SCCR calculations help

[Gssc1414]

I have two questions that I am unsure as to how to perform the correct SCCR calculation. I am finding it difficult to find instructions on how to perform some of these calculations for specific circumstances that I have not yet had to deal with... I am new to this, so please feel free to let me know if I am completely incorrect.

General

The machine in question is a small automation machine. It will be connected to 480vac via a bus-bar (customer to provide a fuse in the disconnect box at the bus-bar).

Question 1

The machine has a 65kA MCCB (Siemens HEG3B030L) as it's main circuit protection. This is what we connect the incoming 480 directly to. From there, a Ferraz Shawmut (mersen) power distribution block (MPDB67653) is fed. The SCCR on the PDB lists that it is 100kA if properly fused. According to the UL data, that 100kA rating can be reached when using a wide variety of fuses, however no specific circuit breakers are listed. So my question is that does the MCCB have any affect on the SCCR of the PDB. I can't see how it would, as i cannot find any data saying using these two in series will yeild any SCCR rating, however I wanted to check to see if I am missing something. Lastly, can I just spec the customer to fuse the machine at the bus-bar with say an RK1 or RK5 (listed in the UL data as increasing the SCCR of the PBD to 100kA) and then I can perform the rest of my calculations with 100kA on the PDB?

Question 2

The same machine goes from the PDB to Class CC fuses, then to a 480vac -> 24vdc power supplies (Murr 85002, 20 amp). I cannot find any SCCR data on this unit. Part of me is thinking that I don't have to include this in my SCCR calculation because this would be considered a control circuit, however I am unsure. If I can exclude this from the SCCR calcuation, does the amp output of the device make any difference (say i were to be using a 40amp instead of the 20 amp)?


Thanks for your time - I really appreciate any help and advice.

 

[kwired]

I have two questions that I am unsure as to how to perform the correct SCCR calculation. I am finding it difficult to find instructions on how to perform some of these calculations for specific circumstances that I have not yet had to deal with... I am new to this, so please feel free to let me know if I am completely incorrect.

General

The machine in question is a small automation machine. It will be connected to 480vac via a bus-bar (customer to provide a fuse in the disconnect box at the bus-bar).

Question 1

The machine has a 65kA MCCB (Siemens HEG3B030L) as it's main circuit protection. This is what we connect the incoming 480 directly to. From there, a Ferraz Shawmut (mersen) power distribution block (MPDB67653) is fed. The SCCR on the PDB lists that it is 100kA if properly fused. According to the UL data, that 100kA rating can be reached when using a wide variety of fuses, however no specific circuit breakers are listed. So my question is that does the MCCB have any affect on the SCCR of the PDB. I can't see how it would, as i cannot find any data saying using these two in series will yeild any SCCR rating, however I wanted to check to see if I am missing something. Lastly, can I just spec the customer to fuse the machine at the bus-bar with say an RK1 or RK5 (listed in the UL data as increasing the SCCR of the PBD to 100kA) and then I can perform the rest of my calculations with 100kA on the PDB?

Question 2

The same machine goes from the PDB to Class CC fuses, then to a 480vac -> 24vdc power supplies (Murr 85002, 20 amp). I cannot find any SCCR data on this unit. Part of me is thinking that I don't have to include this in my SCCR calculation because this would be considered a control circuit, however I am unsure. If I can exclude this from the SCCR calcuation, does the amp output of the device make any difference (say i were to be using a 40amp instead of the 20 amp)?


Thanks for your time - I really appreciate any help and advice.

Question 1 -what is available fault current on supply side of your machine? Is the 65kA breaker customized for your application or is it just the standard breaker they install in that machine. If it is customized for your application then you probably have at least 42kA but less then 65kA available. What is available depends on factors like conductor size, length, motor contribution, and generally gets less the further you are from the source because of impedance of conductors and other components in the circuit.

Question 2 supply side has available current similar to what is at PDB unless there is some length to the conductors. The 24vdc is separately derived and is limited by the "new source" abilities. Chances are the it is much less then the supply side if it is low capacity for control circuits.

 

[Gssc1414]

Question 1 -what is available fault current on supply side of your machine? Is the 65kA breaker customized for your application or is it just the standard breaker they install in that machine. If it is customized for your application then you probably have at least 42kA but less then 65kA available. What is available depends on factors like conductor size, length, motor contribution, and generally gets less the further you are from the source because of impedance of conductors and other components in the circuit.

