Equipment Short Circuit Issue

[Jp807]

New user on this forum but long time reader.

Question I have is in regards to short circuit rating of equipment and a possible solution.

Working on a fast paced project in the pharmaceutical world. One of the equipment manufacturers is based in Europe. They made submittals, we commented and told them their equipment needed to be rated at 65kAIC. Told them multiple times. They are now in manufacturing and left their equipment at 5kAIC. Nothing they can do.

So now we need a solution on the engineering side. The main solution is 480V to 480V transformers for each piece of equipment to limit short circuit below 5kAIC. If not, the fault available at the equipment is around 17k based on the largest feed. This causes a significant cost impact to the project and an electrical room redesign.

Anyone have opinions? We need to limit the short circuit at the equipment to below 5kAIC. I don't believe current limiting fuses or breakers would work but would like to hear some feedback.

Thanks

 

[jim dungar]

I know of installations where changes were made to the conductors, like adding length to the run or changing to aluminum, rather than adding transformers.

 

[Jp807]

I know of installations where changes were made to the conductors, like adding length to the run or changing to aluminum, rather than adding transformers.

Haha, we always joke about that. Just coil a few hundred feet of feeder in the ceiling . Aluminum wouldnt help here.

Unfortunately we have a pretty high fault current. Utility is giving us around 76k from their xfmrs and we have 63k at our main switchgears.

 

[jim dungar]

Haha, we always joke about that. Just coil a few hundred feet of feeder in the ceiling ...

I have had contractors coil 500kcmil copper.

You didn't provide enough circuit details for any specific answer, but you always need to be careful that added impedance does not result in excessive voltage drop.

Current limiting using overcurrent protective devices, is not usually recognized unless it is done by the manufacturer, usually under the guidance of UL. This is especially true when there are protective devices in series.

 

[petersonra]

New user on this forum but long time reader.

Question I have is in regards to short circuit rating of equipment and a possible solution.

Working on a fast paced project in the pharmaceutical world. One of the equipment manufacturers is based in Europe. They made submittals, we commented and told them their equipment needed to be rated at 65kAIC. Told them multiple times. They are now in manufacturing and left their equipment at 5kAIC. Nothing they can do.

So now we need a solution on the engineering side. The main solution is 480V to 480V transformers for each piece of equipment to limit short circuit below 5kAIC. If not, the fault available at the equipment is around 17k based on the largest feed. This causes a significant cost impact to the project and an electrical room redesign.

Anyone have opinions? We need to limit the short circuit at the equipment to below 5kAIC. I don't believe current limiting fuses or breakers would work but would like to hear some feedback.

Thanks

Sounds to me like a contractual problem that should be resolved by lawyers.

If the control panels are UL listed there is not a lot you can do since you cannot relabel the panel and your best bet is the transformer idea.

If they are not UL listed you could do the UL508a calculations for SCCR yourself and see if there are some things you could modify to improve the SCCR. Sometimes all it takes is a current limiting fuse in the feeder circuit to deal with the problem and that CL fuse does not even have to be inside the panel.

 

[Jp807]

Sounds to me like a contractual problem that should be resolved by lawyers.

If the control panels are UL listed there is not a lot you can do since you cannot relabel the panel and your best bet is the transformer idea.

If they are not UL listed you could do the UL508a calculations for SCCR yourself and see if there are some things you could modify to improve the SCCR. Sometimes all it takes is a current limiting fuse in the feeder circuit to deal with the problem and that CL fuse does not even have to be inside the panel.

Yes I would agree. That will be up to the owner and the manufacturer to hash out if they decide to go that route. Just looking to see what the most viable solutions are.

The manufacturer is stating 5kAIC is what the equipment is rated for, so have to design to that. Was really hoping there might be a solution I don't know of but it sounds more and more like transformers are our only solution.

 

[Jraef]

...
The manufacturer is stating 5kAIC is what the equipment is rated for, so have to design to that. Was really hoping there might be a solution I don't know of but it sounds more and more like transformers are our only solution.

Much more likely; the mfr is lazy and does not want to even investigate what it will take to get a higher rating, even though it is relatively simple to do. In most cases, it's as simple as selecting components from the SAME MFR so that they have UL listed Series Ratings for SCCR, but that then means they cannot negotiate with separate suppliers for the cheapest possible parts on each sub-component. So for example Overload Relays have notoriously low SCCR ratings (typically 5kA), but when paired with a contactor and circuit breaker made by the SAME mfr as the OL relay, have a Series Rating of 65kA. But that means they cannot get a Schneider motor starter and connect it behind a Siemens or ABB breaker that was $1 cheaper than the Schneider breaker, they need to go with Siemens + Siemens, or Schneider + Schneider, or ABB + ABB etc.

In rare circumstances, as Bob stated, you can even get away with looking at the ratings of the components when protected by specific current limiting fuses and use that in their UL listing of the panels. It really isn't that hard FOR THEM. It is however all but impossible for it to be dealt with in the field. It sounds to me as though all of the right steps were taken, up until someone accepted their BS response that they couldn't do it. They can, they just didn't want to bother themselves.

Transformers is one option, so are Current Limiting Reactors, which in some cases end up costing less.

