SCCR for servo DC bus

[olsondavidj]

I've recently run in to an odd situation that I'm not sure how to handle. We are building a machine that uses 2 servo drives: the first is fed via main 3ph AC power, and the second drive is powered via the DC bus from the first drive.

This particular drive manufacturer supports this configuration, and they recommend using fuses on the DC bus, so we have fusing on both legs of the DC bus going between the 1st and 2nd drive. Due to the voltage involved, we are using PV fuses/holders that only carry DC voltage ratings.

Now I'm calculating the SCCR for the panel and I'm not sure how to include these components, if at all. My first instinct is that the DC bus is part of the power circuit, and therefore anything in there must be included in the SCCR calculation. But being that the DC bus is generated by a listed component that already has a marked SCCR (i.e. the servo drive), I don't know if this means it's already accounted for in the device's rating. As an aside, the manufacturer's instructions do not list the size/type of fusing to use on the DC bus.

Do I need to include any DC bus components in the SCCR calculation? Or can I safely ignore these? If they're included, how do I rationalize the DC rating with the panel labeling requirement? So if the voltage portion of our SCCR is otherwise good for 480Y/277V max, how does the 1000 VDC rating of the DC components figure in?

 

[Jraef]

The DC bus is accounted for in the first rectifier, so the input AC to that power source is your only concern from an SCCR standpoint. Nothing else down stream of that counts.

 

[olsondavidj]

Thanks, that's the answer I was hoping for!

So the follow-up of course is: why? Isn't the DC bus still part of the power circuit? As I mentioned, I couldn't find anything in 508A, but I could have easily missed it.

And then is this always the case for the load side of any servo/VFD rectifier? For example, if I had a terminal block for field wiring on the load side of a VFD, wouldn't that need to be included in the SCCR calculation as well? That doesn't seem right to me, but the logic would follow since the outputs of the VFD/servo are powered from the same DC bus.

 

[Jraef]

When you have power conversion equipment like VFDs or Servos, everything downstream of the rectifier can ONLY get power FROM that rectifier. It's kind of like a Separately Derived Source in that the amount of fault current in that down stream power circuit is now limited to what the conversion equipment can deliver. Part of the UL listing of that device will include that aspect of it, so if the VFD or Servo amplifier is UL listed, the UL listing INCLUDES testing and listing of the output power circuit's capabilities in regard to short circuit.

If your servo is not UL listed, you are going to be stuck with the "courtesy" untested SCCR values anyway, which for something like a small servo are going to be only 5kA.

 

[olsondavidj]

So I understand the logic, and it makes perfect sense to me.

However, I don't follow the "legal" (code/standard) allowance for it. I've been googling like crazy and I see a lot of discussion about this scenario along similar lines to what you're saying, but I have yet to find any references to codes or standards to support the conclusions.

I just re-read 508A supplement SB (3rd ed.) and I didn't see any exception that says items downstream of a servo can be ignored, regardless of listing status. SB4.2-SB4.3 lists calculation methods for components downstream of current limiting devices, but the closest thing I can see as applicable is SB4.2 regarding power transformers, which of course a servo is not. I also double checked the definitions in the front of 508A, and I didn't see any wiggle room. Branch circuits and power circuits are pretty clearly defined.

So even if there is something in the UL listing/testing for the servo that ensures the downstream current is limited, 508A SB doesn't appear to recognize it based on my reading. Right or wrong, I don't see how I can justify this from a code/standard standpoint.

Fortunately this particular application has a 15A J fuse in the feeder, so it's pretty easy for me to boost the SCCR (aside from a few pesky circuit breakers). But I want to make sure I fully understand this so I can properly apply this information in the future.

 

[topgone]

So I understand the logic, and it makes perfect sense to me.

However, I don't follow the "legal" (code/standard) allowance for it. I've been googling like crazy and I see a lot of discussion about this scenario along similar lines to what you're saying, but I have yet to find any references to codes or standards to support the conclusions.

