"U" Pockets

[Boutch]

NFPA 780 para 4.9.4.1 says "Conductors shall maintain a horizontal or downward coursing free from "U" or "V" pockets" Does someone have an electrical explanation of the pocket effect?

 

[bphgravity]

The "pocket effect" is created when a down conductor from a strike termination device extends vertically down from the air terminal, deflects at a horizontal surface and then travels in a roughly vertical fashion again.

This can create a bend that results in less than a 90? angle which is not permitted and must be avoided. Also, the LPS idea is to take the lightning currents attached at a high structural location and direct it as vertically straight down to earth as possible. Only short horizontal runs should be used when necessary.

 

[Boutch]

What are the consequences and why the energy can't follow any shape in the conductor? Will the energy follow the "U" path or will it exit the conductor to make damages?

 

[stevearne]

Not electrical instructor topic

Not electrical instructor topic

This is not an Electrical Instruction topic, so I will move this thread into a more appropriate area - Please feel free to continue your discussion there.

 

[rcwilson]

What are the consequences and why the energy can't follow any shape in the conductor? Will the energy follow the "U" path or will it exit the conductor to make damages?

A "U" is half or 3/4 of a loop. A loop of conductor creates an inductor that has some impedance to current flow (inductive reactance). At 60 Hz, the impedance from a half loop is hardly anything, but inductive impedance is directly proportional to frequency and lightning's high and sudden current looks like a high frequency currrent even if it only lasts one cycle. The loop becomes a noticalble impedance at higher frequencies.

The "U" may only add 0.1 ohms impedance at 10 kHz, but lightning stroke currrent may be 15,000 or even 30,000 amps. Ohms law says that there will be 0.1 x 30,000 = 3 kV across that loop. That could be enough voltage to cause a flash over to buiding steel or a gutter that is not designed to carry current which could lead to a fire or other damage.

Also, that voltage rise across the "U" means the protected building will now see 3,000V more than it should.