[TowerTalk] 220v trench proximity to 7/8” feedline trench?

[jimlux]

On 8/7/20 11:59 AM, Ed Post wrote:
> Need some guidance as it relates to the proximity of a new 220 V power line (trenched and in PVC conduit) running to my crank up tower (3 HP motor) about a 500 foot run from my barn and 2 runs of 7/8” hardline (partially trenched through fields and not through the woods) all terminating at the tower base.
> 
> What is the minimum that the 220v trench should be separated from the 7/8” feedlines trench? they will run parallel for about 250’ then go their separate ways, 220V to barn, 7/8” to the shack.
> 


 From an electrical code standpoint two conduits could be run in the 
same trench. However, I'm not sure "the code" contemplates direct burial 
coax (i.e. is it like Class 2 wiring?), is Coax, with it's grounded 
outer layer and outer jacket essentially a de-facto conduit? Does it 
even matter - the code essentially requires that line power be separated 
by a physical barrier from "not line power" - the conduit for your 220V 
is that barrier.

 From a "RFI and practicality" standpoint - that's a lot more 
interesting. -  If the AC wires were carefully braided or twisted, so 
that the mean distance from each conductor to the coax shied were the 
same, then there would be no net coupling via capacitance or inductance. 
   However, they won't. So there *will be* some line frequency on your 
coax.
THe coax shield does a fine job for electrostatic shielding, but I don't 
know that it shields the magnetic fields.  And in any case, if you 
induce a current in the shield, and at some point, there is a "low 
impedance" (at 50/60 Hz) connection between shield and center, you could 
induce a voltage (or current) on the center conductor.

Is it a big voltage or current, or a small voltage or current? Depends a 
lot on the circuits.   You also have to worry about ground fault or 
leakage currents, unbalanced loads, etc.  There are dozens and dozens of 
pages written about how power lines can cause "odd stuff" to happen.

It is unlikely your RF will be coupled into the power line, because of 
course, you've invested in plenty of 31 mix cores and have fully choked 
the coax at appropriate places.   A lightning transient is something 
different.  But might be rare enough, or of small enough effect.

For myself, I'd not suffer analysis paralysis - I'd would assume that 
some line current/voltage will be induced and make sure it can be safely 
filtered, shunted, discharged or what have you. So you don't blow up 
your radio equipment or get shocked standing in wet grass with bare feet 
and handling the feedlines.  I wouldn't do anything special about 
transient suppression on the power lines other than what is typical in 
your area. Here in SoCal, land of no lightning, that would be "nothing" 
- if I lived in NM,Colorado, the plains, or eastern seaboard, where 
there's probably a thunderstorm as I write this (and if not this 
afternoon, then yesterday or tomorrow), I'd be a bit more cautious.