LV Panelboards / MCC

[hivolt]

I've searched and searched and have not been able to get a satisfactory answer. For main lug only (MLO) panelboards/MCC, how high can I set the upstream feeder power circuit breaker for short-circuit protection? Let's say that the panelboard is stamped 65 kA (@ 3 cycles per UL). What is the highest instantaneous setting that can be applied to the upstream power circuit breaker? Furthermore, can I disable the instantaneous element all together and just use short time based on calculated I^2 x t?

It is interesting that panelboards/MCCs that are supplied with a main breaker, the instantaneous trip is set somewhere at 25% of kAIC rating. This is due to the limitations of molded case circuit breakers.

I've gone to the mfgrs, but all they will say is that the equipment is tested per UL.

Any suggestions and basis for these suggestions is greatly appreciated.

 

[don_resqcapt19]

You must have a MLO panel that has a short circuit rating at least equal to the short circuit current available at that point in the circuit. An upsteam breaker cannot be used to limit the short circuit current at the panel unless you are installing a series rated system.
Don

 

[jim dungar]

You can set the INST of your protective device to any level that you want. All the NEC (and UL) cares about is the the equipment short circuit rating (i.e. bus bracing) is sufficient for the SCA available.

Protective device Short time and Inst settings are based on the full load current rating of the device and have nothing to do with their short circuit current (AIC) rating.

Long time, Short time, and Inst settings decide when the protective device will operate based on time versus current. AIC ratings say how much current the device can interrupt with failing violently.

 

[hivolt]

Additional info

Additional info

Maybe I'm missing something. Let's say we have a 65 kA MCC/panelboard and we calculate the available short circuit to be 40 kA.

Does this mean that I do not even need an instantaneous element set? Since UL tests these only at the rated value (65 kA for 3 cycles, I^2xt = 211672500), I would think that I would at least have to set the short time element to 36 kA @ 10 cycles ( I^2xt = 211672500 ).

If not, and I am already not setting the instantaneous element, how high can I set the short time trip?

Let's say that the MLO MCC is 1200 A and 65 kA, and it is fed by cables with an ampacity of 1200 A. Also, let's say that the upstream power circuit breaker is also rated at 65 kA @ 30 cycles and has a short time range of 4 x Is to 12 x Is. If I set the short time to the maximum (14,400 A) a maximum delay band (0.5 sec), what happens to the MCC if one of the MCC breakers faults on the bus side and a maximum fault (40 kA) is maintained until the short time element trips (0.5 sec)? Have I not protected the MCC properly and/or have I violated any code?

On the other hand, if my max setting is 12 x Is (14,400) maybe I should determine the maximum trip time at the maximum fault (40 kA @ 8 cycles, I^2x t = 211672500 ). Therefore, I would set the short time at maximum and not to exceed 8 cycles (0.13 sec).

 

[ron]

A panelboard and MCC are not rated to withstand short circuit current for 30 cycles like Switchgear is. There does happen to be one or two manufacturers that can build 30 cycle Switchboards, but that is another story.
The upstream ocpd for the MLO panelboard/MCC must have instantaneous so as to limit the time that the panelboard/MCC is exposed to a fault.
If there was an upstream 1200A CB protecting the MLO panelboard/MCC, you can set it as high as you like, but it cannot be turned off. So if you set it to its max of 12 (assuming it can go that high), then the CB will trip @ approx 14400A within the rating "time period" of the panelboard/MCC.
When selectivity is critical, and I'm using CB's, I specify a lot of switchgear, as it is 30 cycle rated and I can turn off the instantaneous for the CB's the feed it, and its own main.

 

[ron]

In your example, the board will be exposed to 40kA and it must clear in the time period that the equipment can handle it, 3-6 cycles or so. The 65kAIC means that it can handle up to 65kAIC for 3-6 cycles or so, then .... bang. Instantaneous setting of the OCPD upstream will clear the fault nicely within the time period if there was instantaneous available for the OCPD.
If you turn the instantaneous off, then 40kA will be carried by the panelboard/MCC until the shortime picks up, which is longer than 3-6 cycles. I smell smoke ....

 

[hivolt]

Thanks guys. So if the MCC has a rating of 65 kA for 3 cycles, can I assume it can handle 40 kA for and 8 cycles and 25 kA for 20 cycles? Basically keeping constant I^2 x t?

This has been my logic, but I could not find anything in ANSI, NEMA or UL that supports this. The only reference is in UL that the rating is good for 3 cycles.

I can't get anything more from MCC/panelboard mfgrs either.

 

[hivolt]

what if I can set the short time to trip in 8 cycles (0.13 sec)?

 

[petersonra]

Thanks guys. So if the MCC has a rating of 65 kA for 3 cycles, can I assume it can handle 40 kA for and 8 cycles and 25 kA for 20 cycles? Basically keeping constant I^2 x t?

This has been my logic, but I could not find anything in ANSI, NEMA or UL that supports this. The only reference is in UL that the rating is good for 3 cycles.

I can't get anything more from MCC/panelboard mfgrs either.

I don't believe you can make that assumption.

There are two different things you have to protect against - short circuit current and overloads.

The I^2t side of the equation is the overload protection.

 

[ron]

Hivolt,

So if the MCC has a rating of 65 kA for 3 cycles, can I assume it can handle 40 kA for and 8 cycles and 25 kA for 20 cycles? Basically keeping constant I^2 x t?

