Neutral Bus Clearances

[stretched2thin]

I apologize in advance for my ignorance on this topic....

If a neutral bus is solidly grounded, yet the neutral bushing connected to that bus (on the transformer) has a BIL rating of 150 kV, do you still have to worry about neutral-to-ground clearances between the neutral bus and grounded steel on equipment (such as transformer tanks)?

It is my belief that you still have to maintain the 12 inch clearance required for 150 kV BIL. I believe this is the case because the "solidy grounded" connection of the neutral bus won't appear to be solidly grouded to high frequency surges and might cause unintended arc-overs. I think it would be a mistake to reduce that clearance. Am I wrong?

 

[alehman]

Neutral conductors on medium voltage systems are commonly provided with 600V class insulation. If the neutral is bonded to ground at the switchgear, I would think this would be adequate.

 

[mc5w]

A lot of times the neutral on a substation transformer secondary goes through a current transformer before being bonded to ground. The neutral bushing only has to takes the voltage drop that occurs during ground faults and LIGHTNING.

Most of these neutral bushings are not equipped with a lightning arrestor because the typical installation is solidly grounded.

For ungrounded and resistance grounded secondaries I definitely would connect a lightning arrestor to the neutral bushing.