EMF Hotspots...

[xformer]

Working on a project in telecommunications building. Project uses PVC conduit for branch circuits and EGC protection. Engineer told us that we had to use plastic bolts and nuts with our metal conduit straps to prevent interference with the telecom circuits... Any truth to this?

[zbang]

Never heard of such a thing. Is there rebar in the concrete? What about metal equipment racks?

[dicklaxt]

Telecom Cables used in highly sensitive service are usually twisted pairs with individual shields and then an overall shield,shields should only be terminated at one end.Some component manufactures recommend different ways.

The method I have used most is to terminate the individual shield on the card and the overall shield land on the chassis ground and this is adequate for EMI/EMF.....

The metal nuts and bolts you speak of IMO is not something that needs to be considered.EMI/EMF is interference of one cable to another, ieower cable in close proximity to an instrument cable.,large metal structures also may give some trouble but shielding is usually adequate.

Shielded twisted individual pairs or triads are often run in rigid metal conduit with no ill effects if the shields are handled properly.I have Cat5 cable running all over my house with out any problems,tho I have attempted to keep the electrical cables separated and tried to cross only at 90 degree angles rather than run parallel.

I am interested to hear others and their thoughts on your question.
dick

[rcwilson]

Metal straps on strut with a metal bolt creates a conductive loop around the conductors. The loop is an inductor that adds impedance to the circuit. It is like those little doughnuts of magnetic material that are fastened on the power supply cords for our laptops, digital cameras and USB devices. The inductance helps mitigate surge currents, especially high frequency ones.

The telecom cables may be carrying some high frequency signals that are affected or attenuated by the inductive loop of the conduit straps. Using plastic bolts breaks the conductive loop.

The specifications for the main signal grounds in telephone Central Offices used to require the braided ground cable to be run in PVC with PVC bolts, etc to avoid the inductive loops.

Some lightning protection standards require the same thing because that steep-fronted lightning voltage surge acts like a high frequency signal. Any inductive loop adds some impedance to the ground path. A 1/4 ohm impedance at 20,000 amps lightning current results in a 5 kV voltage drop.

But in the grounding application only one conductor is going through the loop; the return conductor (the earth) is outside the loop. With the telephone pairs, both the conductors are inside the loop so I don't see there being that much of an effect since the net current is theoretically zero.

I wonder if the ground cable standard got misapplied to the telecom cables.