What is the impact of distance on induction in electrical cables?

[VAIVAW]

TL;DR Summary

I have a cable caring 660v 4800 amps below an 18g comm line designed for 24v connections. I am inducting on to the 18g line. How do I calculate voltage increase with the distance from the line?

So if my 18g cable is 18 inches above the 660v line what will my V be one the 18g line? compared to if it is 24 inches per say

 

[anorlunda]

Is the 18g cable a twisted pair, or a coaxial cable? Is there a metal conduit around the 660v line?

 

[VAIVAW]

No conduit around the 660. The 18g is 5 wire + a shield

 

[Tom.G]

Moving from 18 to 24 inch separation will probably reduce the coupling by 40%. That's just based on the inverse-square law, where doubling the distance decreases field strength by a factor of four.

Please realize that there may be other issues. For instance if the data interface is not differential signalling, a difference in the Ground voltages between Transmitter and Receiver may be part, or all, of the problem. That could occur even with differential signalling if the Ground references have a large difference.

Other possible solutions: (from simplest to Ouch!)

• Disconnect the shield at the transmitter end, leave connected at receiver.
• Use optical couplers at the receiver to keep common mode interference out of the receiver. (Common Mode is when the same interfering signal is on both wires of a differential signal circuit.)
• Use an optically isolated transmitter.
• Replace the data cable with an optical fiber link.

Or change to a radio link.

{edit}
This one is not very practical, try only if you can not go to optical fiber or radio link. Put a tuned filter in each of the data lines at the receiver, RLC filter tuned to the interfering frequency. If the power line is 3-phase you may also need filtering at the third harmonic, three times the line frequency.
{edit}

Cheers,
Tom

 

[tech99]

The shielding on the comms cable seems to afford zero protection against the magnetic coupling and the conductors sound not to be balanced, so no protection from that either. Also be careful about ground loops (as mentioned).