Breaker and Equipment Withstand ratings

[philly]

I posted a similar thread before regarding power breaker withstand ratings but it is now closed so I wanted to re-visit this topic again.

I understand that a typical power breaker has both an Interrupting Capacity (usually couple of cycles) and withstand capacity (usually 30 cycles). In looking at a typical power breaker I see that it has a IC of 65kA and a Withstand Or Short time capacity of 50kA (I assume Withstand and Short Time are more or less refer to the same thing). So I understand these ratings to mean that the breaker can interrupt a fault up to 65kA and tolerate this fault for the couple of cycles that it takes to open the breaker and clear the fault. If a breaker does not clear the fault within the first few cycles (Instantaneous) then the rating implies that the breaker can tolerate a fault up to 50kA for 30 cycles until the short time element clears the breaker. This would be a case where the Instantaneous setting was turned off, or was above the maximum fault current so it didn't pickup and thus the fault would need to be cleared by the breakers short time.

For instance lets say that with the breaker ratings above the INST setting is turned off but the fault current available at the breaker is 53kA. In this case since the INST would not open the breaker, the breaker would need to withstand this fault current for 30 cycles until the short time element picked up. However in this case the 53kA fault current is above the withstand rating of the breaker so I'm guessing that the breaker would fail or receive a failed rating in an evaluation. In this case the INST setting could not be turned off on the breaker. Am I on the right track here?

This same case seems like it would apply to a situation where the breakers INST setting is turned off, or set above the maximum fault current for coordination purposes. In either of these cases the engineer must then look at the maximum fault current to determine weather or not the breaker can withstand the maximum for 30 cycles. Is this correct? I'm not sure how plausible the example that I gave above is but I guess it could happen.

I also believe that equipment such as MCC's and Switchboards have similar withstand ratings? In other words an MCC has a 3 cycle and 30 cycle rating? Is this also a consideration when turning off the INST setting on a feeder breaker?

 

[Jraef]

I think you might be missing the point.

The ability for the power breaker to hold on for 30 cycles, is not to allow ITS thermal element to trip, it's to allow a LOWER level breaker, closer to the fault, to trip. That's what a coordination system is all about; clearing a fault as close to the occurrence as possible so that you disrupt as little as possible.

So if you have a load center breaker feeding a fault, fed by a panelboard breaker, fed by a switchboard breaker, fed by power breakers in substation switchgear, you don't want the SUBSTATION SWITCHGEAR power breaker to clear every fault. You want the fault cleared at the load center breaker FIRST, and if it doesn't, THEN the panelboard breaker, and if it doesn't, THEN the switchboard breaker , and only if IT doesn't clear it, THEN the substation switchgear power breaker. But all of that time, that power breaker is SEEING the effects of the fault current and has to avoid becoming shrapnel while it waits for the lower level devices to act.

 

[templdl]

An Interrupting rating is simply the devices capability it interrupt a fault as a stand alone device and live to tell about it as another day.
A withstand ration is where the device is in a circuit that may be subject to a fault and must "withstand" that fault until a down stream OCPD interrupts said fault.
With an OCPD such as a power breaker it has the ability to stay closed for and extended length of time without damage before it needs to interupt, this window of time may be considered a withstand rating which is limited in time.
This extended time is very important in that the time delay is used for coordination where an OCPD with just a kaic really must open instantaneously as it will be subject to damage if it doesn't.The power breaker has the ability to have a time delay when subjected to a fault in order to allow sufficient time for the down stream OCPD to clear the fault.

 

[philly]

I think you might be missing the point.

The ability for the power breaker to hold on for 30 cycles, is not to allow ITS thermal element to trip, it's to allow a LOWER level breaker, closer to the fault, to trip. That's what a coordination system is all about; clearing a fault as close to the occurrence as possible so that you disrupt as little as possible.

So if you have a load center breaker feeding a fault, fed by a panelboard breaker, fed by a switchboard breaker, fed by power breakers in substation switchgear, you don't want the SUBSTATION SWITCHGEAR power breaker to clear every fault. You want the fault cleared at the load center breaker FIRST, and if it doesn't, THEN the panelboard breaker, and if it doesn't, THEN the switchboard breaker , and only if IT doesn't clear it, THEN the substation switchgear power breaker. But all of that time, that power breaker is SEEING the effects of the fault current and has to avoid becoming shrapnel while it waits for the lower level devices to act.

