11 KV Cable Trench

You may have to wait until I can get back to my NEC code book on Tuesday (and some free time at work ) to allow me to cite the appropriate code section. But my memory says that you can install monitor and control cables (M&C) in the same tray as power distribution cables if the insulation of the M&C meet a certain criteria. You will certainly have to contend with more interference from the power cables into your M&C lines. This interference can be solved by using shielded twisted pair wires along with the appropriate send and receive electronics. If this 11 KV are high voltage instrumentation biasing voltages, then there is absolutely no problem putting M&C cabling in the same tray. You may have to partition the cable tray (making one tray into two) with a grounded metallic barrier if the possible current through the 11KV exceeds a certain value. This will assure a shorting failure will trip the protection device (ie. fuse, circuit breaker).

Sir:

Thanks for your reply.

The 11 KV I'm referring to, is the main power supply to the facility. There will be two separate cable trays, one for power and the other one for control and monitoring.

My main concern is the safety of the unauthorized low voltage personnel working in the cable trench underneath the 11 KV switchgear.

Thanks and best regards,

Rudy C. Fernando

Unless they're going to be doing some cutting or burning of any large material that might get through the insulation of the 11KV cables, I do not see a problem working under the cable tray. Certainly the integrity of the insulation sheath of the 11KV cables should first be visually inspected. A break in that insulation will require a reassessment before continuing.

With this being your input power line feed. It dawns on me now that regardless of what the NEC standard permits, you probably should contact your utility provider to inform them of the work you intend to do. Particularly since the tray and cables are likely to technically be their property.