I just re-read 508A supplement SB (3rd ed.) and I didn't see any exception that says items downstream of a servo can be ignored, regardless of listing status. SB4.2-SB4.3 lists calculation methods for components downstream of current limiting devices, but the closest thing I can see as applicable is SB4.2 regarding power transformers, which of course a servo is not. I also double checked the definitions in the front of 508A, and I didn't see any wiggle room. Branch circuits and power circuits are pretty clearly defined.

So even if there is something in the UL listing/testing for the servo that ensures the downstream current is limited, 508A SB doesn't appear to recognize it based on my reading. Right or wrong, I don't see how I can justify this from a code/standard standpoint.

Fortunately this particular application has a 15A J fuse in the feeder, so it's pretty easy for me to boost the SCCR (aside from a few pesky circuit breakers). But I want to make sure I fully understand this so I can properly apply this information in the future.

Fortunately for you, there is a standard for your case. Try reading the Violet Book (IEEE Std 551) section 3.16.8 Static Regenerative Drives

"The inclusion of static regenerative drive data will be necessary in the first-cycle calculations. (Note that non-regenerative drives are not sources of fault current and need not be considered.)

 

[Jraef]

I think maybe you are looking for more granularity in the concept than you will find. If you look for the definition of SCCR in the NEC, it has to do with the prospective short circuit current at the point at which a device is connected to a power system. In a drive, only the RECTIFIER is connected to the line AC power system. Everything down stream is Not connected to to that AC system, it is connected to the DC Bus. So the prospective short circuit current those down stream devices can possibly see is no longer related to what is happening on the AC side. Ergo, irrelevant.

 

[olsondavidj]

Fortunately for you, there is a standard for your case. Try reading the Violet Book (IEEE Std 551) section 3.16.8 Static Regenerative Drives

Correct me if I'm wrong, but it looks like IEEE 511 is for calculating the AVAILABLE fault current at any point in a distribution system. I'm not really familiar with that standard so I could be wrong here.

What I'm trying to do is calculate the SCCR of a panel that's installed in an existing distribution system that has a known available fault current. I'm not trying to calculate how much fault current the panel can put back on the system.

I think maybe you are looking for more granularity in the concept than you will find. If you look for the definition of SCCR in the NEC, it has to do with the prospective short circuit current at the point at which a device is connected to a power system. In a drive, only the RECTIFIER is connected to the line AC power system. Everything down stream is Not connected to to that AC system, it is connected to the DC Bus. So the prospective short circuit current those down stream devices can possibly see is no longer related to what is happening on the AC side. Ergo, irrelevant.

Your logic is sound, but I would have to disagree with the conclusion. If we consider a transformer with an isolated secondary (so the secondary has literally ZERO electrical connection to the AC supply), there is still an available fault current on the secondary that must be considered in the SCCR calculation. So this is not strictly about what the supply system can provide, but also what the panel can provide internally. I would argue that a drive is the same situation: while there may be no direct connection to the AC supply, there is still an amount of current that can be pulled by a short circuit on the secondary. Therefore, the components on the secondary must be rated to withstand that fault current.

The problem here is that UL508A SB doesn't seem recognize that fact, at least not that I can see. We see fuses, breakers, and transformers are recognized as current limiting devices, but drives are not. So I don't see that I can ignore any device on the secondary of a drive in my SCCR calculation, regardless of how little current the drive is capable of providing.

So again, I fully understand your point of view, I just don't see any justification from a code/standard point of view. This doesn't mean that UL is right, just that it fails to address the situation.

And the final problem here that I see is that NEC requires that a panel's SCCR is determined by testing (cost prohibitive for most situations), or by an approved calculation method. NEC only lists UL508A SB as an approved calculation method, so lacking any other method allowed by an AHJ (which I can't bank on because our panels go everywhere), I'm stuck with 508A.

