elderly bus cables

[polartownjunkie]

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?

 

[waross]

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

 

[polartownjunkie]

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

 

[waross]

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

 

[polartownjunkie]

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

am i doing the math correctly for energy lost?

 

[itsmoked]

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

 

[Zogzog]

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.

 

[polartownjunkie]

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

 

[itsmoked]

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

 

[waross]

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

 

[itsmoked]

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

 

[waross]

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

 

[Zogzog]

"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.

 

[7anoter4]

Hi polartownjunkie
I think you need Water Cooling Cable.
See

http://www.watteredge.com/pl/order_wccd.htm

Best Regards