UL 508 Panel Protection

[uwryguy]

We are looking to build a UL508 panel that will essentially only have a VFD, a disconnect, and an As-i communication module. The question is my customer says that they are putting all of there branch circuit protection in a central power distribution box and our box will be at a remote location near the motor. Will this be ok from a UL standpoint if the circuit protection for the VFD is not located inside of the enclosure where the VFD is located?

 

[LionelHutz]

Yes, it is OK. UL does allow the circuit protection (fuses or thermal-mag breaker) to be located outside the cabinet. We build cabinets without them all the time. The disconnect only case would be handled the same way as a cabinet without any protection supplied.

 

[jraef]

However!

Under the new 2005 NEC, article 409, you are now required to show the SCCR (Short Circuit Current Rating) on the outside of the enclosure. If you do NOT put the CB or fuses in the box with the drive, your VFD is likely rated for only 5kA fault current, lower than most industrial systems will permit.

Even if you do put a breaker in the box with the VFD, you MUST use the specific breaker that the VFD was tested and listed with (if any), otherwise the same 5kA rating will be the maximum you can get. What that usually means is that if your VFD supplier is also a CB manufacturer, they are tested together for a higher SCCR. If not, your VFD may not have ANY rating behind a CB, r can only be used behind a competitor's CB. Again, one brand or model of CB will NOT translate to a similar one, it must be exactly as tested.

Fuses are another matter because you can usually show that the let-through of a current limiting fuses is lower than the withstand of the VFD. The caveat there is that still, the VFD needs to be tested in combination with the fuse or else you will need to make a convincing technical argument to the AHJ that you have coordinated all the numbers yourself.

So:
Step one, inquire as to whether your state has adopted the 2005 NEC yet. If not, you don't absolutely have to follow the above. If so, read on.

Step 2, determine your Available Fault Current for this drive's circuit.

Step 3, ask your VFD manufacturer for any documents pertaining to it's SCCR rating. If they have it tested for a higher rating and it is above the available fault current, you are golden, just as long as you make sure ALL of the components used in their listing are rovided in your panel.

Step 4, if they have no SCCR rating, the default for an un-tested drive is 5kA, which now puts the onus on you or your customer to mitigate so that the AFC at the terminals of your VFD is 5kA or less.

 

[jraef]

Geeze, my spell checker s broken I guess!

 

[itsmoked]

Looks fine to me.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com