency your calculation is correct.

But you asked why a 400A VCB.

I think he is asking why a would a 50A motor have a 400A breaker protecting it.

 

[mooyong]

I think he is asking why a would a 50A motor have a 400A breaker protecting it.

Yes, I mean that why a would a 50A motor have a 400A breaker protecting it.

 

[Knobhead]

I think he is asking why a would a 50A motor have a 400A breaker protecting it.

Ahh it all becomes clear now,

400A is the current carrying capacity of the VCB, protection is provided by a combination of the Current Transformer (CT) ratio and the Overload (O/L) relays.

Protection relays are normally rated at 5A = 100%

So for your 800KW motor with a Full Load Current (FLC) of 49A you could use 50/5A CT’s with the O/L’s set at say 110% or 100/5A CT’s with the O/L’s set at 55%. Both will give 110% overload protection.

By changing the C/T ratio the O/L’s of the VCB can be set to anything up to 400A

The transformers show in the schematic 1500KVA @ 11000V will have a FLC of 79A so you could use 100/5A C/T’s with the O/L’s set to 90% this will give slightly above 110% O/L protection. (The protection relays I’m used to dealing with are the old PBO & TJM10 inverse time mechanical relays, so setting them to within 1 or 2% isn’t easy)

Sorry for the confusion.

 

[nguyenkhoadung9]

Other than efficiency and voltage drop (at CB) your calculation is correct
Tony was right, just forlow tony's instruction

 

[GMES]

Thread closed due to it being 6 years old.