×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Substation Fault Mechanical Effects - Ground Chain "Whip"?

Substation Fault Mechanical Effects - Ground Chain "Whip"?

Substation Fault Mechanical Effects - Ground Chain "Whip"?

(OP)
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!

RE: Substation Fault Mechanical Effects - Ground Chain "Whip"?

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.

RE: Substation Fault Mechanical Effects - Ground Chain "Whip"?

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

Keith Cress
kcress - http://www.flaminsystems.com

RE: Substation Fault Mechanical Effects - Ground Chain "Whip"?

(OP)
Ditto to David; what physical process causes this to happen?

RE: Substation Fault Mechanical Effects - Ground Chain "Whip"?

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.

RE: Substation Fault Mechanical Effects - Ground Chain "Whip"?

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.

RE: Substation Fault Mechanical Effects - Ground Chain "Whip"?

Quote:

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

RE: Substation Fault Mechanical Effects - Ground Chain "Whip"?

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

RE: Substation Fault Mechanical Effects - Ground Chain "Whip"?

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

Keith Cress
kcress - http://www.flaminsystems.com

RE: Substation Fault Mechanical Effects - Ground Chain "Whip"?

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_1... 6.2.2.

RE: Substation Fault Mechanical Effects - Ground Chain "Whip"?

"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

RE: Substation Fault Mechanical Effects - Ground Chain "Whip"?

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources