NON LIM Breakers?

[glenn bradley]

I am shopping for some 20 amp breakers for my soon to be installed 220v service. I see non lim (which I assume means non-limiting) breakers and 'regular' breakers. The non lims carry a different part number and are about $2 more. Cost is not the issue I am just not sure what I am looking at. The non lims also specify "for replacement only". What's up?

[Mark Hubler]

This is referring to non-current limiting breaker. Non-current limiting breakers trip after about 1 1/2 cycles, current limiting breakers trip at about 1/2 cycles.

[glenn bradley]

What do you all use in your shops? Only about 4 out of 40 different types were offered as non-lim.

TIA

[Greg Funk]

This is referring to non-current limiting breaker. Non-current limiting breakers trip after about 1 1/2 cycles, current limiting breakers trip at about 1/2 cycles.
For clarification, I believe the current limiting breakers don't trip any faster but they will clear the fault faster (i.e. within 1/2 cycles) once the breaker has tripped.

I think current limiting breakers are more applicable in an industrial environment where you potentially have greater fault currents available than in a residential shop.

Greg

[Rod Sheridan]

A current limiting circuit breaker has a faster tripping time than a non current limiting breaker.

During a short circuit, the current will rise to a mamximum value, determined by the circuit impedance, and will flow until the circuit protection (fuse or breaker) interrupts the current flow.

In a house, the maximum current probably wouldn't exceed 20 times the breaker or fuse rating, due to the circuit impedance. (This is an aproximation, the actuall value would have to be calculated for each branch circuit).

Obviously at 20 times breaker rating (example a 30 ampere feeder with a 600 ampere fault current), the amount of current multiplied by time and therefore energy delivered to the fault would be much greater if the breaker were slower to trip.

This is over simplified, and engineering members of the forum will be very familiar with these issues and calculations, as Arc Flash injuries and their prevention have become part of the electrical codes.

So, the condensed version? A current limiting fuse or breaker, opens faster, to reduce the amount of current and energy delivered to the fault. This would be a very good thing if the fault was a person with a conductive tool working on a live circuit. Nobody wants to look like Wile E Coyote on a bad day.

regards, Rod.

[SCOTT ANDREWS]

What do you all use in your shops? Only about 4 out of 40 different types were offered as non-lim.

TIA


Glenn
I looked at my breakers and it doesn't spec what type it is.I bought mine at lowes right off the shelf.They are the Square D brand.My panel is a Siemens which accepts Square D breakers.If your using a Homelite panel,make sure you get the right type of breaker for that panel.They have the QO and the normal style breakers.The QO breakers have the little broom handle type clips on the back.I made this mistake on my dad's sub panel.Bought the breakers and panel not paying attention,(not unusual) and had to return the breakers for the QO style.I like the non QO style better.FWIW,I did a search on both types of breakers,current limiting breakers from what I read seems to be more for sensitve type of equipment like computer stuff,special lighting,etc.I'm no electrician,but nothing has burned down yet in my shop.Good luck.
Scott

[Greg Funk]

A current limiting circuit breaker has a faster tripping time than a non current limiting breaker.

During a short circuit, the current will rise to a mamximum value, determined by the circuit impedance, and will flow until the circuit protection (fuse or breaker) interrupts the current flow.

In a house, the maximum current probably wouldn't exceed 20 times the breaker or fuse rating, due to the circuit impedance. (This is an aproximation, the actuall value would have to be calculated for each branch circuit).

Obviously at 20 times breaker rating (example a 30 ampere feeder with a 600 ampere fault current), the amount of current multiplied by time and therefore energy delivered to the fault would be much greater if the breaker were slower to trip.

This is over simplified, and engineering members of the forum will be very familiar with these issues and calculations, as Arc Flash injuries and their prevention have become part of the electrical codes.

So, the condensed version? A current limiting fuse or breaker, opens faster, to reduce the amount of current and energy delivered to the fault. This would be a very good thing if the fault was a person with a conductive tool working on a live circuit. Nobody wants to look like Wile E Coyote on a bad day.

regards, Rod.
I agree with everything up until the point where you imply that the current limiting breaker will trip faster if the fault was a person. A person has relatively high impedance and will not source enough current to trip a breaker quickly. A 20A current limiting breaker with a 60A fault will still take 1-3 seconds to trip. You'd be dead before the breaker tripped. I'd be surprised if most shop owners with 240V power are at risk of Arc Flash injuries as I believe these occur in industrial environments and they have standards available which are used to identify equipment which has a potential for Arc Flash injuries.

Greg

[Rod Sheridan]

Hi Greg, I didn't mean to imply that the person would be the conductive path.

If a person using a conductive tool were to cause a fault with the tool, the resulting arc would involve the person as far as burn injuries were concerned.

Anything that reduces the energy delivered to the person such as current limiting protective devices, or fire retardant clothing is a good thing.

Obviously at the end of the branch circuit, the exposure will be lower, than working in the panel, however if I were working in an energized panel (not reccomended) I would be wearing proper equipment.

I have a well burned set of insulating gloves, and leather outer gloves that I keep as as a teaching tool for course. They were damaged by a measuring accident involving a meter and a range receptacle in a house. Arc flash injuries occur at surprisingly low power levels.

It's nice to see that people are becomming more aware of electrical safety in their shops.

Regards, Rod.

[glenn bradley]

Thanks Scott and others. Sounds like 'plain ole breakers' is the standard in the workshop.

[Greg Funk]

I have a well burned set of insulating gloves, and leather outer gloves that I keep as as a teaching tool for course. They were damaged by a measuring accident involving a meter and a range receptacle in a house. Arc flash injuries occur at surprisingly low power levels.

It's nice to see that people are becomming more aware of electrical safety in their shops.

Regards, Rod.
Ron,

My only experience was to cut through a live cable. Apart from melting my cutter I had no damage. A friend wasn't so lucky and ended up losing his leg when a concrete pumper truck he was standing beside touched a high voltage line. The arc flash knocked him out and caused severe burns on most of his body.

Greg