Hoist and Crane Contact Conductors SCCR - UL508A + NFPA70 Art. 610

[Zeno Black]

I am working on a UL508A + NFPA70 Art. 610 compliance for a stacker crane system that uses power rails as contact conductors (bus rails) in combination with brush collectors. The manufacturers of the crane and the bus rails are in Europe and we just found out that they certified the power rails under the UL857 standard for Busways. Looking at UL508A in its Cranes section, it does not provide guidance on how to consider power rails/contact conductors for the SCCR calculation. Since conductors do not have an SSCR, one would think that "contact conductors" as referred to in NFPA70 Art. 610.21 would be considered as conductors/wires. However, crane contact conductors are busbars in contact with collectors which might lead someone to consider them as a busbar or a busway more than a conductor or a wire.

After weeks of back and forth, the power rail manufacturer accepted that they did not test the product for Short Circuit and the SCCR of their product is the minimum standard: 10kA. This represents a problem given the fact that the crane has an FLA of over 130A and at that amperage level, there is no chance we can use current-limiting fuses or breakers for limiting the peak let-through current at less than 10kA. Now, there is still the question, can we consider the contact conductors/bus rails as wires and do not even include them in the SCCR calculation?

I have been trying to find better resources for the electrical design of cranes and hoists but haven't found good ones. If anyone knows of an association or organization dealing with this kind of machinery standards, your share would be really appreciated.

 

[petersonra]

Ul508a only covers the control panel. Anything external to the control panel doesn't have to be considered for the short circuit current rating of the control panel.

 

[Zeno Black]

Hello petersonra, thanks for the quick reply. I think that is a misconception since most people believe that because supplement SB4 is in UL508A the SCCR calculation is limited to what is inside the enclosure but UL508A defines that the SCCR of the panel is determined based on the components in the power circuit, which extends outside of the panel in most cases as the power circuit includes everything up to the loads. Now, I have came across this discussion several times and the logic tells me that if for example a control panel is rated at 65kA based on the peak let through current of a feeder overcurrent protection and its corresponding SCCR then the peak let through current shall be less than the SCCR of any component in the power curcuit (feeder or branch circuit). Also, the definition of branch circuit says that this is the circuit between the load and its overcurrent protective device, which might or might not be in the same control panel leading me to believe that if I have a panel feeding other panels it is necessary to consider components remotely to the main panel for the definition of its SCCR. Would you disagree?

 

[petersonra]

Read carefully what the rest of the standard says. The part of the standard that describes the short circuit current requirement does not override anything in the rest of the standard. Other than a few things that you are allowed to have external such as external overcurrent protection, nothing beyond the field terminals is part of the ul508a listing.

Having said that, the fact that the UL 508a listing only applies to the control panel itself does not mean that you might not have a problem with something Downstream of the control panel. But it's not a problem with the listing of the control panel.

 

[Zeno Black]

So, I can list a panel at 65kA even when in the event of a shortcut let’s say an IP67 VFD mounted on a conveyor will certainly catch on fire because the VFD’s SCCR is 10kA but that is ok because it is outside the panel?

 

[paulengr]

So, I can list a panel at 65kA even when in the event of a shortcut let’s say an IP67 VFD mounted on a conveyor will certainly catch on fire because the VFD’s SCCR is 10kA but that is ok because it is outside the panel?

As far as UL 508A is concerned, yes. It’s a Listed assembly,

The VFD has it’s own Listing though and that includes required external short circuit protection. So the violation isn’t the industrial control panel itself but the VFD. There are VFDs on the market with fairly good front ends that don’t need aggressive short circuit protection in many cases. That would be like blaming the breaker manufacturer for a short circuit issue.

Short circuit and arc flash are two areas where the ultimate solution is an engineered approach that nobody is willing to pay for.

 

[bwat]

So, I can list a panel at 65kA even when in the event of a shortcut let’s say an IP67 VFD mounted on a conveyor will certainly catch on fire because the VFD’s SCCR is 10kA but that is ok because it is outside the panel?

In essence, yes. The panel’s rating is about the panel. Not what the panel feeds. If the VFD was inside the panel, then the VFD’s rating is considered when rating the panel, because it’s part of the panel.