Question 2 supply side has available current similar to what is at PDB unless there is some length to the conductors. The 24vdc is separately derived and is limited by the "new source" abilities. Chances are the it is much less then the supply side if it is low capacity for control circuits.

kwired,

Thanks for your response... however I am not sure I follow everything:

Question 1 - Why does the available fault current affect what the SCCR of the panel would be? I understand how that the SCCR of the panel needs to be able to handle the available fault current, however I was not aware that the available fault current affects the calculation of the panel's SCCR. The MCCB is rated for 65kA from Siemens - it's a standard product. So the customers bus-bar (no idea what the avalible fault current is, and neither does anyone I talk to at the company) will drop to our machine and connect into the MCCB. I just need to calculate the machines SCCR rating (MCCB and down) to determine how to mark the panel. My question about the fuses just relates to with the PDB lists in the UL card...and that if it was correct or acceptable to use the 100kA SCCR rating for the PDB if I were to spec out (on the machine tag) that it needs to be fused at the bus-bar using an RK1/RK5 fuse (with a specific current). This way the SCCR of the PDB would be in compliance to the UL rating, however I am not sure if you can do this as the fuse needed to achieve this 100kA is located outside of the panel.


Question 2 - You lost me here... So I would or would not need to include this power supplies in the SCCR rating? If i do, how do I determine the SCCR this information is not available? Would i need to use the general specs from UL, like 5kA?

 

[kwired]

kwired,

Thanks for your response... however I am not sure I follow everything:

Question 1 - Why does the available fault current affect what the SCCR of the panel would be? I understand how that the SCCR of the panel needs to be able to handle the available fault current, however I was not aware that the available fault current affects the calculation of the panel's SCCR. The MCCB is rated for 65kA from Siemens - it's a standard product. So the customers bus-bar (no idea what the avalible fault current is, and neither does anyone I talk to at the company) will drop to our machine and connect into the MCCB. I just need to calculate the machines SCCR rating (MCCB and down) to determine how to mark the panel. My question about the fuses just relates to with the PDB lists in the UL card...and that if it was correct or acceptable to use the 100kA SCCR rating for the PDB if I were to spec out (on the machine tag) that it needs to be fused at the bus-bar using an RK1/RK5 fuse (with a specific current). This way the SCCR of the PDB would be in compliance to the UL rating, however I am not sure if you can do this as the fuse needed to achieve this 100kA is located outside of the panel.


Question 2 - You lost me here... So I would or would not need to include this power supplies in the SCCR rating? If i do, how do I determine the SCCR this information is not available? Would i need to use the general specs from UL, like 5kA?

Maybe I don't understand the question. Are you trying to determine SCCR of something or at a particular point or are you trying to determine available fault current? They are related yet are different. Available fault current is there regardless of SCCR of equipment.

I will say your panel needs to be able to handle whatever fault current is available at its input terminals, that current is still available beyond the input terminals unless you have current limiting devices immediately at the input. Only thing going to lower that value is natural resistance/impedance of conductors and components or intentionally inserted resistance/impedance.

Second question- you have a separately derived source. The input of that source is able to produce a fault equal to what is available on the primary voltage. The secondary side of that source is limited by the impedance of the source itself. If it is a rather small source for controls only the secondary side will have a low available fault current to begin with, and chances are less then 5kA which essentially means SCC of those controls is not generally much to be concerned about, at least for safety reasons.

 

[Gssc1414]

Maybe I don't understand the question. Are you trying to determine SCCR of something or at a particular point or are you trying to determine available fault current? They are related yet are different. Available fault current is there regardless of SCCR of equipment.

I will say your panel needs to be able to handle whatever fault current is available at its input terminals, that current is still available beyond the input terminals unless you have current limiting devices immediately at the input. Only thing going to lower that value is natural resistance/impedance of conductors and components or intentionally inserted resistance/impedance.

Second question- you have a separately derived source. The input of that source is able to produce a fault equal to what is available on the primary voltage. The secondary side of that source is limited by the impedance of the source itself. If it is a rather small source for controls only the secondary side will have a low available fault current to begin with, and chances are less then 5kA which essentially means SCC of those controls is not generally much to be concerned about, at least for safety reasons.

I am trying to determine the SCCR so that we can mark the panel, per NEC requirements.

I understand that the equipment needs to handle the available fault current, unfortunately we are an OEM machine builder. I have no control over the customers available fault current. I also, no matter who i speak with cannot get an answer as to what the available fault current is at their facility. So, at this point - Aren't I only legally responsible for providing an appropriately marked panel? Additionally we cannot control where the end user installs the equipment, so that this point wouldn't it be up to the customer to ensure that the SCCR of the panel meets the available fault current for their system?

