Arcing Current Calculation: VCB or VCBB?

Status
Not open for further replies.
R

RiseFromYourGrav

Member
Location
Chicago, Il
I work for a small switchboard builder, and we are working on a 2000A 208V 100kAIC switchboard that the customer requested to have a fused bolted pressure switch for a main. We figured there would be a fuse out there that would keep us compliant with NEC 240.67, but the arcing energy at that level of fault current is not very high, so it's hard to find a fuse that could clear that in 0.07 seconds (and the fault current is very likely a lot lower than 100kA). My question is: within a multi-section low voltage switchboard, can we just say it is a VCBB configuration if we barrier the line/load lugs, or does the rest of the bus being not barriered require us to go to VCB? There's a fuse we can use that clears 18kA in 0.07 seconds, and the VCBB configuration would take us to 19kA based off of the easy Bussmann calculator (32mm gap, 100kA fault current). If there is another equation that might suit us better knowing the exact size of the switchboard and whatnot, that would help. We would then have to go back to our customer for more information to get the real fault current and see if the fuse would still work. In the future, we're going to all breakers at this voltage/ampacity combo...
 
RiseFromYourGrav said:
I work for a small switchboard builder, and we are working on a 2000A 208V 100kAIC switchboard that the customer requested to have a fused bolted pressure switch for a main. We figured there would be a fuse out there that would keep us compliant with NEC 240.67, but the arcing energy at that level of fault current is not very high, so it's hard to find a fuse that could clear that in 0.07 seconds (and the fault current is very likely a lot lower than 100kA). My question is: within a multi-section low voltage switchboard, can we just say it is a VCBB configuration if we barrier the line/load lugs, or does the rest of the bus being not barriered require us to go to VCB? There's a fuse we can use that clears 18kA in 0.07 seconds, and the VCBB configuration would take us to 19kA based off of the easy Bussmann calculator (32mm gap, 100kA fault current). If there is another equation that might suit us better knowing the exact size of the switchboard and whatnot, that would help. We would then have to go back to our customer for more information to get the real fault current and see if the fuse would still work. In the future, we're going to all breakers at this voltage/ampacity combo...
Click to expand...

In the absence of information always go with the conservative approach; i.e. worst case and then state your assumptions to the customer. Keep in mind in reality VCB, VCBB and HCB may all exist simultaneously in an enclosure. It’s not about isolation between line and load but more about how the arc blast will eject out of the enclosure due to the conductor arrangement and any conductor obstructions. In my opinion you should never look for reasons to get away with lower incident energy values to comply with a rule, but that is just me. You are safer requesting actual data and calculating the incident energy of each enclosure having different dimensions at minimum and maximum fault generation conditions. Take the worst case from there. If there is a violation, upsell a solution to bring the customer into compliance.


Sent from my iPhone using Tapatalk
 
mbrooke said:
I thought it couldn't take place at 120/208? Hence why I ask.
Click to expand...

Gotcha. Arcing does take place on 240-V systems and below but is generally regarded as self-extinguishing (unlikely to be sustained past 1/2 cycle). Keep in mind that arc flash for these systems can still be hazardous under certain conditions which is why the IEEE 1584-2002 125 kVA transformer exception was removed in the 2018 standard.


Sent from my iPhone using Tapatalk
 
Using VCBB will almost certainly result in higher incident energy than VCB. The VCB test setup had three electrodes that were open at the bottom end of the electrodes. VCBB test setup had the three electrodes ending at an insulating barrier at the bottom. HCB was based an end-on exposure to the three electrodes mounted in a horizontal configuration. There's not much guidance in IEEE 1584-2018 about which configuration to use. With some equipment, all three conditions could occur, depending on the location of the fault. So you may need to calculate more than one configuration and use the worst case incident energy.

Dave C.
 
Status
Not open for further replies.
Top