High clearing time causing high IE

[deltawhy]

Hello,
I am doing an analysis on a system with an 800A 600V bus. The arc-fault current is calculated to be lower than my instantaneous setting on my circuit breaker, but I cannot lower the inst. setting as it will overlap the transformer inrush current. The arc flash energy is thus really high as the clearing time is not optimal. Are there reliable mitigation techniques (keep in mind this project is on a tight budget) so as to keep the incident energy at a reasonable level?

Thanks

 

[ScottyUK]

Is the inrush for an upstream transformer or downstream? It's not apparent from your post.

If you can't adjust the settings downward then you could look at some form of unit protection to get rapid fault clearance in the protected zone. How tight is 'tight'? Are you worried about dollars or hundreds of dollars?

 

[deltawhy]

It is a 750kVA Tx feeding the 600V 800A switchboard. I know of the optical options for mitigation, but that would require solid state tripping for my breaker and additional equipment.

 

[LiteYear]

'scuse my ignorance, but how do you expect to supply in-rush current to a transformer if the current available to an arc fault is lower? Or, as ScottyUK muses, is the transformer upstream, and therefore the CB should be downstream of it and set to satisfy the arc-fault limit instead?

 

[deltawhy]

The circuit breaker is in the switchgear enclosure. The available fault current is much higher at the secondary terminals of the transformer and would have no problem tripping very quickly, but the secondary feeders from my transformer into the building is a couple hundred feet. This greatly decreases the fault current. Therein lies the problem, unless I am overlooking something.

 

[Zogzog]

What type of breaker is it?

 

[deltawhy]

Siemens, 800A frame thermal mag with adjustable inst. trip.

 

[Zogzog]

Ouch, that makes is hard. Some options:

1. Replace breaker with modern version that has arc flash switch
2. Use "maintenence setting", lower INST setting if work is being done downstream.
3. Use a remote operator