SCCR and Servo Drive Calculations

[ElectEng]

Hi all,

New person to Mike Holt forums. These questions are in regard to some confusing (at least on my part) things in sccr. I have a machine I'm retro-fitting, and it has two panels/enclosures, one is a main enclosure, the other is what I'll call a motion enclosure. The main will house a 100 amp fused disconnect, a mainline contactor, some motor starters/overload relays, and a small ethernet fieldbus coupler with I/O modules used for reading pressure switches, limit switches, etc., 460V 3Ph powers the machine. The second panel (motion), has servo drives (230V, 3Ph.) another remote bus coupler set-up, a fused disconnect 460/120v power supply, and a 30kVA 3.84%Z 460V/230V 3 Ph. drive transformer (servo) that is mounted on the outside of the panel. The drive transformer is wired to the mainline contactor, although they are approx. 30 machine feet away and reside in/on different panels. The two ethernet bus couplers I listed here, are ethernetted to two of the servo drives used in the motion cabinet. There is approx. 12kA RMS symmetrical amps fault current available to the machine where it will sit, the peak let-through current of the LPJ 100 amp fuses in the feeder (100 amp fused disconnect) will be approx. 5800 amps at that fault current. According to what I'm understanding of the calculations of sccr, the available fault current on the primary side of the drive transformer will be approx. 6500 amps, and approx. 1900 amps on the secondary side.
Here are my questions, am I correct in my statements about the primary and secondary fault currents on the servo transformer, because it looks to be saying (SB 4.3.1) with a transformer greater than 10kVA to "Use lowest sccr of secondary circuit components". Does that mean the sccr of the servo drives connected on the secondary side of that transformer are now the sccr to use for the circuit and it sidesteps the calculations that were done in my above paragraph? Also is it correct in just using the two fuses and transformer that the 460V/120V fused disconnect power supply uses in fault current calculations?

Any help would be welcomed.

 

[Jraef]

If your incoming power is 480V to the first panel, the SCCR for that panel is based on the 480V figure, up to and including the fuses feeding any transformer. But not beyond the fuses THROUGH the transformer. The SECOND panel that is fed BY that transformer would have a new SCCR based upon the AFC from that transformer. It's a new panel, the rules start over.

Now, with 6.5kA AFC on that panel, you are going to have to be careful about coordinating the protective devices ahead of the servo amplifiers, otherwise you might be at 5kA, too low for the supply.

 

[ElectEng]

If your incoming power is 480V to the first panel, the SCCR for that panel is based on the 480V figure, up to and including the fuses feeding any transformer. But not beyond the fuses THROUGH the transformer. The SECOND panel that is fed BY that transformer would have a new SCCR based upon the AFC from that transformer. It's a new panel, the rules start over.

Now, with 6.5kA AFC on that panel, you are going to have to be careful about coordinating the protective devices ahead of the servo amplifiers, otherwise you might be at 5kA, too low for the supply.

Thank you for the reply Jraef,

Are you referring to the time-current curves of the perspective fuses to be used for servo protection and the opening time being to slow on those?

I included the information on the fieldbus couplers for another question, didn't want to ask to much too soon :happysad:, the only thing those are connected to besides a 24v DC power supply are two of the servo drives as stated earlier, through an Ethernet cable. Would those come into any concerns being they are connected to the servo drives, for sccr.

 

[Jraef]

Are you referring to the time-current curves of the perspective fuses to be used for servo protection and the opening time being to slow on those?

No. What I mean is that your servo amplifiers, the POWER portion of the servo system, will probably have an SCCR rating by themselves when protected by some specific fuse or breaker. If not, then the best you can get is the "courtesy" listing at 5kA for untested devices. If that's the case, you having 6.5kA available is not going to cut it for a 5kA rated device. So you need to carefully determine what the Servo amplifier is listed WITH if it has a higher SCCR and you use ONLY those devices. The let-through of fuses etc. is relevant only to the ones who are testing and listing the device, i.e. the manufacturer, or a UL panel shop applying a UL label for the entire assembly per UL 508A using Supplement SB. You as an individual in the field cannot apply a label stating that. So you cannot use current limiting fuses on your own to make something acceptable that was not otherwise tested and/or listed as acceptable from an SCCR perspective (unless you failed to mention that you ARE a UL 508a listed panel shop).

