Fused disconnect or fuse block for arcflash mitigation

[VinceS]

Local arcflash study at a industrial manufacturing company returned horrible results at secondary side of isolation transformers, and step up transformers. There are 200' or greater runs at 150amps 240vac 3ph supplying foreign equipment, the step ups 240/380vac

Mitigation is to add fuses. I suggested fused disconnects, and the suggestion of fuse blocks came up as cost savings. Are there any reasons this is a bad idea?

Is there a reference for other than NEC for design of power distribution for reduced arcflash?

Thanks in advance

 

[iwire]

I was not aware the installation of fuses could reduce arc fault energy.

I am puzzled and wonder where this will go.

 

[petersonra]

Local arcflash study at a industrial manufacturing company returned horrible results at secondary side of isolation transformers, and step up transformers. There are 200' or greater runs at 150amps 240vac 3ph supplying foreign equipment, the step ups 240/380vac

Mitigation is to add fuses. I suggested fused disconnects, and the suggestion of fuse blocks came up as cost savings. Are there any reasons this is a bad idea?

Is there a reference for other than NEC for design of power distribution for reduced arcflash?

Thanks in advance

you are going through multiple relatively small transformers and end up with more IE?

if this is the case, have an engineer familiar with such things take a look at where you might be able to put in some faster CBs.

what difference does it make anyway? you can't work on it live so just stay out of these cabinets when they are on.

 

[wbdvt]

Having seen this before in industrial areas where there is foreign equipment that requires different voltages than is common in the USA. The problem is that the transformers insert additional impedance which will reduce the fault current and corresponding arcing current. This will usually result in a slower trip time on the protective device on the primary side of the transformer. Depending on the protective device, the instantaneous setting may be lowered to reduce the incident energy. This is something that should have been looked at in the study, if applicable.

You may be able to reduce the incident energy by using fuses on the secondary side of the transformers. This would have to be studied but since you have a study with a corresponding model, it should take the engineer long to look at that option. I have used a A6D class fuse to reduce incident energy previously.

The problem is that many of these machine cabinets contain control circuits and PLC's which sometimes need to be worked live for troubleshooting, so yes there is a need to be in the cabinets while energized.

 

[augie47]

You might also take a look at 240.40 and how that will apply to your situation.

 

[VinceS]

The idea is to reduce HRC level 4 and 3 requirements to a level 2 or less. This makes troubleshooting less cumbersome when in HRC 2 or less.

 

[VinceS]

Yep, 240.40 does indeed apply to the step up 380vac transformers.

 

[steve66]

What size transformers? Below a certain rating (I can't remember if its 75KVA, or 112 KVA), you might be able to give it a 0 category, even if the calculations come out higher.

 

[firelient]

What size transformers? Below a certain rating (I can't remember if its 75KVA, or 112 KVA), you might be able to give it a 0 category, even if the calculations come out higher.

125k VA 120v/240v transformers can be level 0