ECN Forum Forums General Topics of the Trade General Discussion Area Adjustable trip/ Inverse time Breakers

What is the difference between these two, and other than motor loads, where do most of you guys use these type breakers?

I recently came across a couple of these adjustable trips in a MDP, that fed subpanels in commercial units, and can't figure out why someone used these in this application.

Thks..


Dnk...........

Dnk:

IIRC, adjustable-trip breakers simply give an adjustable range around the "nameplate" rating, so you can better protect the load. Great for motors. (For example, a 100a rated breaker may have a range from 75-105 amps.) [NOT necessarily a real-life example, you get the idea.]

Inverse-time breakers are a different animal, if I understand them correctly it means the the time from start of overload condition to trip point is inversely proportional to the degree of overload. In other words, the greater the magnitude of overload, the faster the trip time.

Enquiring minds would like to know....

[This message has been edited by mxslick (edited 07-03-2005).]

[This message has been edited by mxslick (edited 07-03-2005).]

Adjustable trip breakers in sizes below 400 amps are rare. There are small frame breakers where the long time trip element can be changed, but they are not adjustable by turning a setting on the breaker. The may have an adjustment for the instantaneous trip for short circuits and ground faults, but this does not change the long time trip rating. These are known as thermal magnetic breakers, with the thermal part providing the inverse time overload protection and the magnetic part providing the instantaneous trip function for short circuits and ground faults. There are also instantaneous trip breakers (also known as magnetic or magnetic only) used as a part of a combination type motor starter, have adjustments for the instantaneous trip point. Other than the motor circuit protectors (instantaneous trip) most breakers have an inverse time function as do many types of fuses.
Don
Don

This one is right up my alley.
First, inverse time is another name for thermal magnetic. It means that if there is a lot of current, such as a short circuit, the magnetic or instantaneous portion of the characteristic will open the breaker fast. An overload would activate the thermal portion and will take a bit more time to trip the breaker open. So the amount of current is inversely proportional to the time it takes to trip.
There are many different adjustments available from different model/style breakers. Instantaneous adjustments are very helpful in reducing arc flash incident energy and protection of a motor during startup (allowing the protection characteristic to more closely follow the starting current without tripping). The long or short time is adjusted, along with instantaneous, to assist in the OCPD closest to the fault/overload to trip first (selectivity).

Thanks to all.........

Dnk...