Short Circuit Withstand Series Ratings for MCC's

[philly]

Is it possible that an MCC and its components can be series rated with an Upstream feeder breaker similar to a panelbaord?

I know that paneloards and switchboards and their internal breakers can be series rated with an upstream feeder breaker if the combination of breakers is a tested and listed series rated combination documented by the manufacturer. For example 35kA rated breakers in a panelbaord can achieve a higher rating of say 42kA or 50kA if they are series rated with an upstream breaker that is a listed by the manufacturer for achieving this series rating for the downstream breakers.

Is it possible to achieve a similar series rating for an MCC? I am looking at an MCC that is slightly overdutied (42kA rating vs 45ka available fault current) and looking for solutions for getting the MCC to pass the equipment evaluation without having to replace it. Usually with a small overduty situation like this you can simply make the feeder cable longer to bring the fault current down however in this case the MCC is fed with BusDuct so that's not an option?

The MCC in this case is a Siemens MCC with a 42kA rating listed on nameplate and is fed from an upstream CH power circuit breaker. Does anyone know if it is possible to series rate the MCC somehow or have other suggestions for getting MCC to pass evaluation without replacing?

 

[jim dungar]

Does anyone know if it is possible to series rate the MCC somehow or have other suggestions for getting MCC to pass evaluation without replacing?

Motor control centers have one SCCR rating for their structure (horizontal and vertical bussing) and a different rating for each 'bucket'. The typical starter bucket contains plug-on stabs, short circuit protection (breaker or fuse style), a contactor, and an overcurrent relay. All of these bucket components are effectively series combination rated.

It is very rare to find multiple levels of series ratings, so it is likely that replacement is the probable outcome. Of, course you can try to reduce the amount of available fault current.

 

[smoothops10]

Even if you did find such a combination 240.86C would probably prevent you from using it as the MCC probably serves several motors.
Also consider if there is a need for selective coordination. I imagine it may not be possible using series rated combinations.
I don't know of any simple solutions for your case.

 

[Jraef]

Series ratings of circuit breakers are accomplished, for the most part, by the manufacturer of the equipment doing some very expensive destructive testing to prove to UL that the let-through of the upstream device is going to result in the down stream devices being able to withstand the fault. So when you see a series rating on circuit breakers, it is generally going to only be valid when the mfr is the same for both sides. Who would pay for the testing of a Siemens MCC behind an Eaton breaker? Siemens might do it for their breaker, or Eaton might do it for their MCC, but neither gains anything by blowing up their gear to allow a competitor to get part of the order.

In some cases, especially with MCCs, the breakers may be brand-labeled by one or the other anyway, so that may end up working out if it just so happens you stumble across that combination. In your case, smaller (150A) frame Siemens breakers are actually made by Eaton. But it's only that one frame size, so it's unlikely you can take advantage.

Current limiting fuses are a more likely solution for getting a series rating after the fact, but it may depend on whether your AHJ will accept a report from a PE stating the suitability. I had a very similar situation as yours a few years ago and tried that, the AHJ said no. We ended up changing the cable length by re-routing it to take a longer route around a building. It was enough to drop it from 46kA to 42kA. Cost a bundle for the extra cable, conduit and re-trenching, but less than a transformer with a higher impedance. Your situation is so similar to mine, you might want to take a look at it.

 

[Bugman1400]

Speaking of MCCs, does anybody have an example MCC spec with soft starters and SCADA interface? Guess I should mention that MCC is Medium Voltage (4160V).

Thanks!

 

[Jraef]

Speaking of MCCs, does anybody have an example MCC spec with soft starters and SCADA interface? Guess I should mention that MCC is Medium Voltage (4160V).

Thanks!

"SCADA interface" could mean anything. Most MV MCCs can come with any number of industrial networking communications options, so how that would interact with a SCADA system will depend a lot on what the SCADA system is. Some SCADA systems however will just want a few discrete I/O and maybe an analog input for the motor current.

I'll send you a PM with an MV MCC spec that includes MV soft starters and discusses industrial network options.

 

[philly]

Current limiting fuses are a more likely solution for getting a series rating after the fact, but it may depend on whether your AHJ will accept a report from a PE stating the suitability.

My understanding with current limiting fuses is that they are typically mis-applied due to the mis-conception in the industry that you can install any old current limiting fuse ahead of a piece of equipment to limit the let-thru current to that equipment. I believe the correct way to evaluate these fuses in years past was to take the fuses let-thru curve and use the "up over and down" method in order to determine the fuses let thru.

