Circuit interrupting rating

[brother]

Just trying to get a better understanding of of breakers. What is meant by the 'interrupting rating' ?? for example, Most of the breakers you buy like a 20 amp or 30 amp single pole 120/240 have it stamped on the side "10,000 A interrupting rating'.

I use to think this was the amps it would instantly open at a very SHORT time. if its 20 amps that i sees then it would take longer open up at. were talking mili seconds of course.

Please correct me if im wrong.

 

[480sparky]

A 10,000AIC breaker means it can open the circuit if there's 10,000 amps or less going through it. It may not open if there's more.

 

[augie47]

brother, you pretty well describe it. the interupting rating does refer to the current it will safely interrupt. In a "short" situation the curent flow can reach levels of 10,000, 50,000, even over 100,000 amps during the few cycles it takes for the breaker to open. That number is mostly dependant on what the utility can supply.
Failure to use a breaker with a high enough rating can result in catostrophic damage if it is called on to open.
The interrupting rating is a number which needs to be addressed in any electrical installation, howver, on most resid. installations it is well below the 10,000 AIC of a "standard" breaker.

 

[480sparky]

....Failure to use a breaker with a high enough rating can result in catostrophic damage if it is called on to open......

Meaning, if a 10KAIC breaker doesn't open with 50KA going through it, something's gotta give. That's when you see the Big Blue Zot and hear the Big Bang.

It ain't purty....

 

[crossman]

In the training videos we have, if you put a 50,000 amp bolted fault on a breaker with a short-circuit rating of 10,000, the breaker DOES indeed open the circuit. Only thing is, the breaker explodes like a bomb, sending molten metal and plastic and sparks and smoke and brilliant flashes of light all over the place. It is fun to watch.

Same thing for fuses which are subjected to several times what they are rated for.

 

[Lxnxjxhx]

breaker explodes like a bomb

breaker explodes like a bomb

Same thing on a smaller scale if you take a single strand of stranded lamp zip cord and wrap it around the prongs of a plug.
If you're looking at it when you plug it in, it looks like a grapefruit-sized white ball.

It's a great practical joke, blackens the outlet, scares the daylights out of most people, makes a strange noise, makes you want to pursue a career in electricity.

Two strands may trip the breaker.

 

[brother]

then help me understand more here. if its a '20 amp single pole breaker' why is it seeing or letting almost '10,000 amps' come through??

 

[ron]

Brother,
It will begin to open at some amperage >20A. The more current the faster it opens (inverse time).
If the calculated maximum short circuit current is 10,000A or less, the CB can be used. The circuit breaker will safely open the circuit during any detected current between 21A and 10,000A without catastrophic failure. Again the more current the faster with the fastest time reached at usually 10-12 times the trip rating.

 

[480sparky]

then help me understand more here. if its a '20 amp single pole breaker' why is it seeing or letting almost '10,000 amps' come through??

A short circuit will cause whatever amperage the transformer can supply to flow through the circuit for a few cycles before the breaker can open. It doesn't matter whether it's a 15a breaker or a 8,000a one. If the transformer can supply 50,000 amps, that's what will flow through the circuit until the breaker can react.

If the breaker isn't rated for 20,000 amps, then it may fail to open (contacts weld shut, etc.). Then the entire circuit turns into, literally, a fusable link. Whatever the weakest link is will blow apart first.

Breakers are mechanical, not magical. A 20a breaker will not open at 20.0001 amps.

 

[Lxnxjxhx]

why is it seeing or letting

why is it seeing or letting

I guess a good analogy is a pressure regulator. It's supposed to regulate a high "pressure" (10,000 amps) down to something lower (tripping eventually on a 120% overload) If the upstream pressure is too high (100,000 amps) the regulator fails. It acts as a regulator within a limited range. All bets are off above that range and the thing may act pretty strangely (although a breaker under these conditions does "open", in a manner of speaking).