I understand the concept of coordination that you discuss, I guess I was referring to a hypothetical case where a 53kA fault occurs at the actual switchgear bus and the main breaker has its INST setting turned off. In this case the only thing that is going to clear the breaker is the breakers Short Time element wherever it is set (maximum of 30cycle delay). So if a 53k fault occurs and the Short time delay is at 30cycles wont this breaker fail since it is carrying 53kA for 30 cycles and is only rated for 50kA?

What if the STD was less than 30 cycles say .1sec? Can it carry more than its rated 50kA for a shorter duration?

 

[GoldDigger]

What if the STD was less than 30 cycles say .1sec? Can it carry more than its rated 50kA for a shorter duration?

The "correct" answer is that we have no idea, since the manufacturer never tested that.

 

[LMAO]

So if I understand "withstand" vs. "interrupt" ratings correctly:

• Withstand and interrupt ratings apply to OCPDs
• Only withstand (and not interrupt) rating apply to non-OCPD conductors like disconnect switches and bus bars.
• Withstand rating of an OCPD is always greater than its interrupt rating.

 

[templdl]

I understand the concept of coordination that you discuss, I guess I was referring to a hypothetical case where a 53kA fault occurs at the actual switchgear bus and the main breaker has its INST setting turned off. In this case the only thing that is going to clear the breaker is the breakers Short Time element wherever it is set (maximum of 30cycle delay). So if a 53k fault occurs and the Short time delay is at 30cycles wont this breaker fail since it is carrying 53kA for 30 cycles and is only rated for 50kA?

What if the STD was less than 30 cycles say .1sec? Can it carry more than its rated 50kA for a shorter duration?

I'm not familiar with a manufacturer that has a product of which you have described where the instantaneous trip can be "turned off." If it could the breaker would be damaged should such an event occur. Case in point, molded case switches years ago were manufactured as breakers with no overload or short circuit elements and could have been applied where they would be exposed to fault currents that exceeds their withstand rating.
For years now they are now manufactured with an instantaneous only (magnetic) element which opens the molded case switch such that its withstand rating would not be exceeded.

 

[big john]

I'm not familiar with a manufacturer that has a product of which you have described where the instantaneous trip can be "turned off"...

That's a standard option on electronic trip units for power circuit breakers. However you cannot defeat both "short time" and "instantaneous" trips simultaneously. It's common to see switchgear installs with instantaneous disabled because it makes coordination much easier.

Having said that, I'm not familiar with a standard that rates circuit breaker withstand separately from AIC on LVBs. I've only ever seen withstand listed on switchgear and MV breakers.

 

[philly]

I'm not familiar with a manufacturer that has a product of which you have described where the instantaneous trip can be "turned off." If it could the breaker would be damaged should such an event occur. Case in point, molded case switches years ago were manufactured as breakers with no overload or short circuit elements and could have been applied where they would be exposed to fault currents that exceeds their withstand rating.
For years now they are now manufactured with an instantaneous only (magnetic) element which opens the molded case switch such that its withstand rating would not be exceeded.

Ok it sounds like I was confusing Power Circuit breakers with Insulated Case breakers. In looking at the Cuttler Hammer Magnum DS Power Breakers I now see that the Short time withstand rating is the same as the interrupting rating. So for one of these breakers having a 65kA interrupting rating, it also has a 65kA short time rating, so even if you turn the Instantaneous setting off for coordination the short time rating will still be able to withstand the same fault current as if the Instantaneous setting was on. It appears that only for higher interrupting ratings (100kA) on these breakers that the short time rating differs from the interrupting rating.

Now I notice that the Cuttler Hammer Insulated Case SB breakers have a short time withstand rating that is less than the interrupting rating, similar to the scenario I described HOWEVER these breakers have a fixed Instantaneous trip even if the Instantaneous setting is turned off. So for example an 800A frame breaker can have an interrupting rating of 65kA but it only has a short time rating of 20kA. The breaker has an Instantaneous override at 18x the breaker frame which would be 14.4kA and thus below the short time rating. So it appears with these breakers that even if the Instantaneous setting is turned off, the breaker will still trip (at INST override) at a value below the short time rating thus protecting the breaker and the short time rating. So in the scenario I painted earlier it would be impossible to have a 50kA fault current that would not be cleared by the INST setting and thus last for 30cycles and damage the short time rating because the breaker would automatically trip at 14.4kA

I think this makes sense now.

