Arc Flash - Transformer Neutral Reactor

[Dumbo2929]

I have a strange question for those familiar with IEEE 1584 and NFPA 70E. A big boss at a big A&E company recently made the following statement. He said that he recommends neutral reactors on service entrance transformers (480Y/277V) because it will limit the arc flash exposure.

It is true that the netral reactor will limit line-to-ground fault current, but how does this lower the arc flash hazard, when the three-phase bolted fault produces the worstcase fault current and highest incident energy.

Does anyone have any thoughts on this? I rarely see neutral reactors on commercial service entrance transformers, and if this statement was true, I would suspect to see a lot more.

 

[unclebob]

It's not a good idea because if you put a neutral reactor or resistor on the transformer, you will be unable to use 277 V loads, only mono or triphased 480V. The code prohibits it.

 

[davidbeach]

The NEC only allows high impedance grounding under certain, very specific, conditions and it is not applicable for most locations. As unclebob states, it would also eliminate the availability of 277V single phase. But the bigger problem for arc flash is that the calculations for PPE are all based on the 3-phase fault current; reducing the single phase to ground current will not provide any benefit on the hazard calculations.

 

[cuky2000]

Reviewing the application of neutral reactors, here are a summary of main advantages and disadvantages:

Advantages: Ground-fault currents touch, and step voltages are reduced; overvoltages are smaller compared to ungrounded systems; and intermittent arc voltages are avoided.

Disadvantages: Use of neutral reactance increases neutral voltages during ground faults and requires higher insulation of the transformer neutral and high level of ground-fault current is required (~25 to 60% of 3 phase-fault) to minimize the risk of high transient overvoltage.

I personally do not see the advantage of specifying neutral reactance to lower the risk of arc flash hazard.

 

[Dumbo2929]

Thank you. I wanted to confirm that this was a silly concept to reduce arc flash hazard.

 

[davidbeach]

Dumbo2929, I wouldn't go that far. The reduction in ground fault current will greatly reduce the likely onset of an arcing fault. Most arc faults in gear start as phase to ground and the plasma allow a rapid flash-over to the phase to phase and three phase. Since the 3 phase fault current is not reduced, the arc flash hazard level remains unchanged; but the possible conditions leading to an arc flash incident are significantly reduced. Same PPE, much lower chance that it will ever need to prove its worth.

 

[tinfoil]

Although it is the worst-case, and must be allowed for in a design, the actual chance of a three-phase fault is quite low relative to the much more common SLG fault, so the manager may be correct, from a probability/risk analysis point of view.

Having said that, any reactor large enough to make a difference is going to screw up the use of single phase loads. But this might make sense if all of the loads are phase/phase connected, or are balanced 3-phase.

 

[Dumbo2929]

I may have mispoke, the silly concept is that a neutral reactor will lower the Hazard Risk Category. I understand that we do these calcs based on 3-phase bolted faults, which are unlikely and reducing the magnitude of line-to-ground faults is always a good thing for safety.

 

[KJvR]

Another tought that might not be valid for this discussion. Arcing is best under high X/R ratios (inductive sources). An arc is extinguished during the zero crossing of the current. Using a reactor will increase the X/R ratio (reduce power factor) and the voltage at the fault would be maximum during the current zero crossing. Using an Neutral Earth Resistor will force a small X/R ratio and it will be more difficult to contain an arc because the voltage (at the fault) is almost zero during the current zero crossing. (Same principles as shunt reactor / capacitor switching)