Control Cable Tray distance to ISOPhase Bus

[TonyLL]

How close can (or should) a control cable tray cross perpendicular to a 13.8kV ISO-Phase Bus? What about a 20kV ISO-Phase bus? What about parallel to the ISO-Phase bus?
Any suggestions will be greatly appreciated;
Thanks,
Tony

 

[ScottyUK]

This probably isn't a direct answer, but we have experienced heating problems on scaffold erected within maybe 6' of a 16kA IPB structure. The bus voltage isn't the issue - the electric field is suppressed very effectively by the earthed outer shield of the IPB. The heating problem exists because of eddy current heating caused by the external magnetic field, which is itself caused by the current in the bus: a high current bus at low voltage will have a a greater external magnetic field than the same bus running at high voltage and low current.

In theory there is no external magnetic field in an IPB installation because when current flows in the centre busbar an image current of equal magnitude and opposite phase flows in the outer shield. On a long straight IPB the cancellation is almost perfect with very low leakage field, but cancellation is less effective at discontinuities such as elbows and tees. At the ends of the IPB structure are main interphase bonds which carry very heavy current, creating their own external magnetic field. They are typically formed from heavy box or channel section aluminium welded to the outer shield. Cancellation is usually poor at termination boxes on bushings because these are typically outside of the main IPB interphase bonding, and the electrical bonding is usually light between the termination boxes resulting in the conductive path for the image current having significant impedance. There should be no intermediate cross-connection between the shields of the IPB structure other than at these main bonds.

To minimise interference pickup, keep as far away from the IPB as you can, and do not cross near the ends of the IPB where the external field is strongest. Be very careful with grounding of cable armour and screens - you may form a shorted turn through the armour and the body of earth which will carry current if stray flux passes through the loop.

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