Chiller Low SCCR 1

[CanisLupus]

Hi,

We've run into an issue where the chiller we bought is rated at 5kA SCCR and the available short circuit current at the switch gear is 52kA. The manufacturer says there isn't anything they can do to change the rating, we can't modify it ourselves without losing our warranty, and the breakers they used do not have a any tested series rating available.

I attached the chiller schematic along with the part numbers for the electrical components for reference.

One option we are considering is specifying a 480V-480V transformer with a custom impedance to limit the fault current down to 5kA. We're thinking of specifying a 320KVA transformer with an impedance of 7.7%. Using the MVA method I get a 77% voltage drop at the chiller during starting. Anything I am not considering or should be aware of?

The specs for the chiller are as follows,

460V
MCA = 383.9
ICF = 1380.6

Thanks,
CanisLupus

 

[jraef]

The long term operating cost of that high impedance will be borne by the end user now, but given the situation there really isn't much choice (although I have used current limiting reactors instead of a transformer because they can be cheaper). The only other option is to have the control panel completely rebuilt by a listed panel shop using the proper components to get an SCCR of at least 52kA, while forcing the supplier to honor their warranty because they sold you equipment that cannot be legally connected per the NEC (a violation of a basic implied warranty).

This situation is happening a lot now. For future reference, I have started adding this line to all of my specifications (when I get the chance to write them):

Electrical equipment shall bear a minimum Short Circuit Current Rating (SCCR) listed by an NRTL (Nationally Recognized Testing Lab acceptable to the local Authority Having Jurisdiction) and shall be [25,000A] [45,000A] [65,000A] or otherwise commensurate with the Available Short Circuit Current at the installation site. Lower SCCR listed units implying that the use of Current Limiting Fuses ahead of them in the field, in lieu of an actual appropriate SCCR rating, are not acceptable.


Without that statement or something like it in the tender specifications, suppliers will often cut corners and use the cheapest components they can find with no concern for this issue whatsoever, letting the burden fall on the end user or installer (as you have just experienced). It is NOT AT ALL DIFFICULT to attain a higher level of SSCR, it just means that they must be more careful about the components they select so that they have series listings. But without being forced to comply, suppliers are always worried that "the other guy" will get the order by ignoring it and being cheaper.



" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[CanisLupus]

Unfortunately the equipment was consciously ordered with the lower SCCR without any electrical input. That made me look at our other chiller installations and they too are rated at 5kA SCCR. I am definitely going to add that line to all our specifications in case we (electrical guys) don't get consulted in the beginning.

The chiller manufacturer keeps telling me to install a current limiting fuse upstream and all would be good, but I can't find a series rating on the chiller breakers, contactors, and overloads to do so.

I am not familiar with specifying a current limiting reactor, anything I should know about when specifying and where to buy from?

Any issues with the specified transformer if we go that route?

 

[waross]

77% voltage drop implies 23% voltage at the motor.
Considering that the voltage drop will drop the voltage to the motor, the motor starting current will be less than the DOL starting current and the voltage drop will be less.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

I can't see most refer compressors successfully starting with even a 30% voltage drop.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[wroggent]

I recently went through this with an air compressor. I went with a transformer one size larger than I needed to (impedance still selected appropriately). It took about 4 attempts to get the breaker to stay closed. This was a 300kVA transformer on a 300A breaker. That’s a mistake to watch out for I guess.

I understand your pain with not being consulted before purchase.

Even more recently at another plant they had also bought a new compressor. It has the same SCCR issue but this time I kept my mouth shut. ¯\_(ツ)_/¯

 

[jraef]

There is a persistent myth surrounding the use of current limiting fuses. That CAN be done, but not by installers in the field. You could (in theory) hire a registered PE to undertake evaluating the withstand rating of all of the devices in the power circuit of the panel, then compare that to the let through current of current limiting fuses in all scenarios (not just a bolted fault). It's essentially the same thing a panel shop would do, except the PE takes on the role of the component manufacturer in determining the series capabilities, and is doing so without testing (as the mfrs would). In reality, most PEs I've talked to about this idea will not want to stake their license on it, meaning if they were wrong and something catches on fire, it's their name on the drawings, yet they didn't actually design the system.

As to the reactors, I used these guys when I did it. They were very helpful. Not cheap, but less expensive than a transformer.

http://gilbertelectricalsystems.com/page.aspx?ID=1017

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[CanisLupus]

Oops I said that backwards, I meant a 23% voltage drop. I made one last ditch effort to see what it would take for the vendor to up the SCCR, hopefully they can do something because I really do not want to go with the transformer option. Jraef, thanks for the suggestion on the reactors, I contacted them and will see what they have to say.