 

[Jp807]

Much more likely; the mfr is lazy and does not want to even investigate what it will take to get a higher rating, even though it is relatively simple to do. In most cases, it's as simple as selecting components from the SAME MFR so that they have UL listed Series Ratings for SCCR, but that then means they cannot negotiate with separate suppliers for the cheapest possible parts on each sub-component. So for example Overload Relays have notoriously low SCCR ratings (typically 5kA), but when paired with a contactor and circuit breaker made by the SAME mfr as the OL relay, have a Series Rating of 65kA. But that means they cannot get a Schneider motor starter and connect it behind a Siemens or ABB breaker that was $1 cheaper than the Schneider breaker, they need to go with Siemens + Siemens, or Schneider + Schneider, or ABB + ABB etc.

In rare circumstances, as Bob stated, you can even get away with looking at the ratings of the components when protected by specific current limiting fuses and use that in their UL listing of the panels. It really isn't that hard FOR THEM. It is however all but impossible for it to be dealt with in the field. It sounds to me as though all of the right steps were taken, up until someone accepted their BS response that they couldn't do it. They can, they just didn't want to bother themselves.

Transformers is one option, so are Current Limiting Reactors, which in some cases end up costing less.

Oh, trust me, they are lazy and downright incompetent. They had calculations like basic ampacity calculations from kVA and voltage incorrect. They have been a problem for all engineering disciplines on the job, not just us electrically.

The saving grace is as the electrical engineer of record we did our job and diligence, it wasn't a miss on our part. But now we need to tell the client and deal with the design changes.

I appreciate your input! I will have to investigate current limiting reactors. I have never dealt with them.

 

[wbdvt]

Is the fault current the utility providing you the available fault current or infinite bus? If they are providing infinite bus you may find that getting the available fault current will result in lower levels.

However, it looks like you are stuck with using either transformer or CL reactors to limit the fault current.

 

[Jp807]

Is the fault current the utility providing you the available fault current or infinite bus? If they are providing infinite bus you may find that getting the available fault current will result in lower levels.

However, it looks like you are stuck with using either transformer or CL reactors to limit the fault current.

It's the available fault on the secondary of their transformers. We have two switchgear each served by 3750kVA xfmrs.

I also reached out to Eaton and SquareD and both have confirmed as of this morning that transformers are really the only solution. So now have to deal with working the redesign. We now need 5 transformers, (4) at 300 kVA and a 225kVA and an elevated platform to fit it all. Figure adding a few $100k to the project cost because a manufacturer screwed up.

Key take away as engineers. Review everything and comment on everything. At the end of the day this could have been a huge change order that could have been the fault of us as the Engineer of record. Thankfully we caught it and elevated it. Now it's on the manufacturer.

 

[David Castor]

Small transformers may be your best option at this point. But a 300 kVA transformer will have much more than 5 kA of fault current at 480 V.

 

[Jp807]

Small transformers may be your best option at this point. But a 300 kVA transformer will have much more than 5 kA of fault current at 480 V.

Yeah agreed but we will feed a distribution panel off the transformer and then the branch circuits off the panel will drop the short circuit enough. The branch feeds will be around 400' in length.

 

[David Castor]

As you're probably aware, voltage drop could be an issue if loading is heavy. Also, the impedance of 300 kVA transformer is highly variable. There's not a set standard value. There is a range.

 

[Jp807]

As you're probably aware, voltage drop could be an issue if loading is heavy. Also, the impedance of 300 kVA transformer is highly variable. There's not a set standard value. There is a range.

Yes, we would specify the %Z requirement on the single lines. Most of our standard feeder sizes can handle a fair amount of voltage drop on the 480V side. But agree that it's something that needs to be checked. Nothing worse than having to upsize a feeder for voltage drop then gain more short circuit from the increased feeder size. But should be minimal.

 

[d0nut]

Make sure to inform the owner about the losses in your system that the transformers will introduce as well. These losses will cost the owner money for the life of the installation and would not be present if the manufacturer had properly designed the equipment.

 

[jim dungar]

Yes, we would specify the %Z requirement on the single lines...

They didn't follow your specifications for the SCCR of the equipment, how do you know they will follow the %Z requirement?
300kVA is not usually a special order transformer, most vendors are likely to offer a standard design, unless they are paying close attention.

 

[Jp807]

They didn't follow your specifications for the SCCR of the equipment, how do you know they will follow the %Z requirement?
300kVA is not usually a special order transformer, most vendors are likely to offer a standard design, unless they are paying close attention.

The transformers will be supplied by the contractor and will go through their own submittal process. Standard impedances on a SquareD 300kVA transformer range between 5 and 5.9 %. At 5% that's a maximum available fault of 7,225A on the secondary for a 480V-480V xfmr. With 400' branches we will be okay considering the sizes of the feeds. It will drop.

 

[Jp807]

Small transformers may be your best option at this point. But a 300 kVA transformer will have much more than 5 kA of fault current at 480 V.

SquareD dry types at 300kVA range between 5 and 5.9% Z. At 5% it would be 7,225A maximum. We'd be dropping below 5k with the smaller sized branches and long distance of the branches.