Now that said, I think your logic would be valid if I were to send my panel out for testing. I could make the assumption that any fault on the output of the drive will draw minimal current, and therefore ignore it in my design and still be confident that it would pass testing.

 

[Jraef]

I think you are conflating calculating the AVAILABLE fault current with calculating the SCCR, which is not about what devices CONTRIBUTE, but what they can WITHSTAND. Maybe it will help you to know that the SCCR terminology replaced the older concept of "Withstand Rating", because it was felt to be too ambiguous in some cases. So since the devices DOWNSTREAM of a rectifier can "withstand" whatever can be delivered by the rectifier, there is no need to worry about them in the context of what else may happen up stream in the rest of the panel, they are effectively isolated from it. And to your other point, you do NOT consider the secondary of a transformer that is in the panel when doing an SCCR determination, in fact you do not consider it at all...

SB4.2.1 All power circuit components, including disconnect switches, branch circuit protective devices,
branch circuit fuseholders, load controllers, motor overload relays, terminal blocks, and bus bars, shall
have a short circuit current rating expressed in amperes or kiloamperes and voltage.
Exception: Power transformers, reactors, current transformers, dry-type capacitors, resistors, varistors,
and voltmeters are not required to have a short circuit current rating.

Also, in the general body of UL508A:

49.1 The input terminals intended to be connected to each source of supply shall be rated in volts, total
full-load amperes, ampere or horsepower rating of the largest motor (when multiple loads are controlled),
number of phases when other than single phase, and the frequency.

49.5 The input terminals in 49.1 shall have a short circuit current rating. The short circuit current rating
shall be determined based upon the requirements in Supplement SB.

 

[topgone]

Correct me if I'm wrong, but it looks like IEEE 511 is for calculating the AVAILABLE fault current at any point in a distribution system. I'm not really familiar with that standard so I could be wrong here.

What I'm trying to do is calculate the SCCR of a panel that's installed in an existing distribution system that has a known available fault current. I'm not trying to calculate how much fault current the panel can put back on the system.

It is what it is! You will come up with an SCCR after you shall have determined the fault current available at that point of common coupling plus the fault current contributions of loads which are "sources of fault currents" connected to it. Crystal!

 

[olsondavidj]

I think you are conflating calculating the AVAILABLE fault current with calculating the SCCR, which is not about what devices CONTRIBUTE, but what they can WITHSTAND. Maybe it will help you to know that the SCCR terminology replaced the older concept of "Withstand Rating", because it was felt to be too ambiguous in some cases.

Correct, when I say SCCR I am talking about the "withstand" of the panel, which is what I'm aiming to calculate here. I understand the difference, but I may not have been 100% clear.

So since the devices DOWNSTREAM of a rectifier can "withstand" whatever can be delivered by the rectifier, there is no need to worry about them in the context of what else may happen up stream in the rest of the panel, they are effectively isolated from it.

Well that's the question then, right? You make the assumption that downstream devices CAN in fact withstand the drive's available fault current, but what makes that true? The typical case is that they are sized based off the load current (e.g. field terminals per 28.3.2), but that doesn't necessarily mean they're sized for the available fault current that the drive can supply. I'm sure the drive can limit the output current to some degree, but that's not recognized by UL 508A that I can see. In any case, it's almost certainly going to be a number larger than the nominal output of the drive. How much larger? Probably depends on the manufacturer and the price point.

Said another way, the purpose of UL 508A SB is to calculate the SCCR for the panel as a whole to make sure nothing blows up in the event of a short circuit. As part of that, wouldn't you need to take in to account the ability of the devices downstream of a drive/rectifier to withstand the current generated by a short circuit? You can have a short anywhere, so of course the answer is yes, right? Then the follow-up of course is: how do you calculate that? Is the available fault current a function of the panel's AC supply? Probably not, it's more likely a function of the drive's current limiting ability.

