What is the min. striking distance between energized parts and covers?

[kh2]

This question came up during the rewind of a 13.8 kV machine 40 MVA, Hydro. The new bars were elongated by about 6 inches. As a result the air baffles (covers) became closer to the overhang portion of the bars which reduced the clearance between the live parts and the covers. The Contractor moved the covers away from the bars by the same 6 inches to keep a safe striking distance not less than 2.5" for this voltage.

Is there a formula, standard, or guideline to follow for deciding on the minimum striking distance at different voltages?

Appreciate your comments on the subject.

 

[PWR]

For live part clearance to ground, the NEC Article 490 value of 5 inches for nominal 13.8kV equipment is a reasonable value.

 

[edison123]

kh2

If you move the air baffles/guides away from the winding, will not the air flow path affected ? Don't you think the fan suction / delivery paths will be disturbed ? A better alternative could be to replace the metallic baffles with fiber glass molded ones.

* If a little knowledge is dangerous, then I am the safest person in the world *

 

[kh2]

Thank you PWR and edison 123 for your responses.

NEC Article 49 is addressing bare conductors. Our case is about a fully insulated conductor.

I was told that the striking distance for 13.8 kV insulated stator bars has to be 2.5" regarless of the material of the covers (metal or fiber glass).

The elogation of the bars and the adjustment of the clearance to the covers to keep it the same as before did not have much effect on the air flow. This assumption is based on practice (no calculations), since we did the same at another powerhouse and the air flow and temperature readings did not change as a result of this modification.

I am still interested to know the purpose behind requiring a certain space for the striking distance in spite of the full insulation of the conductors. If it is for the air flow it may be called a different name such as air flow clearance.

Thanks for the discussion.