Arc flash incident energy

[adamscb]

Forum,

I've always noticed that medium / high voltage systems always have lower arc flash incident energy levels than 480V systems. Why is this? Is it due to the use of microprocessor-based relays, therefore the trip times are very low? Whereas with 480V systems, thermal magnetic breakers take longer to trip?

And by medium / high voltage systems I'm talking about 4160V and 13.8kV mostly.

 

[don_resqcapt19]

The incident energy is much more a function of current than of voltage...the low voltage systems typically have much more current.

 

[jim dungar]

Is it due to the use of microprocessor-based relays, therefore the trip times are very low? Whereas with 480V systems, thermal magnetic breakers take longer to trip?

Don is correct.

You are also correct in that faster relaying may come into play. However if the protective device is operating instantaneously, the rest of its characteristics don't really matter.
Another thing is, the high incident energy places for 480V equipment are usually found on the secondaries of step down transformers, because the primary protective device needs to be set very high to handle inrush currents.

 

[adamscb]

The incident energy is much more a function of current than of voltage...the low voltage systems typically have much more current.

I guess the main thing I’m struggling to understand though, is that MV/HV systems have higher fault currents, is that correct? Wouldn’t that lead to higher incident energy levels?

 

[topgone]

I guess the main thing I’m struggling to understand though, is that MV/HV systems have higher fault currents, is that correct? Wouldn’t that lead to higher incident energy levels?

The current is computed as power divided by 1.732 and the system voltage. With lower voltage, the computed current will be much higher than if the voltage was MV or HV.

 

[don_resqcapt19]

I guess the main thing I’m struggling to understand though, is that MV/HV systems have higher fault currents, is that correct? Wouldn’t that lead to higher incident energy levels?

In the typical industrial application the MV fault currents are much lower than the 480 volt fault currents. In the case of 5kV and 480 volt systems, the 5kV fault current is about 1/10 of the 480 fault current.

 

[wbdvt]

The fault currents are generally lower in MV systems than in LV systems. Some other factors are the electrode spacing is generally larger and working distances are greater in MV systems than LV systems.