In cross bonding, why is there a voltage gradient between grounded and bonded points of sheath

[engr5678]

In a cross bonded system, if the sheath of each minor sections are physically connected and earthed at the ends of a major section, why is there a increasing voltage gradient between the earthed point to the point where the sheath are bonded? A logical guess is because of the resistive nature of the cable sheath resulting in a voltage rise as it is further away from ground. Is this the main reason?

 

[7anoter4]

If it is about to Continuous Cross Bonding, then in this system the sheaths are cross bonded at the end of each minor section throughout the whole cable route. The three sheaths are bonded and grounded at the two ends of the route only.
In each minor shield section is a build-up voltage. The sum of all three built-up voltages per each main section is -theoretically-zero.
See IEEE 575 Fig.12

 

[DBL-E]

Are you referring to a measurement of the sheath voltages at the midpoint of the earth cross-bonding locations? It due to the induction from the current carrying phases, and each phase has its vector that you will measure a different induced sheath voltage from. as you get closer to the earth cross-bonding, you get voltages back to zero. Similar things happen for GIS, especially if there is electrically significant distance from one bond to the next.

 

[waross]

The voltage gradient is a fact of life.
Cross bonding is not a cause but a mitigation.
Proper cross bonding will limit the maximum voltage reached by the voltage gradients and will reduce or eliminate sheath currents caused by unrestrained sheath voltage gradients.
Sheath currents cause heating that results in a reduction of the maximum allowable current in the cables.
Do a search for "induced potentials in parallel conductors".

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Ohm's law
Not just a good idea;
It's the LAW!

 

[newelecteng]

In a cross-bonded system, the increasing voltage gradient from the earthed point to the bonding point is caused by the induced voltage from the AC current flowing in the cable conductors. While the earthed points hold the sheath at ground potential, the magnetic field induces voltages in the sheath along its length, creating a potential difference that increases as you move away from the earthed point toward the bonding point.