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# Vacuum circuit breaker sizing 3

## Vacuum circuit breaker sizing

(OP)
Hello friends

I’m new to MV circuit breaker sizing . Could someone help me size the MV vacuum circuit Breakers A,B, C and D below please
We have a 11kv cable suppling a MV switchgear (A) which is also supplied by a 2mva (B)generator .

The switchgear is supplying two substation with 2.5mva and 3mva transformers(0.4kv/11kv)(C)and (D) respectively - circuit breakers before transformer .

http://files.engineering.com/getfile.aspx?folder=d...

Thank you

### RE: Vacuum circuit breaker sizing

All four should be identical, probably 1200A. The relays determine where the trips happen, not the breaker.

### RE: Vacuum circuit breaker sizing

(OP)
Hi David
Could you please show how the size was calculated?

### RE: Vacuum circuit breaker sizing

Generally the smallest practical size. 800A might be possible in some product families.

(OP)

### RE: Vacuum circuit breaker sizing

The maximum current you can see is less than 300A - both loads together. The smallest available breaker is much larger. Not much to calculate. You also need to pick an interrupting rating high enough for the application, that depends a lot on what the external system is like. That generator, if it were the only source of fault current - wouldn’t even get to the breaker thermal rating.

It’s just a matter of looking at what’s available and picking one, expressing those load values in Amps is about the only possible calculation in this case.

### RE: Vacuum circuit breaker sizing

See below sample of Vacuum Circuit Breaker sizing. While the demand load current is only 335.26 Amps, the selected VCB rating is 1200 Amps.

Actual application rating of Vacuum Circuit Breaker depends on the CT ratio and the relay settings.

Vacuum Circuit Breaker (VCB) is electrically operated switch. There are no overcurrent devices built in. The overcurrent protection and control comes from using current transformer (CT) and protective relays that are mounted in the switchgear assembly. Power is derived from a control source, batteries or control power transformer external to the breaker assembly.

### RE: Vacuum circuit breaker sizing

(OP)
Thank you friends , if the rating is that high despite the small Amerage of the load , how the tripping is carried out?

### RE: Vacuum circuit breaker sizing

(OP)
LET me give this example , for a 11kv system, if a MV switchgear is supplying 6 2mva transformers ( connected in serious ) , what would be the right size for the vacuum circuit Breaker of the main feeder the the first transformer is connected to. Would 630A be sufficient or too small ?

### RE: Vacuum circuit breaker sizing

A medium-voltage circuit breaker cannot trip on overcurrent by itself. It requires the use of accessories to be able to trip on overcurrent. Typically, these accessories are required:

1. Protection relay, usually overcurrent relay.
2. Current transformer
3. Power supply, usually a battery bank.

The current transformer senses the current flowing in the circuit breaker. Its secondary output is fed into the overcurrent relay. The overcurrent relay can be set to trip when the current input from the CT secondary reaches a particular level which is commensurate to the overcurrent value at the primary side of the CT. When this condition arises, a trip contact of the relay will energize the trip coil of the medium voltage circuit breaker causing it to trip.

The power supply provides operating power to the overcurrent relay (for electronic relays) and power to trip the circuit breaker when required.

### RE: Vacuum circuit breaker sizing

Moreover, any circuit breaker with rated current and interrupting rating higher than the load current and the available fault current respectively at the circuit breaker location can be used. Economics dictate the use of breaker ratings closest to the actual load and fault values.

### RE: Vacuum circuit breaker sizing

2
You're asking questions about equipment that suggests you should not be choosing said equipment. You need to understand more than choosing enough amps to properly select medium voltage power equipment. You have to understand the power, short circuit levels, control and protection scheme, switching scheme, wiring, installation, etc to get it right.

### RE: Vacuum circuit breaker sizing

(OP)
Thank you friends
Lionel- could you please give me a short introduction about these points and any suggest any good materials or websites to cover them .

Also could you answer my question above and confirm the correct circuit breaker size given the indicated sizes of transformers

### RE: Vacuum circuit breaker sizing

No. You'd have to take a multiple day course just as an introduction.

### RE: Vacuum circuit breaker sizing

FYI, towards the top of this page below the subject name and above the responses is a greyish bar with buttons. Try clicking the FAQ button. There is a FAQ with a list of reference materials.

### RE: Vacuum circuit breaker sizing

(OP)
Thank you

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