Neutral Cut Out, Necessary Or Not?

[jjustice]

On a 3-phase 4-wire system, is it necessary to run a disconnect on the neutral line when feeding a standard distribution bank of pole-mounted transformers? I can see how this could be a safety precaution, in the event of an adjacent phase-neutral fault while the bank in question is being serviced. Are there any other issues that could arise from not running one?

 

[waross]

It is common practice to solidly connect the neutral with possibly one exception.
There are some legacy wye-delta banks in some areas. These banks normally do not have the primary wye point connected to the system neutral. If the primary wye point is connected to the system neutral then any voltage unbalances or phase angle errors will result in disproportionately high circulating currents.
However, wye delta banks with a floating primary neutral may develop high transient voltages during energization. As a way to avoid subjecting customers to switching transients, some utilities have added a fourth fused cutout in the neutral conductor. When a bank is to be energized by closing the fused cutouts on the pole, a fuse is installed in the neutral cutout and the cutout is closed. Then the three phases are closed. Finally the neutral fused cutout is opened and removed. The fourth fuseholder is stored in the service truck and only used when a bank is to be manually energized in the field.
The wye/delta transformer bank may not be a good solution for distribution service.

Bill
--------------------
"Why not the best?"
Jimmy Carter