Earthing of supervisory screen

[flickstar]

Hey Folks,

I work for a distribution company and have recently noticed in our standards that the screens on our aerial supervisory cables are required to be earthed at each end. I have always thought that for screens preventing electromagnetic induction in cables, it was important that the screen only be earthed at one point to ensure that the entire screen is at the same potential. I would also have thought that by earthing the screen at both ends, you would be creating a low resistance path through which earth fault currents could flow.

Can somebody please indicate why our screens would be earthed at both ends? Are my thoughts - as outlined above - incorrect?

Your ideas would be greatly appreciated!

 

[dpc]

In an overhead circuit where your supervisory cables are in close proximity to high voltage circuits, there is a risk of the shield (screen)reaching a fairly high voltage if grounded at only one end. This may explain the requirement for grounding at both ends. This is also fairly common for shields on medium-voltage power cables.

For control and instrumentation circuits, the idea of grounding at only one end is to avoid circulating current which can be a source of common-mode noise in the signal wiring. But this depends on the type of signals.

Actually, if your supervisory cables are running between substations, or long distances, you really need to be concerned about ground potential rise (GPR)) during a substation ground faults and you may need to add some type of GPR protection, similar to that used on incoming telephone company circuits.

 

[busbar]

 
Although regional philosophies and practices are bound to vary, for shielded control cables in utility environments, “one end grounded only” is not universal. IEEE Std 525 …Cable Systems in Substations, §6.4.4 Metallic shielding of control cables states..

When shielded control cable is used, grounding the shield at both ends is recommended. … If only one end of the shield is grounded, large transient shield-to-ground and conductor-to-ground voltages may be present at the ungrounded end…

Grounding a shield at both ends allows shield current to flow. The shield current resulting from magnetic induction creates a counter-flux which will tend to cancel the flux that created the shield current. The net effect of the shield on the lead is to reduce the noise level.

For power frequencies, “both ends grounded” can reduce magnetically coupled noise to conductors within shielded cable.

 

[corrosivo]

Flickstar

By Grounding both ends of the shield you will have circulating currents in the signal wires, the reason is that the shield will be in parallel with the power ground that connects both equipments. When there is a flow of current in your shield you will have magnetic field right next to your signal wires, depending of the type of signal, Z of the shield and if the wires are not twisted this might affect the quality of the signal and longevity of the front end circuits.

DPC pointed a valid issue with common mode noise and GPR.
This might cause an increased failure rate of the communication equipment or cards. This will depend of the ruggedness of the front end circuits and the power supply as the excessive circulating currents under certain conditions can destroy your equipment. You will have to comply with the standard and at the same time break the ground loop you are creating. Hope this helps. Regards

 

[cuky2000]

GENERAL COMMENT
Shielded cable for overhead application is less common than underground installation. Several indicators suggest that shielding cable for OH installation improve system reliability and safety.

http://www.hendrix-wc.com/pdf/e_tip3.pdf

QUESTION: ………I have always thought that for screens preventing electromagnetic induction in cables, it was important that the screen only be earthed at one point to ensure that the entire screen is at the same potential.
COMMENT: “Shields bonded or grounded at only one point will have a voltage built up along the shield. The magnitude depends on several factors (mutual inductance to other cables, the current in all the conductors, distance to the grounded point, etc).

QUESTION: ………Can somebody please indicate why our screens would be earthed at both ends?
COMMENT: This voltage rise may cause discharge or create an unsafe condition for maintenance personnel. The usual safe potential in the US is about 25 volts for cables having nonmetallic covering over the shield. For short lengths or where special bonding arrangements are used, grounding at one point only may be satisfactory.

QUESTION:……I would also have thought that by earthing the screen at both ends, you would be creating a low resistance path through which earth fault currents could flow.
COMMENT: The insulation shield, including metallic component, should be designed to provide a defined path for the flow of charging current and fault current in the event of insulation failure Calculation should be performed to verify the withstand capability of the shielding in the event of a ground fault. The Insulated Cable Engineers Association (ICEA) Publication No. P-45-482 details the specific shield performance parameters required for various cable constructions.
More sophisticated installation suggest the use of surge arrester to protect shield during fault conditions and minimize losses during normal operation.

NOTE: See the enclose site for SC Shielding Current Formulas:

http://www.okonite.com/engineering/shielding-currents.html

http://www.kerite.com/catalog/catalogfiles/short_circuit_limitations.htm

 

[cuky2000]

My previous comments are applicable to current carrying power conductor application.

After I read carefully your post, I have some dough regarding the “screen” (shielding) application that you mentioned in your post.

Could you please clarify what you mean by “screens aerial supervisory cable”? Is this cable used for power or for communication, control, signal or SCADA application?

Thanks