I have several bits of information that stated that this method is not completely accurate due to the fact that the breakers in the downstream equipment can present a dynamic impedance to the circuit if they being to start opening for a fault on their load side. Molded case breakers for example may start contact parting very soon after the inception of a fault, and although they wont clear the fault before the upstream current limiting fuse does, the parting of the contacts will create a dynamic impedance that will reduce the fault current seen by the fuse and therefore the up over and down method may not be accurate.

I know there is a provision in the NEC for a PE to evaluate this like you mentioned but I haven't come across many that would sign of on this solution due the complexity of the evaluation. Like you mentioned large manufacturers pay lots of money for these types of tests, so I don't know how a PE would be expected to make such a similar judgment based on even less information.

I had a very similar situation as yours a few years ago and tried that, the AHJ said no.

In this case its located in an existing facility so an AHJ wouldn't really be involved and the decision would fall solely between the engineer and customer. I'm curious though on new construction does an AHJ typically review a short circuit study to see if equipment has passed or failed? How else would he know something failed without reviewing some sort of analysis?

We ended up changing the cable length by re-routing it to take a longer route around a building. It was enough to drop it from 46kA to 42kA. Cost a bundle for the extra cable, conduit and re-trenching, but less than a transformer with a higher impedance. Your situation is so similar to mine, you might want to take a look at it.

I had originally through these MCC's were fed via Bus Duct but it turns out they are fed with cable so lengthening the cable may be a solution. I'll have to play with the cable lengths in the model to see how much I need in order to reduce fault current. May be tough with an existing installation however.

 

[Jraef]

Just to clarify, the AHJ was asked in advance if he would accept it and said no. He never actually evaluated anything, he rejected the entire concept. I've had others tell me they got it accepted, that's why I had tried.

It was a greenfield project, hence permits were pulled.

 

[petersonra]

Just to clarify, the AHJ was asked in advance if he would accept it and said no. He never actually evaluated anything, he rejected the entire concept. I've had others tell me they got it accepted, that's why I had tried.

It was a greenfield project, hence permits were pulled.

I think that in the case of new stuff the AHJ is not allowed to accept a PE report.

240.86 Series Ratings. Where a circuit breaker is used on
a circuit having an available fault current higher than the
marked interrupting rating by being connected on the load
side of an acceptable overcurrent protective device having a
higher rating, the circuit breaker shall meet the requirements
specified in (A) or (B), and (C).
(A) Selected Under Engineering Supervision in Existing
Installations. The series rated combination devices shall be
selected by a licensed professional engineer engaged primarily
in the design or maintenance of electrical installations.
The selection shall be documented and stamped by
the professional engineer. This documentation shall be
available to those authorized to design, install, inspect,
maintain, and operate the system. This series combination
rating, including identification of the upstream device, shall
be field marked on the end use equipment.
For calculated applications, the engineer shall ensure
that the downstream circuit breaker(s) that are part of the
series combination remain passive during the interruption
period of the line side fully rated, current-limiting device.

 

[kaichosan]

Is it possible that an MCC and its components can be series rated with an Upstream feeder breaker similar to a panelbaord?
...

Is it possible to achieve a similar series rating for an MCC? I am looking at an MCC that is slightly overdutied (42kA rating vs 45ka available fault current) and looking for solutions for getting the MCC to pass the equipment evaluation without having to replace it. Usually with a small overduty situation like this you can simply make the feeder cable longer to bring the fault current down however in this case the MCC is fed with BusDuct so that's not an option?

The MCC in this case is a Siemens MCC with a 42kA rating listed on nameplate and is fed from an upstream CH power circuit breaker. Does anyone know if it is possible to series rate the MCC somehow or have other suggestions for getting MCC to pass evaluation without replacing?

Since the calculated available fault current is 45kA and it's within 10% over the SCCR of 42kA, you might want to review and re-calculate the short circuit study. Most short circuit studies are conservatively on worst case scenarios starting with the utility's system fault contribution and X/R ratio used, cable lengths, type of loads, etc. For various estimated conditions and assumptions, a +10% is usually acceptable.

Series rating, as previously mentioned, will not apply since you have different manufacturers for the protective devices.

Fuse current limiters on the secondary breakers is possible, or increasing the cable length.