Another analogy would be a bulletproof vest. It can't help but let some of the bullet's kinetic energy into the wearer, but at very high energy levels the vest is destroyed along with the wearer.

Probably many things we take for granted are only good within a limited range, including us, and as long as we stay in that range we never find out what happens above that range.

 

[iwire]

A short circuit will cause whatever amperage the transformer can supply to flow through the circuit for a few cycles before the breaker can open. It doesn't matter whether it's a 15a breaker or a 8,000a one. If the transformer can supply 50,000 amps, that's what will flow through the circuit until the breaker can react.

Very well put. :smile:

For what it's worth a 10,000 A interrupting rating is at the very ow end of the scale. Many times you will need breakers with much higher rating.

Things like conductor sizes and length from the source greatly effect the required rating.

 

[480sparky]

Very well put. :smile:

For what it's worth a 10,000 A interrupting rating is at the very ow end of the scale. Many times you will need breakers with much higher rating.

Things like conductor sizes and length from the source greatly effect the required rating.

Typically 22KAIC.

 

[zog]

then help me understand more here. if its a '20 amp single pole breaker' why is it seeing or letting almost '10,000 amps' come through??

The 20A is a "Frame size" rating, thats the amount of current it han handle 24/7, it is based on heat disapation capabilities of the breaker.

 

[crossman]

Brother:

This is my addition to what the others have already said. I hope it helps.

Circuit breakers and fuses act as conductors while the circuit is under normal conditions.

Current flow conforms to Ohm's Law.

Fuses and breakers do not open instantaneously when they experience fault current through them. It takes a little while before they can open up to stop the current flow.

Fault currents also conform to Ohm's Law. The total impedance value must include all of the factors involved - the internal impedance of the source, impedance of service, feeders, etc. This fault curent can be extremely high due to the low impedance of short circuits.

The initial fault current through a breaker or fuse can be much greater than what the breaker or fuse is rated to trip at. For example, a 100 amp CB under fault conditions could have 8000 amps (arbitrarily chosen figure from me) flowing through it for a few cycles before the breaker trips.

And depending on the impedance of the total path, in other scenarios, that 100 amp CB could have 40,000 amps flowing through it until it trips open.

The interrupting rating of a CB or fuse is an indication of how much fault current the OCP can safely shut off.

If the Interrupting rating is exceeded in a fault, the OCP can be destroyed and can cause serious damage to persons and property. Like I said earlier, there are videos that show what can happen. The improper use of OCP Interrupting ratings can create a bomb!

There are formulas and tables which can be used to calculate the maximum fault available at the OCP device. They involve the source internal impedance, the voltage, the type of system, length of conductors, and type of wiring methods used.

These calcs aretypically made by enginners who specify the proper OCP while the job is in the planning stages.

 

[charlie]

Breaker Explodes Like a Bomb!

Breaker Explodes Like a Bomb!

Same thing on a smaller scale if you take a single strand of stranded lamp zip cord and wrap it around the prongs of a plug.
If you're looking at it when you plug it in, it looks like a grapefruit-sized white ball.

It's a great practical joke, blackens the outlet, scares the daylights out of most people, makes a strange noise, makes you want to pursue a career in electricity.

I know we are all electrical professionals; however, everyone who reads these posts is not necessarily an electrical professional. I would hope that no one would be stupid enough to actually follow your advice. Assume for a moment that there was enough impedance in the circuit to delay the overcurrent device for the circuit from opening; have you caused someone to start a fire? OK, assume the available fault current was high enough to do as you have suggested; has the practical joker been aware that maybe there shouldn't be anything around that would catch fire?

I know I am coming down with both feet but this is something for either an accident or a test bench, not to be done on purpose to someone with limited electrical knowledge.

 

[templdl]

Arcing fault or bolted fault that is the question.

Which one is more common, an arcing fault or bolted fault?

What will perform better in detecting and quickly responding to and clearing and arcing fault?

