Article 470

[bkludecke]

What does NEC article 470 refer to? What are these things and what do they do?

 

[marti smith]

Bob-on-the-left-coast: I have never installed any of these, nor am I familiar with their use. I think it's a great question. But here is a tidbit I found (and you'd probably find oodles if you googled):

If the motor starting operation results in a voltage sag that causes tripping of equipment within the facility or at other customer facilities, you can use one of the following methods to reduce the voltage sag.

1. Use resistance and reactance starters. These initially insert an impedance in series with the motor. After a time delay, the starter bypasses this impedance. Starting resistors may be bypassed in several steps while starting reactors are bypassed in a single step. This approach requires the motor be able to develop sufficient torque with the added impedance.

Hopefully someone here that is familiar with these will add in.

 

[antenna2001]

Check 250.36. The high impedance that they are talking about there are those resistors and reactors. Usually, the center of a Y-connected transformer is grounded through a resistance, a reactance, or a combination of both. That prevents high fault currents from flowing.

If you ground the transformer without impedance (the most common type of grounding) then very high currents flow under fault conditions.

Each system has advantages and disadvantages.