If the equipment is rated at 65kA for 3 cycles, I know of no document that indicates it would be rated at any amount of fault current for more than 3 cycles. I think that stretches the testing arrangement too much. 3 cycles is the max ; 65kA is the max. Without relisting/retesting the equipment at UL, you can't interpolate the results.

 

[kingpb]

You cannot arbitrarily pick trip currents and times, that is why it is called Time Current Coordination (TCC). A short circuit analysis needs to be done, and the trip settings need to be selected based on protection of the equipment.

The IEEE Buff Book is an excellent reference on how to set the protection. If in doubt, don't guess or make assumptions, get a person trained in protective relaying invovled.

Also, don't forget about Arc Flash. This can have an effect on the settings as well!

 

[hivolt]

kingpb,
I'm probably not being very clear. I am not asking about coordination or short circuit calculations. I am asking about protection of 480 V MCCs / panelboards which are supplied M.L.O.

I understand the 65 kA rating for 3 cycles. However, if I am using a 480 V power circuit breaker with an adjustable instantaneous trip and I adjust it to the maximum (let's say 40 kA), what happens when the a fault occurs (with some resistance) and does not exceed 30 kA.

I am protecting the 65 kA at 3 cycle damage point, but how much time can it take for the long time or short time trip elements to operate before the MCC/panelboard is damaged at 30 kA? How low must I set the instantaneous trip unit such that for lower level faults I will still protect the panel?

Again, when a panelboard is spec'd with a molded case main breaker, the main breaker will have an instantaneous override. For this particular panelboard, the breakers I have been seeing have an inst. override at 17.5 kA. This tells me that the panel at least has a short time withstand below 17 kA. If you look at the long time trip curve, it can be as high as 5-10 seconds. This tells me that the panel can take 17 kA for 10 seconds and 65 ka for 3 cycles. It seems to me that a damage curve can exist somewhere between these two points. What is my max short circuit was 25 kA?

To go one step further, what if the same panel is spec'd without a main. The panel mfgr does not specify that this certain breaker (with 17.5 kA inst override) should be used upstream. What if I choose a breaker with a 20 kA inst override. Better yet, what if it is fed by an upstream 480 V power circtui breaker with an adjustable instantaneous trip element? Can my panelboard handle just under 20 kA before the inst trip? Can I get an breaker with an adjustable inst trip that I can adjust say to 25 kA....what about 30 kA? How can this be determined and on what basis?

To go to the final step, if we say that the panelboard can withstand 25 kA (pick some number above a standard inst. override) for some time, and my maximum short circuit is under 25 kA, then I would much prefer to disable my instantaneous trip and use the short time trip element which would provide me with better coordination of downstream instantaneous breakers.

 

[hivolt]

Still unclear

Still unclear

Thanks for all the input. I wanted to clarify that this is a question about coordination or arc flash.

Coming back to the protection against short circuit, a couple of comments were made regarding that the equipment is tested at 65 kA and it is tested at 3 cycles. This is the same answer that I am getting from the panelboard mfgrs. But this still doesn't answer the question, sorry for being stubborn or dense. This is because the answer implies that any overload condition would also require an instantaneous trip within three cycles. We all know that this is not realistic.

Follow this logic please. When a panelboard is spec'd with a molded case main breaker, the main breaker will have an instantaneous override. For this particular panelboard, the breakers I have been seeing have an inst. override at 17.5 kA. This tells me that the panel at least has a short time withstand below 17 kA. If you look at the long time trip curve, it can be as high as 5-10 seconds. This tells me that the panel can take 17 kA for 10 seconds and 65 ka for 3 cycles. It seems to me that a damage curve can exist somewhere between these two points. What is my max short circuit was 25 kA?

To go one step further, what if the same panel is spec'd without a main. The panel mfgr does not specify that this certain breaker (with 17.5 kA inst override) should be used upstream. What if I choose a breaker with a 20 kA inst override. Better yet, what if it is fed by an upstream 480 V power circtui breaker with an adjustable instantaneous trip element? Can my panelboard handle just under 20 kA before the inst trip? Can I get an breaker with an adjustable inst trip that I can adjust say to 25 kA....what about 30 kA? How can this be determined and on what basis?

To go to the final step, if we say that the panelboard can withstand 25 kA (pick some number above a standard inst. override) for some time, and my maximum short circuit is under 25 kA, then I would much prefer to disable my instantaneous trip and use the short time trip element which would provide me with better coordination of downstream instantaneous breakers.

I'm not trying to be difficult, I sincerely want to know.

 

[kingpb]

#1. Don't confuse breaker ratings with panel/equipment bus ratings.
#2. Breaker's need to be selected/adjusted to protect the equipment. Not the other way around.
#3. The withstand ratings you are talking about are symmetrical rms values, and the asymmetrical rms is what the equipment can actually handle. But they are unpublished because there is no set value, being that they are tested using a specific X/R ratio, and short circuit power factor. X/R ratios vary for every system, and at every point on every system.
#4. The 1/2 cycle asymmetrical peak current withstand capability of the device or equipment is even much higher.

Keep in mind that because of fault current decay, by the time 3 cycles has gone by, the fault current has already decayed well below the initial fault level. By this time, if the system is coordinated properly, the appropriate devices have already started the tripping cycle. Even if it is a 5 cycle breaker (low voltage), your talking about times of 0.05 sec (3 cycle) vs. 0.083 sec. So for the bus to be able to withstand a 65kA rms symm. total fault current for 3 cycles, is actually longer then it really needs to.