An analogy would be like a switchboard or panelboard. Its rating is its rating. The panelboard doesn’t get a lower SCCR when you supply something downstream from this panelboard that has a lower rating.

The whole installation may fail, but it’s not because of the panel’s rating.

 

[Zeno Black]

I would agree, but as an OEM and as a designer of a whole solution the liability is on us so in practicality the VFD, or the contact conductors in this case would have to be considered for the rating of the panel, so we can ensure that the installation is secure and we comply not only with UL508A but more importantly with NFPA70.

 

[paulengr]

I would agree, but as an OEM and as a designer of a whole solution the liability is on us so in practicality the VFD, or the contact conductors in this case would have to be considered for the rating of the panel, so we can ensure that the installation is secure and we comply not only with UL508A but more importantly with NFPA70.

As an OEM NFPA 70 does not apply to you. That is a site specific design and installation requirement. That is why for instance the arc flash label requirement is on the installer, not the manufacturer. You have to respect it or there would be no market to sell to but the Listing is 508A.

However when you undertake responsibility to design for a specific installation and make performance guarantees is when liability occurs. If you just supplied say light bulbs, it’s on the end user. If they put 600 V on your 120 V lamp (labelled as such) it’s their fault. People can sue you for anything. Nothing can be done about that except tort reform. As a motor shop frequently customers accuse us of motor issues when it’s the load that is the problem.

It’s also perfectly legal to make ANSI equipment and switchgear is often ANSI rated but not UL as an example. It may not even be Listed. It is acceptable for use where NEC does not apply (mining, power plants, maritime, utilities). Listing only exists as a formal inspection process on the seller instead of inspections by the buyer.

Now if the manufacturer also does the installation (design/build) then you own it and must follow NEC.

If you sold the VFD and the panel as a package this could be an issue. It might be legal to sell a 65 kA panel and a 5 kA VFD but you are at best misleading with the panel label. To me the choice would be to advertise the lower rating on the panel (5 kA) since it is AT LEAST that, or put current limiting in the panel to allow the 65 kA rating. Then there is no confusion.

 

[petersonra]

Is any ansi switchgear listed?

 

[petersonra]

So, I can list a panel at 65kA even when in the event of a shortcut let’s say an IP67 VFD mounted on a conveyor will certainly catch on fire because the VFD’s SCCR is 10kA but that is ok because it is outside the panel?

It does not matter what the rating of the downstream equipment is when you calculate the rating of the UL listed control panel.

Just so you know an IP67 VFD is technically only a type one enclosure unless it's also UL listed for something else

 

[paulengr]

Is any ansi switchgear listed?

Yes. But they are UL 1077. So they are components, not stand alone assemblies. As an example UL 845 is low voltage metal enclosed switchgear assemblies. This should make sense because many are just high current switches and rely on external relays and often external DC power for trip functions, while those with built in trio units are user replaceable. This runs completely counter to the standalone philosophy of UL 489.

Also UL.com only follows UL.org standards. So typically an ANSI breaker would be Listed by CSA or ETL. IEEE is not going to give control to UL.

 

[petersonra]

Ul845 is mccs. Ul1077 is supplementary breakers.

I don't see how either has anything to do with ansi rated switchgear.

 

[paulengr]

Ul845 is mccs. Ul1077 is supplementary breakers.

I don't see how either has anything to do with ansi rated switchgear.

Missed it. I hardly ever bother with UL in switchgear.

UL 1558 | UL Standards & Engagement | UL Standard

UL 1558 | UL Standards & Engagement | UL Standard | Edition 5 | Standard for Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear | Published Date: February 18, 2016 | ANSI Approved: November 06, 2019

standardscatalog.ul.com

1558 is metal enclosed gear. This is different from metal clad. They don’t really have a metal clad standard. But metal clad is a more restrictive subset of the metal enclosed design. Contrary to popular belief metal enclosed gear can be far more than just fused switches. Breakers are perfectly acceptable. Both ANSI types can be tested to this standard although UL 1558 is a less restrictive standard.

UL 1077 is for breaker components. Essentially anything you use as a breaker without the limitations of UL 489. ANSI breakers can be tested under UL 1077 but it is more generic than the ANSI standard. So ANSI breakers can be tested under thus standard even though UL says nothing about ANSI.