Requirement for spacing between bus bars in 600V switchgear

[rbyrne15]

Could anyone steer me in the direction of the minimum distance required by code (N. America) between copper busbars in 600V switchgear?

Also, is the requirement for aluminium bus dufferent from that of copper bus? (Assuming Al is used for such an application).

The Standard Handbook for Electrical Engineers gives only the distances for High and Extra High Voltage equipment, and my Google searches yield nothing relevant.

Thanks.

Richard Byrne.

 

[waross]

NEC Clearances, Live Parts, Auxilliary Gutters
Don't forget the mechanical bracing to withstand the magnetic forces under the maximum asynchronous fault current.

 

[cuky2000]

Inside the switchgear, the NEC does not have any directly requirement in term of clearance for the busbars. Somehow, the busbar dielectric clearance is satisfied by performance test requirements by standards such ANSI or UL.

Noted that for short air clearance, the withstand voltage is around 300kV/m (7.2kV/in). For 600V class the switchgear is often tested to witstand a power frequency voltage of at least 2.2kV = (2x600V+1000V).

Most bare busbar configuration in air inside metalclad switchgear complies with this requirement with sufficient safety margin with approximately 1-inch clearance phase to phase or phase to ground.

For insulator covering or supporting the bus such as bushings clearance in term of creepage distance is typically around 2 inches.

 

[rbyrne15]

Thank you both for your answers.

I was once told that 1" was required between adjacent phase bus bars, and given that I learned that the dielectric strength of air is approx. 3kV/mm (7.62kV/in), 1" is more than enough. But I was just curious about what the code requirement was.

Thanks again,

Richard Byrne.

 

[tulum]

In Canada...

1" p-p is required under CSA. Most manufacturers also use 1" p-g to cover faults on impedance grounded systems.

Regards,
TULUM

 

[alehman]

Per UL 891, 1 inch through air, 2 inches on insulating surface. Phase-to-phase and phase-to-ground are the same.

 

[jraef]

If you are building something from scratch, NEC rules prevail. If you are speaking of something built in a factory and listed by a NRTL, Cuky2000 is correct. There is no specific standard for air spacing as long as it passes testing. There are guidelines however in ANSI and IEEE, based on what is known to not pass.

As backup of alehman's response, read #59 of these Powell Technical Briefs,

http://www.powl.com/main/Uploadpdf/ptb_1_91.pdf

But alehman, does it actually say that in UL-891 or are you going from memory? I no longer have a copy of that standard but I used to and I don't recall having seen that.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[alehman]

It's defined in UL 891, Table 24.1
[tt]
btwn live parts of to ground
opposite polarity
through air over surface
0-125V 0.5 0.75 0.5
125-250V 0.75 1.25 0.5
250-600V 1.0 2.0 1.0
[/tt]