Shielded Cable Sheath Grounding with Parallel Conductor

[naturalspry]

In high-voltage electrical substations, one of the requirements that I've heard while working is that shielded control cable should be installed with a parallel conductor the same size as the ground grid conductor in the event of switching surges / faults.

IEEE 80 has a short statement on this phenomenon in section 17.5:

"If the control cable sheath is grounded at widely separated points, large potential gradients in the ground grid during faults may cause excessive sheath currents to flow. One solution is to run a separate conductor in parallel with the control cable connected to the two sheath ground points. The induced current in the separate conductor will induce an opposing voltage on the control cable sheath, thereby minimizing the current in the sheath."

2 quick questions:

1) The language from the standard makes it appear as though the sheath is grounded at both ends, is this an accurate / acceptable practice?
2) How does the 'sacrifice' conductor (for lack of a better term) protect the cables in this case? I'm having trouble understanding this phenomenon in general, but more specifically from an EMF point of view.

Thanks,

naturalspry

 

[Wire-Smith]

In high-voltage electrical substations, one of the requirements that I've heard while working is that shielded control cable should be installed with a parallel conductor the same size as the ground grid conductor in the event of switching surges / faults.

IEEE 80 has a short statement on this phenomenon in section 17.5:

"If the control cable sheath is grounded at widely separated points, large potential gradients in the ground grid during faults may cause excessive sheath currents to flow. One solution is to run a separate conductor in parallel with the control cable connected to the two sheath ground points. The induced current in the separate conductor will induce an opposing voltage on the control cable sheath, thereby minimizing the current in the sheath."

2 quick questions:

1) The language from the standard makes it appear as though the sheath is grounded at both ends, is this an accurate / acceptable practice?
2) How does the 'sacrifice' conductor (for lack of a better term) protect the cables in this case? I'm having trouble understanding this phenomenon in general, but more specifically from an EMF point of view.

Thanks,

naturalspry

1) if the shield is thick enough, it is recommended to ground at both ends. (thick enough for potential transient currents (lightning in some instances))

2)it protects the shield from melting during a fault that would introduce high current on the shield. (depending on design, substations can likely see very high transient currents from faults, so even a thick shield alone is sometimes not thick enough)


IEEE 1100
-says best to ground both ends except sometimes with low frequency common mode dc signals. cable shields can carry unwanted ground current, use a blocking device except at one ground termination (silicon avalanche diodes or ac capacitor rather than rectifier diodes)
-long runs should have shield grounded at multiple points, or use SPD to ground everywhere but source
-electromagnetic shielding ground at both ends


IEEE1143
-grounding both ends allows a counter current that cancels out interference

-when grounding both ends consideration should be given to the shield thickness, need shield thick enough for voltage gradients from faults, 0.19mm aluminum, .13 copper, .15 steel, shield is usually thick enough

these are thicker than normal control cable foil shields