Current Limiting Breaker

[Pitt123]

Can someone explain how a current limiting circuit breaker works? Does the breaker have some other feature besides an instantaneous pickup for clearing a fault quickly thus making it current limiting?

I understand that to be current limiting the fault current must fall the the right of the instantaneous pickup vaule. How is this ensured and how does this limit fault magnitude?

 

[jim dungar]

A current limiting breaker can employ several different methods that will extinguish the arc fast enough to to meet UL's peak current and clearing time values. Most breaker use more than one method.

Ways to do this include:
a) Unlatch earlier (which is similar but may not be identical to what you are calling Instantaneous pickup).
b) Use the magnetic forces, generated by the fault current, to 'blow apart' the contacts so they actually open faster than normal.
c) Use the magnetic forces, generated by the fault current, to 'blow out' the arc faster (one method involves blowing the arc into a more complicated arc chute).

 

[zog]

Some breakers will also use current limiting fuses

 

[Pitt123]

A current limiting breaker can employ several different methods that will extinguish the arc fast enough to to meet UL's peak current and clearing time values. Most breaker use more than one method.

Ways to do this include:
a) Unlatch earlier (which is similar but may not be identical to what you are calling Instantaneous pickup).
b) Use the magnetic forces, generated by the fault current, to 'blow apart' the contacts so they actually open faster than normal.
c) Use the magnetic forces, generated by the fault current, to 'blow out' the arc faster (one method involves blowing the arc into a more complicated arc chute).

Do all these methods clear a molded case breaker in less than 1/2 cycle? What is the typical clearing time of an non current limiting molded case breaker?

 

[zog]

Do all these methods clear a molded case breaker in less than 1/2 cycle? What is the typical clearing time of an non current limiting molded case breaker?

I think I see where you are going with this, you have to be careful with assumptions of clearing times, there is no "typical", it depends on the type of trips the breaker has and the actual fault current. To get clearing times you have to refer to the trip curves.

 

[Pitt123]

I think I see where you are going with this, you have to be careful with assumptions of clearing times, there is no "typical", it depends on the type of trips the breaker has and the actual fault current. To get clearing times you have to refer to the trip curves.

I was referring to the "clearing time" as the time from when the breaker picks up to clearing the fault. For example a 5kV vaccum breaker has a clearing time of 3-5 cycles after initiating a trip command.

So for a molded case breaker if the fault is above the instantaeous pickup how long does the breaker take to clear the fault?

 

[skeshesh]

I agree with Zog. I'm not sure what you mean by "typical" as you brought up a 5KV application, but I think most on this forum would consider LV (600V or less) as a typical breaker. Here's a link to SqD's documentation for their smaller frame molded case breakers:
http://static.schneider-electric.us.../0150 A Frame PowerPact H/0611CT0401R0610.pdf

The trip curves start at page 59 so take a look. The "maximum" clearing time for a 60HZ system seems to be about 1 cycle. Obviously the trip curve will be different (even if slightly) from breaker to breaker and the hard part about power studies is to have the proper tools (software) and adequate experience/technical knowledge (I don't consider myself as such for example) to make the correct assumptions, specially if the system is large and complicated.

Again, be careful not to assume too much and to identify the breaker correctly if it's an existing system. Zog Jraef and some others have extensive experience and can probably help you find information on an older breaker.

 

[jim dungar]

Do all these methods clear a molded case breaker in less than 1/2 cycle?

These are some methods that are used to meet UL 489 which in part says "A current-limiting circuit breaker is one that does not employ a fusible element and that when operating within its current-limiting range, limits the let-through I2t to a value less than the I2t of a 1/2 cycle wave of the symmetrical prospective current."

What is the typical clearing time of an non current limiting molded case breaker?

It is entirely possible, and in fact is not uncommon, for a non-current limiting breaker to open and clear faults in less than 1/2 cycle but still not limit the current enough.

 

[Pitt123]

Ok

So lets say I had (2) 250A frame breaker with a 200A trip unit. One was a standard molded case breaker and the other is a current limiting molded case breaker. Both of these breakers are thermal-magnetic breakers with an adjustable instantaneous setting.

Lets say both of these breakers are set to their maximum instaneous setting of 10x or 2000A and are subjected to a fault of 5000A.

The standard breaker will pickup in the instantaneous region immediately and the breaker will trip instaneously with the time the breaker takes to trip being the "clearing time" of the breaker which I have always understood to be a mechanical function of the breaker which took about 1/2 cycle.

Now the current limiting breaker will also see this same 3000A fault and pickup in the instantaneous region, but what is it about this breaker that will make it react faster to the current and clear faster? Will it also limit the current to below the 3000A? Is it the mechanical makeup of this breaker that causes it to clear faster?

 

[skeshesh]

The simple answer is Yes. Here's a basic paper on the subject from ABB:

http://www05.abb.com/global/scot/scot209.nsf/veritydisplay/d342027bcb18bc9585257584006b2744/$file/1sxu210127g0201.pdf

I'm not an expert of the construction on breakers, but I believe the current limiting capability requires both faster sensing and faster acting. The link above discusses that these features are indeed present. As far as faster acting, it is easier to understand as it's a function of engineering better material. I only spent a moment looking for the link I provided, but if you tried working the google machine a bit you should find more detail. Also if you're an IEEE member it's likely that you'll find a reference in one of their Industrial Application Society or other related publications.

