Cables separation

[thinker]

We have a situation (the site is in China) when we need to run a steel conduit with AC dynamometer signal wiring (Speed and Torque sensors) in the trench cross-section 600x600mm, and this trench has bottom installed power cable 10 kV (feeding other equipment). We definetely will try to separate these runs at a maximum, but are we looking for trouble? May be 10 kV cable should be additionally encased into grounded overall devider/case? Did anybody have a similar problem?

 

[electricpete]

It sounds like a good question for the power forum

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Eng-tips forums: The best place on the web for engineering discussions.

 

[sreid]

Any chance you could run the data signals digitally through fiber optics?

 

[aolalde]

Some of the “good practice” General Provisions applying to all Wiring Systems are: Conductors of more than 600 Volts should not occupy the same enclosure as conductors carrying less than 600 Volts. Communication circuits should not occupy the same enclosure with light and power wiring.

However, the USA NEC opens that possibility on "Art. 300-32 Conductors of Different Systems. Conductors of high-voltage and low-voltage systems shall not occupy the same wiring enclosure or pull and junction boxes.
Exception No 1: In motors, switchgear and control assemblies and similar equipment.
Exception No. 2: In manholes, if extra-low or low-voltage conductors are separated from high-voltage conductors. "

 

[RRaghunath]

If you are not running the instrument cables in the same tray as the HT power cables (for that matter any power cables), you are meeting the codal requirements.

In case the HT power cable is armoured and the armour is earthed at both ends, the arrangement provides a good electro-magnetic shield and avoids possibility of any induction in the instrument / or any other cables nearby.

 

[dbaird]

Northern Telecom IBDN User Manual contains an Appendix D titled
_UTP Separation Guidelines From EMI Sources_. The values are the
same as the cabling pathways standard, EIA-569, table 4.8-5.

Minimum Separation Distance
from Power Source at 480V or less
CONDITION <2kVA 2-5kVA >5kVA
Unshielded power lines or
electrical equipment in proximity
to open or non-metal pathways 5 in. 12 in. 24 in.
(12.7 cm) (30.5 cm) (61 cm)
Unshielded power lines or
electrical equipment in proximity
to grounded metal conduit pathway 2.5 in. 6 in. 12 in.
(6.4 cm) (15.2 cm) (30.5 cm)
Power lines enclosed in a grounded
metal conduit (or equivalent
shielding) in proximity
to grounded metal conduit pathway - 6 in. 12 in.
- (15.2 cm) (30.5 cm)

Transformers & electric motors <------- 40-in (1.02 m) ----->

Fluorescent lighting <------- 12-in (30.5 cm) ---->

Source: Integrated Building Distribution Network (IBDN) User Manual
- Northern Telecom, doc # IBDN-UM-9105, 1991.



David Baird
mrbaird@hotmail.com

Sr Controls Engineer

EET degree.

Journeyman Electrician.

 

[thinker]

dbaird,
Thanks a lot for excellent reference. So far I was able to locate a New Zealand cable separation standard which requires separation between 11 kV cables and communications conduits running in shared trench to be greater than 450mm. Thanks to everybody for responses.

 

[buzzp]

Is HV cable shielded? If so, potential problems are reduced significantly.

 

[eglaude]

Thinker,
You do not mention if the 10KV feeder is in any kind of a raceway. If this area is prone to excavation work in the future, and since as you are running signal wires today there are bound to be other trenches run in the future, you should consider additional encasement of the HV feeder just for safety concerns. I have worked in refineries all my career and we never run underground duct nor conduits without red-dye encasement for.