Breaker Instantanious Settings

[Humble2000]

Dear Friends,

I need to adjust instantaneous settings on a 600AMP magnetic breaker that feeds a 250KVA 600/208V transformer. This breaker only has instantaneous settings.
I don't have any background in this field and appreciate any feedback on this subject.

Thanks

 

[jraef]

If you are in the US or Canada, that breaker cannot be used for feeding a transformer. Mag-only breakers are specifically used for motor starter circuits where there is a separate thermal tripping device dedicated to the circuit (the Over Load Relay). They are not legal for use in any other application. You must use a thermal-magnetic circuit breaker. Maybe if you also have fuses in between the breaker and the transformer you can make a case, but if this is going to be inspected I seriously doubt any inspector will approve that arrangement.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[wbd]

I would ask why you are changing the instantaneous settings on the breaker when you state you have no background in this field. Why are you changing the settings? Is it tripping? Is this tripping new and the breaker has been installed for awhile? (might be indicative of a problem developing) Is it really a mag only breaker or since you don't have any experience with breakers is it a thermal-magnetic bkr?

 

[suriae]

This is something serious,however I presumed the reason why you need to adjust it because it is tripping inadvertently as wbd suggest. My "guesstimate" is this breaker was originally set at Iins=8*Irated, so, if you don't find anything abnormal then you may raise up to Iins=11*Irated BUT mind you take jraef advice!!

 

[DanDel]

What is the manufacturer/model of this device? Is it truly just magnetic trip, or is it thermal/magnetic with an unadjustable thermal unit, and adjustments just for instantaneous (like almost all thermal/mag C/Bs)?

And, as mentioned by others, why do you need to change the settings?

 

[Humble2000]

Breacker is thermal/magnetic breaker.
Two loads were fed from a single breaker,we are seperating the protection for each load.
We are in Canada.This is a new installation and I need to adjust the breaker settings.

 

[waross]

Without a little more information on your application, all I can say is
"Turn the adjustments with a screwdriver."

 

[Humble2000]

Waross,
Thanks for the reply.
Its a 600AMP thermal-magnetic breaker to protect a 250KVA 600V/208. What other information is missing?

 

[DanDel]

How does adjusting the Inst setting separate the protection for two loads?

 

[Humble2000]

We will leave the existing breaker in place feeding the other load and use this new breaker only feed the transformer.

Chiller and transformer were fed from a same breaker. Now, chiller will be on seperate 600AMP and Transformer on new 600AMP breaker.

 

[DanDel]

A 250kVA transformer with a 600V primary will, per IEEE, max out at 10 x FLA for 0.1sec upon inrush, which is 240.6A x 10 = 2406A.

Your 600A C/B typically would have an Inst range of 5x to 10x the rating (3000A - 6000A), so any setting shouldn't cause a nuisance trip, though it will apparently allow a 250% overload of the transformer. This may or may not be acceptable for your application.

The proper way to do this, though, is to plot the C/B curve and the transformer damage curve to verify protection.

 

[jraef]

Is this a 3 phase transformer or single phase? A standard 3 phase transformer in that range is 225kVA, 250kVA is a standard single phase size.

The reason I ask is that if it is 3 phase 225kVA, the FLA will actually be around 217A. If it is 3 phase and 250kVA then it is a custom wound transformer, making DanDel's comment on attaining the damage curve even more important.

If it is a 1 phase 250kVA, the primary current will be around 417A, which makes a difference in your magnetic settings (as far as the amp rating, not the percentages).

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376