How to design circuit breaker capcity

Hi to all,

I have some doubts in choosing of circuit breaker.......

could any one help me for these....

How to design circuit breaker capacity(KA) for Motor & heater?

if i use 37kw /440V motor min how much KA should be used?
if have any simple calculation, pls explain clearly?

thanks for Advance....

You have to design a cascading system and what is up the line (before your circuit breaker) will determine the kA rating you require.
Have a look in the back of a Schneider circuit breaker catalogue - it may help.
I would suggest you use a specialised motor circuit breaker - they generally have a fairly high kA and you can use a lower rated device as the magnetic component is higher and specially designed for motor starting (inrush).

verify first if the load is single-phase or three-phase..
for single-phase:
power/voltage=ampere rating of the breaker
note: for motors always multiply by 2.5 the quotient of p/v
for 3-phase
power/(sq.root of 3 x(voltage))=ampere rating of the breaker
note: for motors always multiply 2.5 to the quotient of p/v

KA IS BREAKING CAPACITY AND THIS ISN'T IMPORTANT FOR PROTECTION YOUR MOTOR OR HEATER YOU MUST BE USE A GV2 OR GV3 (circuit breaker)FOR YOUR MOTOR PROTECTION (MY OFFERR GV3ME80 FOR 37KW )
BUT IN HEATER YOU CAN USE MCB FOR PROTECTION FOR DETRMINE CURENT YOU CAN USE THIS FORMULR P(KW)=VI
KA IS IMPORTANT IN MPS (MAIN POWER SUPPLY) IN YOUR PANEL
IF YOU DONT HAVE ANY IDIA FOR DETERMINE THAT YOU CAN USE A FUSE FOR MPS WITH 100KA BREACKING CAPACITY

SPBDASAN said:

Hi to all,

I have some doubts in choosing of circuit breaker.......

could any one help me for these....

How to design circuit breaker capacity(KA) for Motor & heater?

if i use 37kw /440V motor min how much KA should be used?
if have any simple calculation, pls explain clearly?

thanks for Advance....

Click to expand...

You are standing at the circuit breaker, looking toward the motor and trying to determine the KA rating.
What you need to do is turn around, and look toward the power supply transformer to determine the KA rating.
From memory, don't quote me, as I dont have reference books with me, I believe a 1000 KVA transformer requires a 35 KA circuit breaker if it is close to the transformer.
A breaker in your house should be alright at 6 KA , as it is further from the transformer, and connected to smaller supply cables.
Never compromise on KA rating, as it is what determines whether it will turn off under major fault conditions.

From memory, don't quote me, as I dont have reference books with me, I believe a 1000 KVA transformer requires a 35 KA circuit breaker if it is close to the transformer.

Click to expand...

Depends on the transformer design - I have a 50kA main switch on my house board - 1000kVa tranny and council advise me I need 50kA because of the transformer design.

KA IS BREAKING CAPACITY AND THIS ISN'T IMPORTANT FOR PROTECTION YOUR MOTOR OR HEATER YOU MUST BE USE A GV2 OR GV3 (circuit breaker)FOR YOUR MOTOR PROTECTION (MY OFFERR GV3ME80 FOR 37KW )

Click to expand...

It is very important if the client/boss requires cascading protection - and it depends on what is in front of the control panel.
I am doing a job at the moment whwre I have 3 generators going onto a switchboard. They are about 1750 kVa each but the fault potential level is 80kA!!! Remember, they are basically a mechanical device and really cannot generate a huge kA level - trannies on the end of the grid can be huge.
A proper protection study needs to be carried out to determine the kA level of the MCB - unless the kA level is knowon.
Big bang theory otherwise!!!

BobB The refernce book I was thinking of was The Australian NewZealand Electrical Standards 3000, That I think gives the detail on KA ratings against transformer KVA.

I know all our Main breakers are greater than 50 KA and most are 65KA, with our Large HRC fuses being Rated at 80KA or 120KA in order to make sure faults are cleared

Just be carefull of the latest AS/NZ3000. Maximum of 2 circuits per RCD and the introduction of a section on surge protection.
Also a greater section on generators.
It was supposed to be smaller/simpler - it is bigger and more complex than ever!!!

BobB said:

Just be carefull of the latest AS/NZ3000. Maximum of 2 circuits per RCD and the introduction of a section on surge protection.
Also a greater section on generators.
It was supposed to be smaller/simpler - it is bigger and more complex than ever!!!

Click to expand...

I know its an Electrical standard the trouble is that its written by Lawyers and Accountants

The two pieces of information required for the design of an electrical circuit are the ELI (Earth Loop Impedance)value and the SSC (Short Circuit Current) value, both referenced to the origin of your particular circuit.

As well as choosing the correct protection device for the motor you need to calculate the cable size for the installation which is mainly dictated by the volt drop unless its a very short run. Don't forget the protection required for the cable. The protection for both can be found in one device such as motor circuit breaker but if you use contactor and overload, a seperate device is required to protect your cable.

An excellent reference manual to help is the Moeller wiring manual, you can find it at this address:

http://www.moeller.net/en/support/wiring_manual.jsp

You do not have to buy Moeller equipment, the principles in the manual with help in choosing the correct equipment from any suppliers technical publications.

Regards

Paul

The relationships between transformer sizes and prospective fault currents are only rule of thumb values but they should cover most general distribution systems. Industrial installations are more complicated however because large rotating machines will effectively act as generators for fault current if there is a major fault. This can result in much higher fault currents than the rule of thumb would suggest during the initial moments of a fault and the breaker must be able to handle this.