For a feeder breaker that feeds a downstream MCC without a main breaker do you have to be cognizant of the MCC's short time rating when turning the INST setting off on a power breaker. For example lets say the MCC only has a 30 cycle rating at 50kA (I'm guessing here b/c I'm not familiar with these ratings) and the 65kA feeder breaker had its INST setting turned off. This breaker would allow a 65kA fault to occur at the MCC for 30 cycles and thus exceed the MCC's short time rating. I'm not sure if what I described here is a likely scenario, but hopefully it at least gets my point across. Do these equipment ratings need to be taken into consideration in such cases?

 

[templdl]

Regarding the MCC, are you referring to the MCC bracing or the combination motor starter's kaic ratngs in it since as I understands it there is no main MCC breaker? You may be eluding to a coordination study being done.

 

[kingpb]

The values you are providing are not discrete, other than the max 65kA. If the breaker can handle up to 65kA during sub-transient (Inst), then it can handle anything less; for some period of time.

Simply look at the breaker/device trip curve and I think you will answer your own question.

 

[philly]

The values you are providing are not discrete, other than the max 65kA. If the breaker can handle up to 65kA during sub-transient (Inst), then it can handle anything less; for some period of time.

Simply look at the breaker/device trip curve and I think you will answer your own question.

Yes but this does not appear to be true for an insulated case breaker which as a max 65kA interrupting (INST) value but cannot handle that for some duration of time (only a 20kA rating thus the need for the INST override)

 

[jim dungar]

Yes but this does not appear to be true for an insulated case breaker which as a max 65kA interrupting (INST) value but cannot handle that for some duration of time (only a 20kA rating thus the need for the INST override)

During an interrupting test, no breaker actually sees the full amount of current it is listed for use on based on its 3 cycle (INST) rating.
A 65kAIC rating means the breaker can be installed on a system that can deliver not more than 65kA to the breaker line side terminals. The breaker may actually begin to open at 20,000A and finish clearing while the fault current is still climbing towards its maximum available amount.

However a withstand rating is the full amount of fault current that must be able to flow through the device without it operating.

 

[philly]

During an interrupting test, no breaker actually sees the full amount of current it is listed for use on based on its 3 cycle (INST) rating.
A 65kAIC rating means the breaker can be installed on a system that can deliver not more than 65kA to the breaker line side terminals. The breaker may actually begin to open at 20,000A and finish clearing while the fault current is still climbing towards its maximum available amount.
.

The breaker may see the full amount of fault current if the INST pickup is turned off. On a power circuit breaker like the Magnum DS the INST pickup can be turned off and there is no INST override. So if the avalaible fault current happens to be 64kA the breaker will not open until it's sort time setting kicks in at its detlay time. Tis could be 30cycles before it picks up and interrupts the breaker.

 

[jim dungar]

The breaker may see the full amount of fault current if the INST pickup is turned off. On a power circuit breaker like the Magnum DS the INST pickup can be turned off and there is no INST override. So if the avalaible fault current happens to be 64kA the breaker will not open until it's sort time setting kicks in at its detlay time. Tis could be 30cycles before it picks up and interrupts the breaker.

Which is what the rest of my post said. Withstand means the breaker does not 'operate'.

I understand, all UL489 breakers contain some amount of self protection even when the INST is turned off.
Non-UL489 breakers are not required to have this self protection and require coordination with downstream equipment such as switchboards built to UL891.

 

[kingpb]

The breaker may see the full amount of fault current if the INST pickup is turned off. On a power circuit breaker like the Magnum DS the INST pickup can be turned off and there is no INST override. So if the avalaible fault current happens to be 64kA the breaker will not open until it's sort time setting kicks in at its detlay time. Tis could be 30cycles before it picks up and interrupts the breaker.

This is very possible, but if the withstand is 65kA, then the breaker is not damaged during a fault, and by the time the contacts actually part 30 cycles (0.5sec) until pick-up and another 3-5cycles before contacts actually part; your probably looking at minimum short circuit values that have decayed into the steady state region. Even if you have a very asymmetrical system, the currents that the contacts see upon parting are going to be well below the 50kA.

This of course can all be calculated, and TCC plotted; given the system information.