Question 1

I am merely trying to determine what the SCCR of the PDB would be. It is protected inside of our panel with the MCCB, however the UL file card lists the PDB as having 100kA when protected by an appropriate fuse or circuit breaker. The Mersen documentation lists out several fuses that meet the criteria to achieve 100kA for the PDB, however there is no circuit breakers listed (even though the UL file card says "fuse or circuit breaker"). So with that I was under the impression that because there is nothing in any document that I can find saying that the specific MCCB that we are using inside our panel, that I could not use the 100kA rating. Because of that, I was thinking that if I were the specify that they HAD to use RK1/RK5 fuses at the bus-bar, that I could use the 100kA rating (due to the documentation stating that the SCCR of the PDB is 100kA when protected by an RK1 or RK5 fuse). My hesitation here is that I don't know it that's ok... to use the 100kA rating when the current-limiting fuse is not inside our panel and we are just merely telling the end user that they need to use that specific fuse.

Question 2

Ok, so the available fault current at my input terminals would be also at my power supply... I see what your saying here - But I still don't know if I need to include this in the calculation for the entire panel. I would think these would be similar to transformers:

[FONT=Lucida Grande, Verdana, Arial, sans-serif]"If the transformer is rated 5 kVA or less with a 120 V maximum secondary, as long as the load-side components and overcurrent protective device ratings are rated 2 kA or greater, the interrupting rating of the overcurrent protective device on the line side of the transformer can be applied to the load-side components and overcurrent protective devices (see figure 4)."\[/FONT]

[FONT=Lucida Grande, Verdana, Arial, sans-serif]So with the above statement, we don't include our 480vac -> 120vac transformers in the SCCR calculation of the panel. We simply use the SCCR of the overcurrent protection devices on the line side of the transformers. I guess I was wondering if the same could be applied to our 480vac -> 24vdc power supply.

So here is my thoughts - if UL508A says that you can use the SCCR rating of the overcurrent protection devices as long as your 480vac -> 120vac transformer is under 5kVA and all load side components are 2kA or greater, why couldn't you apply the same thing to the 480vac -> 24vdc power supply?

Sorry if my explanations / questions are not clear... As i said, I am new to this and I am trying to use the correct terminology and be as detailed as possible.[/FONT]

 

[Jraef]

The problem, as I see it, is that you are looking for a tested series combination of the power distribution block from Mersen, along with the circuit breaker from Siemens, in order to avoid having to revert to the default untested values. If that is the case, the underlying issue is that you are looking for Mersen, FUSE manufacturer, to have tested their block with a CIRCUIT BREAKER, which is in direct competition to their mission to sell more FUSES, not to mention that they must compete with Siemens in everything else they do. Series testing is expensive, why would they do that?

What might be a better approach would be to ask SIEMENS for an appropriate power distribution arrangement to go along with their breaker. It might not exist, but if there is a chance that it does, that would be the only way to get it.

Side hint: that particular breaker is actually a brand-labeled Eaton Series G breaker, so if THEY have a listed combo, that might be another source. Alternately, if you must end up changing the breaker because you do not want to have to use fuses, but Siemens has no listed combination either, you could consider changing BOTH devices after finding someone who does offer that tested listed combo.

 

[Gssc1414]

The problem, as I see it, is that you are looking for a tested series combination of the power distribution block from Mersen, along with the circuit breaker from Siemens, in order to avoid having to revert to the default untested values. If that is the case, the underlying issue is that you are looking for Mersen, FUSE manufacturer, to have tested their block with a CIRCUIT BREAKER, which is in direct competition to their mission to sell more FUSES, not to mention that they must compete with Siemens in everything else they do. Series testing is expensive, why would they do that?

What might be a better approach would be to ask SIEMENS for an appropriate power distribution arrangement to go along with their breaker. It might not exist, but if there is a chance that it does, that would be the only way to get it.

Side hint: that particular breaker is actually a brand-labeled Eaton Series G breaker, so if THEY have a listed combo, that might be another source. Alternately, if you must end up changing the breaker because you do not want to have to use fuses, but Siemens has no listed combination either, you could consider changing BOTH devices after finding someone who does offer that tested listed combo.

So basically I need to look at my power distribution block and back figure to see what circuit protection device I need to use? Based on the documentation I have seen so far, everything is listed as having a high SCCR rating if protect by a fuse - I have yet to see a circuit breaker ( I could be missing it or looking in the wrong spot ). I know there are a lot of companies that use MCCB's as a main disconnect... but it looks like no power distribution blocks are rated in series with an MCCB. So does it make sense that I would have to go MCCB -> Fuse -> PDB in order to use the higher SCCR rating OR just use the default 10kA for the PDB if I go MCCB -> PDB??