But here's my point: I can't find anywhere that UL 508A differentiates between the panel incoming supply and the drive's output. Logically yes, the devices downstream of a drive should only have to withstand the current that the drive is capable of delivering. However there is nothing that I can see in UL 508A that allows this. So as far as I can see, UL does not consider the possibility that a drive limits its output current during a short circuit, so they assume that the full available fault current from the supply will be present.

And to your other point, you do NOT consider the secondary of a transformer that is in the panel when doing an SCCR determination, in fact you do not consider it at all...

SB4.2.1 is referring to the components themselves. So the exception is saying the transformer ITSELF doesn't need to have a labeled SCCR. A power transformer is defined in UL as a transformer whose secondary is supplying loads. Therefore the secondary of that transformer is part of the power circuit, which means per SB4.2.1 all the components in that circuit must have an SCCR, and that those components must be considered in the panel's overall SCCR.

With that, SB4.3.1 allows recognition that the transformer provides a current limiting function. So it gives you a means to calculate the fault current available on the transformer secondary. Then it tells you that as long as all the devices on the transformer secondary have a higher SCCR than what the transformer can deliver, you are allowed to apply the transformer's primary OCPD SCCR to that branch.

My hope is that at some point UL will recognize that drives perform a similar function, but near as I can tell they don't do that at all right now. The point of my starting this topic was in hopes that I was missing something, but the more we discuss this and the more I re-read things the less likely that is.

Also, in the general body of UL508A:

I think I see what you're getting at, but I believe it would be a stretch to apply that to this situation. Even so, the devices downstream of a drive have input terminals, so they're still subject to having an SCCR.

 

[Jraef]

You make the assumption that downstream devices CAN in fact withstand the drive's available fault current, but what makes that true? The typical case is that they are sized based off the load current (e.g. field terminals per 28.3.2), but that doesn't necessarily mean they're sized for the available fault current that the drive can supply.

Since 2005 this has been part of the UL 508c listing of power conversion equipment (VFDs, servos etc.); the device must protect anything connected downstream from short circuits on the load side, up to and including the capacity of the device. They don't have to bring it up in 508a because it is already covered in 508c.

This by the way includes the motors connected to the drives, because in the case of AC VFDs if the VFD is over sized for future consideration (as is common) or some other reason, when the NEC began requiring (in 2002) that the upstream OCPD be sized per the VFD maximum Input Current, that resulted in the OCPD required for the VFD being potentially too large for the smaller motor down stream. So UL began requiring that the VFD be listed to provide SC protection for the load side circuit and that everything on that side be capable of whatever the drive can deliver.

 

[olsondavidj]

Since 2005 this has been part of the UL 508c listing of power conversion equipment (VFDs, servos etc.); the device must protect anything connected downstream from short circuits on the load side, up to and including the capacity of the device. They don't have to bring it up in 508a because it is already covered in 508c.

This by the way includes the motors connected to the drives, because in the case of AC VFDs if the VFD is over sized for future consideration (as is common) or some other reason, when the NEC began requiring (in 2002) that the upstream OCPD be sized per the VFD maximum Input Current, that resulted in the OCPD required for the VFD being potentially too large for the smaller motor down stream. So UL began requiring that the VFD be listed to provide SC protection for the load side circuit and that everything on that side be capable of whatever the drive can deliver.

I figured that was the case, but I find it odd that 508A doesn't recognize that fact. Nothing I see in 508A SB allows that... they don't care if it's a drive or a contactor, it's still part of the power circuit so it has to be figured in the SCCR calculation just like any other device.

Typically UL 508A allows plenty of exceptions for devices that are listed under a different UL standard. As an example, I was just looking at cooling fans. Generally per 508A, even the circuit feeding an internal 24VDC circulating fan has to be considered as part of the power circuit since it's a motor load. However, 508A allows you to consider it as part of the control circuit if the fan is listed/recognized under UL 507.