I included the information on the fieldbus couplers for another question, didn't want to ask to much too soon :happysad:, the only thing those are connected to besides a 24v DC power supply are two of the servo drives as stated earlier, through an Ethernet cable. Would those come into any concerns being they are connected to the servo drives, for sccr.

No, nothing in the control circuit counts. SCCR applies to line power (as it enters the specific enclosure) devices only.

 

[ElectEng]

Thank You for answering again.



No not a UL approved panel shop, I am aware of high fault coordination efforts, all the components in the main panel are as such, the drives are not UL listed, so yes the default rating will be attached to the panel. At this time the machine will be located to an area where the AFC calculates to a lower amount that fits in that window.

Wouldn't changing the fuse in the feeder circuit to a low peak, current limiting LPJ80-SP change the amount of peak let-through current and RMS AFC to an amount acceptable to a 5kA device as in this example. The numbers in this case would still be 12 kA at the first panel, but peak let-through would be 4500 amps, and available AFC to 2000 Amps because of the feeder fuse? Isn't that what SB4.3.3 is saying?

 

[GoldDigger]

IMHO, what Jraef is trying to tell you is that YOU (or even a random PE?) cannot make the determination that the lower fuse let through current will allow you to use lower SCCR rating. That would have to come from a manufacturer or NRTL test.

Sent from my XT1585 using Tapatalk

 

[Jraef]

IMHO, what Jraef is trying to tell you is that YOU (or even a random PE?) cannot make the determination that the lower fuse let through current will allow you to use lower SCCR rating. That would have to come from a manufacturer or NRTL test.

Sent from my XT1585 using Tapatalk

Yes, or a PE willing to stake his livelihood on it...

 

[Macbeth]

As semantics go, you are nor “Raising” the SCCR of the Device but “Lowering” the AFC

As semantics go, you are nor “Raising” the SCCR of the Device but “Lowering” the AFC

Since the Servo Drive is on the Secondary side of the transformer the Available Fault Current is:
(1.73 * 30kva)/230v = 226A (Secondary Amps at 230v 3ph)
Available Fault current is:
226A/0.038z = 5938A (Approx. 6ka)

You will need the Let-Through graph for you fuse you are selecting. Using the “Available Fault Current Symmetrical RMS Amperes” Vrs “Let-Thru Amperes”

1. Staring at the bottom of the chart X axis “Available Fault Current Symmetrical RMS Amperes” draw a line upwards from the 6ka value.
2. Where the first line crosses the fuse size, from that point draw a second line to the left.
3. Where the second line crosses the Dead Shot Line, from that point draw a third line straight back down to the down to the X axis “Available Fault Current Symmetrical RMS Amperes” .
4. That value is now your actual “Available Let-Thru” as long as that value is Less than the SCCR ratting of your device your circuit is protected.

It is important as semantics go, you are nor “Raising” the SCCR of the Device but “Lowering” the AFC of the circuit to be lower than the SCCR of the device.

 

[ElectEng]

Yes, or a PE willing to stake his livelihood on it...

I posted another reply but it doesn't look like it made it.

I would like to thank everyone who takes the time to put these forums on and invests in the discussions, it must be a tremendous undertaking. Indeed I don't want to offend anyone by my answers.

It is as Macbeth says, I'm not trying to raise the SCCR of the device, I didn't design or have part in designing it, but I believe SB4.4.3 tells us the AFC can be lowered allowing that particular item to work.
Macbeth used the infinite model for transformer AFC, and the up-over-and-down method, which I used in the second part of my yesterday 3:30pm message. Either method can be used, but I calculated out the AFC on the primary and used that number for the secondary result. Their method can be used if the machine may undergo changes to the electrical power, through machine modification or the power company that supplies power to the facility.
UL508A Supplement SB is used to evaluate a panel if it is not economically feasible to send to UL or another NRTL for testing( Could be a one off, Machine/Panel could be in the field, etc. ). It goes through several steps in doing so, and the step in my reference is to consider the effects of current-limiting devices in lowering short-circuit currents. UL determined a component in a circuit, that an upstream OCPD passed peak current, doesn't exceed the SCCR of that component, is appropriate to use. The OCPD is a Class J fuse, it is marked "current-limiting", it is in the feeder circuit, and the Ipeak passed by the OCPD isn't greater than the SCCR of the component. The step I'm showing here is appropriate for this discussion.