 

[Jraef]

So basically I need to look at my power distribution block and back figure to see what circuit protection device I need to use? Based on the documentation I have seen so far, everything is listed as having a high SCCR rating if protect by a fuse - I have yet to see a circuit breaker ( I could be missing it or looking in the wrong spot ). I know there are a lot of companies that use MCCB's as a main disconnect... but it looks like no power distribution blocks are rated in series with an MCCB. So does it make sense that I would have to go MCCB -> Fuse -> PDB in order to use the higher SCCR rating OR just use the default 10kA for the PDB if I go MCCB -> PDB??

What I meant was, you are fishing for grouper in a fresh water lake. If you want to use a CB as the main disconnect, search for a PDB that is sold by someone who makes CBs, not fuses.

http://www.ab.com/en/epub/catalogs/...High-Fault-Short-Circuit-Current-Ratings.html

http://www.eaton.com/ecm/idcplg?Idc...veAs=0&Rendition=Primary&dDocName=TD01200003E

http://www.schneider-electric.com/d...Text-for-US&docType=Declaration-of-conformity

It appears as though Siemens does not make PDBs, or if then do, I can't find them (and I used to work at Siemens...). That means not only was is a fresh water lake, it was a dead lake.

 

[jim dungar]

My hesitation here is that I don't know it that's ok... to use the 100kA rating when the current-limiting fuse is not inside our panel and we are just merely telling the end user that they need to use that specific fuse.

Every series rating I have seen allows for the upstream device to be 'remote' from the downstream equipment.
However, very few combination ratings are for more than 2 items in series. The addition of your MCCB disconnect is probably problematic.

 

[kwired]

Now that it is more clear what you are asking about- Jraef kind of gave same answer I have, it all depends on what series ratings components have been tested for. If not tested to work together - which just doesn't happen much when you mix components from different manufacturers you generally default to 5kA being all they are worth regardless of what a component may be marked.

Again your control circuit is it's own system, just because you have 65k on a main supply component doesn't mean the control circuit will ever see that kind of current - exception being an accidental crossover from power to controls. If this is class 2 controls they should be arranged so that isn't all that possible.

 

[Gssc1414]

Now that it is more clear what you are asking about- Jraef kind of gave same answer I have, it all depends on what series ratings components have been tested for. If not tested to work together - which just doesn't happen much when you mix components from different manufacturers you generally default to 5kA being all they are worth regardless of what a component may be marked.

Again your control circuit is it's own system, just because you have 65k on a main supply component doesn't mean the control circuit will ever see that kind of current - exception being an accidental crossover from power to controls. If this is class 2 controls they should be arranged so that isn't all that possible.

So are you saying that I do not have to consider the 480vac -> 24dvc power supply in the entire panel SCCR? I do see what your saying and i understand that just because 65k on the main supply doesn't mean the control circuit will see that kind of current... However I am hoping to get a more definite answer.... I do or do not have to include the power supply in my calculations? From what your saying it's looking like I do not have to, however I would like to solidify it and make sure that I am understanding this correctly.

 

[Gssc1414]

What I meant was, you are fishing for grouper in a fresh water lake. If you want to use a CB as the main disconnect, search for a PDB that is sold by someone who makes CBs, not fuses.

http://www.ab.com/en/epub/catalogs/...High-Fault-Short-Circuit-Current-Ratings.html

http://www.eaton.com/ecm/idcplg?Idc...veAs=0&Rendition=Primary&dDocName=TD01200003E

http://www.schneider-electric.com/d...Text-for-US&docType=Declaration-of-conformity

It appears as though Siemens does not make PDBs, or if then do, I can't find them (and I used to work at Siemens...). That means not only was is a fresh water lake, it was a dead lake.

Thanks for the post... This cleared things up for me.

 

[Jraef]

So are you saying that I do not have to consider the 480vac -> 24dvc power supply in the entire panel SCCR? I do see what your saying and i understand that just because 65k on the main supply doesn't mean the control circuit will see that kind of current... However I am hoping to get a more definite answer.... I do or do not have to include the power supply in my calculations? From what your saying it's looking like I do not have to, however I would like to solidify it and make sure that I am understanding this correctly.

Power supplies and control power transformers are specifically excluded from the rules. The fuses (or CB) that feed them would be included though, however that's not usually tough to deal with, as long as you steer clear of those cheap little "DIN rail" breakers.

 

[Gssc1414]

Power supplies and control power transformers are specifically excluded from the rules. The fuses (or CB) that feed them would be included though, however that's not usually tough to deal with, as long as you steer clear of those cheap little "DIN rail" breakers.

Thanks! That's exactly what I was looking for.



I appreciate everyone's time and help with this!