Current Transformer Wiring

[Lightninarrester]

Guys,

These were specifications by customer and they apply to AC and DC.
1. Use shielded cables for CT signals and breaker control signals. Why Shielded?
2. Use 90 degrees rated cable for 600V control cables. Why not 75 degrees for relay and breaker control signals? Isn't the NEC require the 75 degrees rating, I know that should depend on the equipment termination points ratings (Lugs, terminals etc.)
Thanks

[ August 24, 2005, 06:16 PM: Message edited by: Lightninarrester ]

 

[jim dungar]

Re: Current Transformer Wiring

Shielded cable may help reduce erroneous signals on the circuits (keeps them from acting like an antenna). Actually twisting is probably more effective than shielding, but hey I'm not the spec writer.

It is very hard to find 75C only conductors except for NMB and 300V PVC shielded cables. Almost every standard wire is rated THHN, TFFN, MTW, AWM, and SIS are all 90C insulations.

The NEC is a minimum set of requirements, 90.1(B). The NEC is not a design manual, 90.1(C).

 

[steve66]

Re: Current Transformer Wiring

The NEC may require the conductor ampacity to be calculated based on 75 deg terminations, but that doesn't prevent the actual cable from being rated at 90 deg.

Steve

 

[rcwilson]

Re: Current Transformer Wiring

Some substation designers and operators require special shielded cables to prevent interference or damage to control and metering circuits in HV & EHV substations during a fault. The high fault currents and electric fields in a substation can induce noise or misoperation on a long control circuit.

Usually the shield is an overall copper tape or corrugated tape shield that is heavier duty than the usual Mylar foil shield used on instrumentation circuits.

[ August 25, 2005, 05:10 PM: Message edited by: rcwilson ]