Chiller SCCR

A

AMBATISTA

Member
Location
SACRAMENTO, CA
Occupation
EE
I am hoping to get some good teaching/correction on my understanding of SCCR.

I have a project where the chiller mfg states that the SCCR is 5,000A. Our AFC at the chiller is 19,000A on the 240V side. The chiller is fed with a separate 120V circuit for the control board. The chiller is 240V, 3 phase. (see page 35 and 36 of submittal)https://www.shareddocs.com/hvac/docs/1005/Public/06/30HX-16PD.pdf

The wiring diagram shows that most of the control components seem to just be switches, interlocks, relays, with either 24VDC or 120V ratings. I don’t see how the compressor is controlled on the wiring diagram.

The manufacturer has a bulletin, which talks about the possibility of increasing the SCCR. It specifically calls out the replacement of components within the control board (how do I attach PDFs?)

My questions:
  1. Is it clear that the entire machine (compressor motors + the control panel) has an SCCR of 5,000A, i.e. does it not matter if the control panel has a separate feed? If this is so, then I should calc the available fault current on the 240V side.
  2. Is it possible that only the control board is rated at 5,000A? This seems unlikely to me. But if it was true, then could I calculate the available fault current on the 120V side only?
  3. If the control board clearly had a 120V->240V contactor for the compressor, then most likely that contactor would be the component that could experience a fault on the 240V side, which would require us to calculate the available fault current on the 240V side. Is my thinking correct here?
  4. In general (for my own understanding of SCCR), if a control board is fed downstream of the incoming machine power (single point of power connection for the chiller), can you simply provide Class J fuse that would reduce the available fault down to 5,000A before the connection and that solves the problem? Or does that fuse need to be somehow selected/coordinated with the control power components in the control panel?

I do understand another option would be an isolation transformer to reduce the incoming available fault current.

Appreciate any help
 
Unfortunately unless it is a manufacturer listed application (doubtful) install a Class J fuse does not legitimately address the problem.
If there is any appreciable wire distance on the 120v feed to the control board there is likely no SCCR problem with that feed.''

This is an all to often situation and has been discussed frequently on the Forum.
I suggest you review this thread:

Install Current Limiting Fuse to Reduce KAIC

Made up scenario for this discussion: SCCR of a chiller is lower than the available short-circuit current. example: AIC at chiller is calculated at 30kAIC, Chiller is rated for 10kSCCR. What to do? (don't get hung up on this question though, the real question is below) I have been reading a...
forums.mikeholt.com
 
SCCR never applies to control circuits, only power.

Unfortunately, YOU cannot change the SCCR yourself in the field, despite what they tell you. The 5kA value is basically a value that they can apply without doing anything. It’s the minimum amount you can get, meaning the manufacturer put exactly zero effort into making their equipment useable…

Your only viable option is to use a transformer stead of it unfortunately. And let the world know about this crappy manufacturer who couldn’t spend an extra 17 minutes of engineering time to chose power components that have a higher SCCR value already listed by the manufacturers, which is all they had to do.
 
The diagram basically only shows the external wiring connections. power, pump interlocks, flow switches etc. Standard chiller stuff.

What you have is the product data which is basically what you need to know to install the equipment. For more details on the internal wiring....how the compressors start etc read the sequence of operation & the Service Manual would tell you more.
 
The control circuits do not have an SCCR, only the power circuits.

You can't change the sccr of the control panel panel UNLESS it is marked that way on control panel.
 
Jraef said:
SCCR never applies to control circuits, only power.
Click to expand...
Can you please explain this a little more? I always assumed that it was the control components that needed to be protected from a fault. If I understand the other discussions correctly, HVAC equipment with a UL508 control boards are the ones with a listed SCCR.
 
Jraef said:
Unfortunately, YOU cannot change the SCCR yourself in the field, despite what they tell you.
Click to expand...
Understood, I was referring to a bulletin that the manufacturer put out. You can order the chiller with a higher SCCR (they explain that they have higher rated components in the control panel).
Unfortunately for this project, the chiller was already ordered....
 
AMBATISTA said:
Understood, I was referring to a bulletin that the manufacturer put out. You can order the chiller with a higher SCCR (they explain that they have higher rated components in the control panel).
Unfortunately for this project, the chiller was already ordered....
Click to expand...
And now the project gets to pay the manufacturer 100x more than if the equipment was ordered correctly to get them to send someone out to change those components and provide a new label. It will also take at least two months to get them on site, so hopefully your schedule still has some room to slide.
 
d0nut said:
And now the project gets to pay the manufacturer 100x more than if the equipment was ordered correctly to get them to send someone out to change those components and provide a new label. It will also take at least two months to get them on site, so hopefully your schedule still has some room to slide.
Click to expand...
Right, makes sense. I was brought in as a 3rd party to help with possible solutions. So I'm going to say the transformer may be the only option.
 
A transformer will have losses that the owner will be paying forever. Unless I was the one who ordered the equipment, if I was the owner I wouldn't accept adding a transformer as a solution without a substantial credit.
 
AMBATISTA said:
Can you please explain this a little more? I always assumed that it was the control components that needed to be protected from a fault. If I understand the other discussions correctly, HVAC equipment with a UL508 control boards are the ones with a listed SCCR.
Click to expand...
It is not the components that have a SCCR. It is the circuits.

Branch circuits that are not power circuits do not have a SCCR.

Components like contactors that have both a control circuit (the coil) and a power circuit (the contacts) only have an SCCR calculated for the power circuits.
 
I was in a bind one time where a big chiller only had 5kA SCCR. I was able to show the manufacturer that if the upstream impedance was high enough to keep the AFC below 5kA, then it would be too high to allow the machine to operate within voltage tolerances.

They upgraded it to 65kA for no charge.
 
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