Path: utzoo!attcan!uunet!decwrl!wuarchive!mailrus!accuvax.nwu.edu!nucsrl!telecom-request
From: covert@covert.enet.dec.com (John R. Covert  25-Jul-1990 1345)
Newsgroups: comp.dcom.telecom
Subject: Re: What Rate Applies For Phone Used as an Intercom?
Message-ID: <10117@accuvax.nwu.edu>
Date: 25 Jul 90 17:47:00 GMT
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Organization: TELECOM Digest
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X-Telecom-Digest: Volume 10, Issue 516, Message 3 of 9


  From: Dale Neiburg
  Organization: National Public Radio 

In TELECOM Digest, vol 10, issue 509, Phil Earnhardt asks:
 
> As an aside, it was very sad to see the electric and phone lines
> being buried separately. Is there a good reason why they couldn't
> use the same trenches?
 
One reason is a problem that at this very moment has my wife turning
gray before her time.

She's a civil engineer, employed by the water authority in a Virginia
jurisdiction suburban to Washington, D.C.  Their main pumping station
has just added three new pumps (don't know the horsepower, but each is
driven by an electric motor about four stories tall).

Everything is just fine till one or more of the pumps are run at
variable speed.  When that happens, high order harmonics (we're
talking 30th-40th harmonics) are generated back onto the VEPCO power
line and crosstalk into C&P telephone cables buried in the same
trench, with such levels as to overpower phone signals (I assume
non-multiplexed analog, since we're still talking audio-frequency
harmonics).

Incidentally, the power service to the pump station has harmonic
filters on it.  Maybe.  All the consultant has been able to tell her
about what's inside the cabinet is that it's "either transformers or
capacitors", which in turn tells you something about consultants.  All
parties insist that their parts of the system are in spec--it's just
that now and then phone service gets screwed up for about 1/4 of the
county.

This probably wouldn't be a problem for a residential installation,
unless you have one helluva home workshop -- but is one good reason to
keep power and telco separated as far as possible.

Opinions expressed are my own.


Dale Neiburg (202)-822-2402  (Voice only)