Ratio of Upstream / Downstream breakers for better selectivity

Charz

Member
Location
Texas
How the upstream breaker with 2 or 3 times rated higher than the downstream breaker provides a better selectivity? Can anyone explain me with an example.

The achieved selectivity is only for the overload region or also for the Instantaneous region?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
For proper selectively you should address all possible fault currents, overload, short circuit, and ground.
I have found the difference in frame size is a major consideration. The typical coordination problem almost always involves the magnetic portion of the curve. About the only way to change this on thermal-magnetic breakers is to change the frame, because all trip ratings in one frame use the same magnetic element.
 

David Castor

Member
Location
Washington, USA
Occupation
Electrical Engineer
It's difficult to generalize. If you are referring to molded case circuit breakers, then a 3 to 1 ratio between similar breakers should coordinate relatively well in the overload (thermal) range, but probably will not coordinate in the instantaneous range. This depends on the breaker characteristics and adjustability, but also on the available fault current. In general, you cannot assume molded case breakers will coordinate in the instantaneous ranges unless you evaluate the time current curves over the range of available fault current. If you absolutely must have complete coordination at low voltage, then fuses are the best option. Generally a 2 to 1 size ratio between similar current-limiting fuses should always coordinate. For a few types a 3 to 1 ratio is required. This is based on testing by the fuse manufacturers. Of course fuses have other drawbacks.
 
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