Breaker types

[ryan_618]

How do you determine if a breaker is an instantaneous trip or an an inverse time?

Are they marked on the breaker? I've never looked that close.

 

[ron]

Re: Breaker types

The inverse time type trip is also known as the thermal portion of the characteristic curve, also known as the long and short time of the characteristic curve, and also known as the overload protection portion of the characteristic curve. Protection of this type protect after many electrical cycles have past.

Instantaneous type trips are fast acting and protect against short circuits (as opposed to overloads). These type breakers are called magnetic trip type breakers or motor circuit protectors. Some molded case switches (breaker without automatic protection) have some aspect of instantaneous protection characteristics.

 

[brian john]

Re: Breaker types

Most molded case CB's are thermal, magnetic. That is; they have instantaneous and long time trips. Most of the Instantaneous only CB's I've dealt with are in MCC's where the motors have overload protection. The adjustable settings on most of these CB's is for the instantaneous adjustment.

Of course in larger frame CB's electronic trips allow a variety of possibilities.

 

[ryan_618]

Re: Breaker types

The reason that I ask is I've got some motors at a job I'm inspecting that has overcurrent protection in the form of a breaker. 430.52 will be the governing table for OCPD's so I need to know how much larger the OCPD can be, which changes dramatically between the two.

 

[steve66]

Re: Breaker types

Now I'm really confused. If I understand brian john correctly, if I use a standard circuit breaker to protect a motor branch circuit (say a Square D type QO or QOB for example), I can use either circuit breaker column in table 430.52? So for a single phase motor, I could size the breaker at 800% of the full load current?

In the past, I have always been conservative and used the Inverse Time Breaker column.

Steve

 

[ryan_618]

Re: Breaker types

Originally posted by steve66:

In the past, I have always been conservative and used the Inverse Time Breaker column.

Steve

Steve, I'm the same way. I wish everybody did that! I f they used that column and breaker type they will not comply, however if they used the other they will....

 

[don_resqcapt19]

Re: Breaker types

Steve and Ryan,
Look at 430.52(C)(3).
Don

 

[steve66]

Re: Breaker types

Thanks Don. That makes it pretty clear that a QOB breaker would have to be sized per the Inverse Time Breaker column (even if the breaker has an instantaneous trip).

It is clear because a QOB breaker would not have an adjustable setting, and it would not be part of a listed combination motor controller.

Steve

 

[brian john]

Re: Breaker types

430.52 is the maximum size fuse/CB allowed. The instaneous portin of the trip curve is what you are trying to allow for with this table. The longtime trip would not be a consideration if say you used a QOB CB for a motor at the maximum allowable. But the motor is still protected by the Overloads.

 

[tom baker]

Re: Breaker types

A Sq D QOB is an inverse time thermal magnetic CB. An instantenous trip (IT)CB is also know as a motor circuit protector or mag guard. IT breakers can only be used as part of a listed motor controller. Most MCC's with CB's are IT type. We don't want to go there, but for the best motor protection, use time delay fuses.
To tell what type of CB you have consult the mfg catalog. The maximum NEC value for a QOB type breaker is 250%, which is too large in my opinon, 150% is better. Square D makes a really handy Motor Data Calculator, see if your distributor can get one for you.

 

[templdl]

Re: Breaker types

Remember that inverse time/instantaneous, or thermal magnetic, breakers will the UL listed 489 devices and will be labeled as UL where mag only will be labeled as a reverse UL or backwards UL component listed device. The most common application is as a motor circuit protective device. But, there have been mag. only breakers that provide supplementary protection such as for SCRs in drives in the past as an example. There have been a lot of mag only breakers that were specially calibrated for specific applications. You may have run across some older breakers in the field that were manufactured by Westinghouse under a shop order number that were calibrated at a specific magnetic setting +-10%, the setting of which was marked with yellow.
Because mag only breakers have no overcurrent protection they can be damaged from an overload.
I any even event, mag only breakers are not for branch circuit protection by themselves.