 

[templdl]

I've had the privilege of taking apart all of the Westinghouse, now Eaton, CL breakers. The trip units and the speed of how fast they pick up have nothing to do with them being current limiting as they are the same as a standard breaker. CL breakers that use CK fuses have a triggering mechanism that extends to activate the trip bar of the breaker. I have found the engineering of CL breakers to be most fascinating.
The breaker design engineer uses anything form "slot motors" to a reverse loop on the stationary contacts to blow the contacts apart and extinguish the arc in order to limit the fault current. It is the voodoo that is done in causing those contacts to blow apart quickly. The engineers use the magnetism that results from the fault current to their advantage with consideration to the design of the numerous plate that are used to divide uo in extinguish the arc.
Incidentally, all circuit breakers are current limiting to a given point which is why some can be series rated protecting breakers down stream from them.

 

[Jraef]

Here's a bit of trivia on the subject of CL breakers.

The "horseshoe" current path arrangement that is the facilitating design in forcing the contacts open quicker under a fault was invented at Klockner Moeller AG and patented by them in the late 1950s. It was then LICENSED to Westinghouse. When that patent expired in the mid '70s, Westinghouse and everyone else was able to adopt it on their own. That's when the current "C-Line" breakers were re-designed at Westinghouse so they could use their own mechanical design tweaks without having to pay K-M any longer.

Ironic twist: Westinghouse is no more, bought by Eaton, who recently bought Moeller Electric (the outgrowth of the old K-M) as well.

 

[templdl]

Jraef,
Yes, I know it well as I was part of that Westinghouse when Eaton bought the Distribution protection and control business unit and I know those breakers inside and out having the distinct privilege to meet and talk with the old time breaker design engineers before they retired. What a brain trust they were. I even had the opportunity to sit at the warranty returns test bench for a day to both visually inspect and electrically test product and write test reports. That was 20 years ago.

In the more recent times they are able to even take pictures of the arc and how it is extinguised inside the breaker.

 

[Jraef]

Jraef,
Yes, I know it well as I was part of that Westinghouse when Eaton bought the Distribution protection and control business unit and I know those breakers inside and out having the distinct privilege to meet and talk with the old time breaker design engineers before they retired. What a brain trust they were. I even had the opportunity to sit at the warranty returns test bench for a day to both visually inspect and electrically test product and write test reports. That was 20 years ago.

In the more recent times they are able to even take pictures of the arc and how it is extinguised inside the breaker.

I'd love to see those pictures. All I have ever seen is the result of the arc extinguishing in the arc chutes. We had to analyze the path of the arc based on the marks it made on the arc chute leaves as it propagated.

 

[templdl]

Jraef,
I got lucky as I thought that I discarding most of my stuff but found a VHS video which I think shows a lot of neat stuff.
I'll see if I can down load it to Youtube but it may take 2-3 weeks to get to it.
It shows the arcing in an arc chut of a breaker and there is also one showing the arc in a medium voltage vacume breaker.

 

[zog]

Jraef,
I got lucky as I thought that I discarding most of my stuff but found a VHS video which I think shows a lot of neat stuff.
I'll see if I can down load it to Youtube but it may take 2-3 weeks to get to it.

I would love to see that as well

It shows the arcing in an arc chut of a breaker and there is also one showing the arc in a medium voltage vacume breaker.

You mean inside the vacuum bottle?

 

[templdl]

I would love to see that as well

You mean inside the vacuum bottle?

Since the vacuum battle itself doesn't lend itself for taking pictures inside of the pictures look as if they are taken inside a larger vacuum chamber of sorts. It is actually a video as is the ones taken of the arc chute of the breaker.

 

[Pitt123]

I've had the privilege of taking apart all of the Westinghouse, now Eaton, CL breakers. The trip units and the speed of how fast they pick up have nothing to do with them being current limiting as they are the same as a standard breaker. CL breakers that use CK fuses have a triggering mechanism that extends to activate the trip bar of the breaker. I have found the engineering of CL breakers to be most fascinating.
The breaker design engineer uses anything form "slot motors" to a reverse loop on the stationary contacts to blow the contacts apart and extinguish the arc in order to limit the fault current. It is the voodoo that is done in causing those contacts to blow apart quickly. The engineers use the magnetism that results from the fault current to their advantage with consideration to the design of the numerous plate that are used to divide uo in extinguish the arc.
Incidentally, all circuit breakers are current limiting to a given point which is why some can be series rated protecting breakers down stream from them.

So if I understand what your saying its that the trip units in both types of breakers are identical and therefore the fualt is detected and picked up the same however its the design of the tripping mechanism in the current limiting breaker that causes the current limiting breaker to distinguish the arc faster and thus limit the current?

So if were looking at faults in the instantaneous range of the breakers does anyone know the total fault/arc clearing time from fault inception to total clearing on both types of breakers? I know with fuses any fault to the right of where the fuse curve crosses the bottom .01 second line is considered the current limiting range since anything below and to the right of this point is clearing in less than 1/2 a cycle.