×
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

elderly bus cables

elderly bus cables

elderly bus cables

(OP)
i got an 18000 amp per leg ac electric arc furnace with bus cables over 3 years old with tens of thousands hours running time, our bus cable provider suggest we swap them out every 9 months or at 20% increase in resistance, which got me wondering, 20% increase of a cable newly rated at 0.033milliohms could cause some power problems, right?

say, 20% for each of the three legs at 18000 amps

p = (18000^2)*(.000033/.2) = 53460W

ptot = 53460 * 3 = 160.3kW

is that very bad figuring or very bad cables?

RE: elderly bus cables

Can you tell us what is causing the rapid increase in resistance?
Swapping 18,000 amp cables every 9 months is a powerful financial incentive to look for an alternate solution.
Thanks

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

RE: elderly bus cables

(OP)
not necessarily swapping them, rather rotating them and examining the cables resistance, which increases, i am told, as the 8000 MCM cable heats and disintegrates over time, i have a photo of our most recently failed cable, which after 3 years of duty, finally melted in half

RE: elderly bus cables

If you click on "Check out the FAQ
area for this forum!" and scroll down, you will find instructions on posting pictures, thanks to itsmoked.
Is the heat high ambient temperatures and radiated heat or because of high current densities?

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

RE: elderly bus cables

(OP)
all three, not to mention constantly being moved up and down as the electrodes adjust

am i doing the math correctly for energy lost?

RE: elderly bus cables

Math I don't think so.

P = I2 x R

P = 18,0002 x (0.000033 + (0.000033 x 0.20))

P = 12.8kW

Unless I screwed it up..

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: elderly bus cables

I would say do the math, find an acceptable resistance (Based on losses) and test the cables monthly, that way you can replace them when they exceed your limit you have set. Monthly testing will allow you to predict when you will need to replace them. Based on the results you get you may need to test them more or (More likely) less often.

When the cable provided says replace them every 9 months, thats a red flag to me, I am guessing this size cable is not cheap.

RE: elderly bus cables

(OP)
alright alright,
thanks guys, yeh its about 12k for a decent new one, i now see where i went wrong with my math, thanks

i am going to measure the resistence this weekend and will report my finding because theres the chance for some intresting numbers pouring out

RE: elderly bus cables

Just how do you measure for 33microOhms over a distance on a cable like that.   Just contact resistance of the probes is going to be far higher than that.  Then you have the probe leads.

Put 50A DC thru the cable and measure the voltage drop?

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: elderly bus cables

I get a voltage drop under full load of 0.594 volts drop new, and  0.713 volts drop after a 20% increase in resistance.
Is the current stable enough to use the voltage drop under load to determine the resistance?

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

RE: elderly bus cables

So Bill..

Can I watch you skinny down that,(RE1), cable into the vat of boiling steel, and hold the meter probe on the connector?



RE1:  Regularly Exploding (cable)

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: elderly bus cables

I was going to use the 30 foot long telescoping hot stick to hang a probe on the end of the bus cable. I hope that there is a bare spot on the cable near the connector where I can hang the probe and avoid possible joint resistance issues.
Actually, I may try to get my last helper to do it when he wakes up.

The boy was not happy to see this posted on the office wall the next morning. I didn't particularly care that he was not happy.

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

RE: elderly bus cables

"Just how do you measure for 33microOhms over a distance on a cable like that.   Just contact resistance of the probes is going to be far higher than that.  Then you have the probe leads."

Wheatstone bridge, thats exactly the type of application it is for.

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