It has been of my experience that an arcing fault may start with a high current that may decrease as the arcing fault continues and the impedance may increase, that the impedance of arcing fault will be different depending upon their dynamics.
That the magnetite of the current which feeds into an arcing fault as ambiguous at best.
A breaker has a definite long time (thermal) element that respond to I2t and a definite instantaneous (magnetic) element that response a specific peak current value. Will a fuse respond identically or better?
It is of my opinion that a breaker will detect an clear an arcing fault faster should the magnitude of the arcing fault reach that of the magnetic trip value. If not, then the breaker's I2t thermal element my respond which must be compared to the response of that of a fuse in this situation.
I believe the breaker will respond to and clear an arcing fault quicker than a fuse limiting the damage of an arcing fault.

Which one will have a tendency to clear a bolted fault and limit-damaging let through current? It is of my opinion that a fuse by nature will be more likely to be current limiting be reducing the damaging fault current. Again one must qualify each application, that fault current available, the specific breaker's ability to be current limit Vs a fuse, and the bolted fault itself.

What types of faults are more likely to occur, arcing or bolted?
Which OCPD is better suited for each.

 

[Lxnxjxhx]

scares the daylights out of most people

scares the daylights out of most people

Maybe you were off-duty at the time, but is this any worse than the post that talked about shaking the bucket at the end of a boom to scare the other guy?

 

[acrwc10]

Maybe you were off-duty at the time, but is this any worse than the post that talked about shaking the bucket at the end of a boom to scare the other guy?

Still seems kind numb between the ears.

 

[templdl]

Is it common for an OCPD to be subjected to their maximum KAIC or is in imperative that we simply cover it for the rare even that it may need to do so? Yes this is extremely important but where is the real action at, bolted or arcing faults?

It may be like "shaking the bucket at the end of a boom" to some that may not have an appreciation of time current curves of OCPDs when compared to how they will respond to various faults. It's a matter of current and time, arcing fault or bolted fault, which is more likely to occur, what the OCPD will respond to Vs what we want it to respond to?
We all have a tendency to zero in on a bolted fault that is a worse case scenario. But can it ever determine what an arcing fault will be?

Interrupting rating? My point being it is imperative that the OCPDs be rated to clear a fault without destroying itself. It is imperative that we have to cover it. We all hope that we don't have to use the spare tire in our car or truck but is would not be very wise not to have one as we hope that the OCPS will never be exposed to its maximum interrupting rating.

It is of my opinion that arcing faults, which I understand are more common, will not require a breaker to clear maximum fault current available. It must respond quickly if the current is of high enough magnitude to cause a trip limiting damage. As such it is my preference to select a device that with respond to an arcing fault more effectively where I prefer breakers. Ground fault may be better suited to pick up on low level arcing faults that may not reach a magnitude to trip the instantaneous element of a breaker.

The question if a 20a breaker is calibrated magnetically to trip at 10X its rating, (200A) if the arcing faults magnitude was 200a the breaker would clear in a couple of cycles. Will a 20A fuse respond in the same way?
If the current were less than 200a then were talking in the I2t area. Which would respond faster?

If you had a bolted fault which one would clear the fault if applied with its rating? Both would. However, a breaker may be consistent in opening and clearing a fault quicker than a fuse where a fuses performance from what I understand improves as the level of fault increases to a maximum available fault current available which results in the fuse clearing the fault faster than a breaker and limiting the let through fault current.

I have seen an outlet kicked out by a student in a high school and start arcing which I would have expected to have trip a breaker but didn't. Was the breaker defective? No. The magnitude of current was never high enough for the breaker to respond to.
Was the interrupting rating of the breaker a factor? Certainly not.
Which OCPD would have been a better choice, breaker of fuse?
Can it be assured that all arcing faults will be the same, that is are they consistent by nature? Are they predictable.
Can the same be said about true bolted faults? Are they predictable?

Which more often causes personal injury and/or death, arcing or bolted faults? Will a bolted fault develop into an arcing fault?