Why a high magnetic breaker?

Status
Not open for further replies.
mark32

mark32

Senior Member
Location
Currently in NJ
Hello, I'm currently working at an electrical supply house, but have six years in the trade. Today I was going through some Square D items and came across a QO breaker that had a sticker on it that said "High magnetic". I've never heard of such a thing, what's so special about it and when would you use it?
 
Re: Why a high magnetic breaker?

I think you'd find that generally a plain Jane thermal-magnetic MCCB, like a QO, has the instantaneous, magnetic portion of its trip set at appx. 10-times the "handle-rating".

A High-Magnetic version would be more than 10X.
Evidently it is a specialized bkr. for serving some type of load with extremely high inrush.

Not sure what that load would be, since 10X ought to be enough for motors.
 
Re: Why a high magnetic breaker?

If you are talking about the 15-20A units (QO/QOB) then they are typically used for large LV lighting transfomers. These loads have a very high inrush. I am troubleshooting a circuit that has 3-300W units on it, and is recording inrush of over 140A! on a 20A circuit. I hope the HM breakers I have on order will fix the problem.

A close relative of the HM breaker is the QO-HID which has the same trip characteristics as HM, plus heavier duty contacts for handle the large inrush currents that a bank of HID lighting will have. THey are also SWD (Switch Duty) rated for switching directly from your panel, every day.

The Mission Critical, as ron mentioned, are 225A/400A HM breakers that are used to ensure a trip on a 20A circuit won't take out the whole feeder.
 
Re: Why a high magnetic breaker?

Thanks guys for the info. Today I asked the salesman that placed the order (For the 20 amp breakers in question)if he knew what separated them from a typical breaker but he had no idea. Now I know, and I'm probably the only one in the building who does know. Thanks again, Mark
 
Re: Why a high magnetic breaker?

The common breaker has always had a mag. calibration of 10x the rating of the breaker with the exception of small residential breaker which could have been even higher. The residential breakers that I've beed familiar with have been built in there family groups. 15-30a, 31-70a, and 71-100a. Because they are so price sensitive, selling for 3 to $6 per pole you know that the manufacturer's cost may be $1 each at best so a 1, 20, 25, and 30a breaker probably have the same magnetic trip calibration.
Anyway SqD as close as I can tell made their standard breaker to be about 7x and sold their 10x breakers as high mag where manufactures like Westinghouse which I know had a standard breaker as 10x which they had always been and then introduced a low mag breaker as 7x.
 
Status
Not open for further replies.
Top