Developing Fault Scenarios

[ycliff]

I am currently developing a list of fault scenarios for an arc flash study with a system containing over 500 busses and am wondering what the standard for developing these scenarios is? ie: Do I consider a fault at each and every bus for each operating configuration (tie breakers closed, tied breakers open) or do I only consider certian busses? If so, which busses should I be consider in developing the fault scenarios?

 

[davidbeach]

You should be using a software package designed for the purpose. Such software can evaluate all busses for any configuration of the system you select. You need to look at configurations at both ends of the fault current range, sometimes a lower fault current provides a worse arc flash hazard than a higher fault current.

 

[ycliff]

"sometimes a lower fault current provides a worse arc flash hazard than a higher fault current" - Can you elaborate on this? In what instances would this occur?

 

[dpc]

Arc energy is a function of (among other things), fault current and fault duration. In some cases, a slight reduction in fault current can result in dramatically longer fault duration - such as when an overcurrent relay or circuit breaker transitions from an instantaneous trip element to an inverse time trip element. For a decrease of a few hundred amps of fault current, the clearing time may increase from 5 cycles to 5 seconds or longer. This results in much higher arc-flash energy for the lower fault current.

If you don't have a copy of IEEE-1584, you should get one and review the step-by-step procedure outlined in it. That will be a good starting point.

Then you will have to decide what scenarios you need to consider. But it's not an exact science.