How to Calculate Sizing of Vacuum Circuit Breaker

How to calculate sizing of vacuum circuit breaker  electriciansforums.co.uk /threads/how-t /threads/how-to-calculate-sizing-ofo-calculate-sizing-of-vacuum-circuit-bre vacuum-circuit-breaker.21588/ aker.21588/

Discuss How to calculate sizing of vacuum circuit b reaker in the Commerci Commercial al Electrical Talk area at ElectrciansForums.co.uk. Tags: breaker  circuit Thread Status:

Not open for further replies. Please make sure you checkout our forum sponsors, many do discounts for  members and, they keep the forum free to use.

1. Offline

mooyong

Guest 

1/5

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

2. Offline

Knobhead

Guest 

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

3. Offline

mooyong

Guest 

2/5

Tony said: ↑ 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.

4. Offline

Knobhead

Guest 

Other than efficiency your calculation is correct.  But you asked why a 400A VCB.

5. Offline

caddy

Guest 

Tony said: ↑ 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.

6. Offline

mooyong

Guest 

caddy said: ↑ 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.

3/5

7. Offline

Knobhead

Guest 

caddy said: ↑ 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.

Last edited by a moderator: Aug 31, 2010 8. Offline

nguyenkhoadung9

Guest 

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

9. Offline

GMES

Chief Gorilla

Staff Member Trusted Advisor  4/5

Thread closed due to it being 6 years old.

Thread Status:

Not open for further replies.

Share This Page Tweet

Users found this page by searching for: 1.

vcb breaker rating calculation ,

2. electrical vcb calculation , 3. vcb breaker ratting determined , 4. how to calculate the size of vcb Checkout the deals and offers on Power Tools on eBay - Click Here

5/5