Substation Fault Mechanical Effects - Ground Chain "Whip"? 1

[brendax]

Hi there,

I am a mechanical engineer currently working at a transmission substation and have been reviewing safety protocols. One safety rule has to do with ground chains connecting construction equipment to the station ground grid. We have been recommending that crews place sandbags or a tire or something else to hold down the ground chain in the event of a substation fault or other accidental energization. Evidently the ground chain can "whip" when a surge of current flows through it.

However, I cannot identify any physical mechanism that would cause this to happen. I am familiar with z-pinch on multi-conductor lines as well as transformer short-circuit effects, and of course mutual inductance between multiple conductors can cause mechanical effects, but what would cause a mechanical force on one single conductor?

Eg. a 1/2" diameter, 25ohm ground chain suddenly experiences 11,000 amps for 4 cycles.

Any guidance in explaining "why" for this safety rule (if any reasoning exists) would be appreciated. Thanks!

 

[davidbeach]

The magnetic forces want to straighten the conductor. Since it can't all be straight at once it whips around trying to be do what can't be done.

 

[itsmoked]

David; Why does it want to straighten? What's the physics?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[brendax]

Ditto to David; what physical process causes this to happen?

 

[stevenal]

No such thing as a single conductor. Charge is conserved, it must return to the source. The interaction here is between all current carrying conductors, possibly including the ground grid.

 

[davidbeach]

Considering the "single conductor", every bend places two portions of the conductor closer to each other than they would be if the conductor was straight. That bend tries to straighten, creating another bend elsewhere that then tries to straighten.

 

[itsmoked]

Considering the "single conductor", every bend places two portions of the conductor closer to each other than they would be if the conductor was straight. That bend tries to straighten, creating another bend elsewhere that then tries to straighten.

I think I see it. You've got all those magnetic force rings (right-hand-rule) jumping out perfectly perpendicular to the conductor. If the conductor is bent those force rings are forced to overlap on the inside of the bend with some repulsive force generated.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I think you have it Keith. That is my understanding. Now consider that straightening effect driven by the steep leading edge and high current of a lightning strike.


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

 

[itsmoked]

Yeah,I think I'll stand back now, waaay back.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[stevenal]

25 ohms sounds extremely excessive for a personal safety ground. Time for replacement?
Weighting extra cable to control movement does not seem like a very reliable method. In my experience an appropriate length is chosen for the task. See

http://www.usbr.gov/power/data/fist/fist5_1/vol5_1a.pdf

6.2.2.

 

[waross]

"1/2" diameter, 25ohm" This looks like a slipped decimal point, possibly more than one place.
I agree with stevenal. Use an appropriate length so that there are no loops or coils in the ground "chain".

Anecdote. Many years ago, when I was starting out, experienced field workers would "Safe Out" bus bars and cables by wrapping them with a piece of logging chain. Safety grounds are now almost universally made of copper but the name "Chains" persists.

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

 

[davidbeach]

Since this, I was discussing this with someone else who referred to it as the grounds "wanting to explore their environment" during a fault. Best to stay away.