[RFI] Interesting Case of VHF RFI caused by Chilled Water Controller

[David Robbins]

were the cabinets metal or plastic?  if plastic try simply wrapping it in aluminum foil to see if that cuts the radiation distance enough.  if so a metal enclosure or box over the current enclosure could be a more permanent solution.
 
if the cabinet is metal see if the door is grounded, maybe just a jumper to the door to ground it would be sufficient.  if it is grounded only by the hinges try stuffing the perimeter with aluminum foil, if that helps enough then some finger stock to ground it several places around the edge may be a more permanent solution.
 
could you tell if it was getting conducted out of the box on cables?  if so maybe some snap on ferrites outside the box would help.  snap on ferrites inside the cabinet on leads from the bad board may also reduce the radiation.


Dec 2, 2015 02:19:49 PM, utility.rfi.pro at gmail.com wrote:

I recently investigated an interference problem to Amateur Radio VHF
equipment installed at the county EOC. The interference was continuous and
strong across the spectrum from approximately 120 MHz to 150 MHz in the
form of a broadband white noise that didn't break the squelch but produced
a continuous midrange signal strength indication on the affected receivers.
The interference originates from the CPU module of a multi-module control
system for a group of chilled water pumps that are part of the building's
air conditioning system.

The users of the affected equipment requested an investigation by the
electric utility. A bad lightning arrester was found and replaced
approximately a half mile away. The repair did not clear the continuous
RFI. No other sources were heard out to 3/4 of a mile. It became clear the
source in this case was located at or in the building.

Another survey, conducted around the outside of the building, quickly
revealed that the source was located in a concrete wall-enclosed area
outside but at the immediate rear of the building. The "compound" contained
generators, chilled water pumps & controls and the 50 foot radio tower with
a dozen or so antennas including those used by the affected equipment.

Surveying inside the compound, the source was quickly found to be one of
several locked NEMA cabinets mounted on the chilled water pump racks.

Another survey was scheduled with all sorts of county and contractor
personnel. The cabinets were opened and, once again, the actual source was
quickly found. The RFI originates from the CPU module of the ALERTON (A
Division of TRANE) chilled water pump control system. The large cabinet
housed several modules mounted in 4 rows of 3 or 4 modules. Each module
served a specific purpose and all modules were wired together by a variety
of wires and cables.

This is a commercial installation. The RFI radiates from the source to a
distance of approximately 150 to 200 feet. The Alerton CPU module probably
meets FCC Class B requirements. The RFI would likely not be a problem
except for the fact that the radio tower is situated approximately 30 feet
from the NEMA cabinets. All of the antennas on the tower are within 30 to
80 feet of the RFI source.

The majority of the communications equipment at the EOC facility is 800
MHz. There is limited 150 MHz VHF and 450 UHF radio use. None of these are
affected. Only the Amateur Radio equipment is affected to a significant
degree. The county has few options for correcting the problem. The radio
tower can't be moved. The manufacturer of the control system will likely
not help especially since the contractor does not understand the problem
and the county doesn't seem overly anxious to spend much money to help
resolve the interference.

The county radio shop did offer to use existing resources to create a
remote control setup that will practically duplicate the function of at
least one affected VHF Amateur Radio.

To locate this source I used the Tecsun PL660SLV almost exclusively. My
search wasn't so much a direction finding effort as a signal
strength/proximity search. That is, I tuned the receiver to 133.325 MHz AM
that produced usable signal strength indications and useful audio. (When
close-ish to the source a distinctive noise pattern modulation could be
heard that sounded like data pulses.) As I got closer and closer to the
source, the signal strength increased and the clarity of the received audio
improved. Shortening the antenna acted as an adequate attenuator to keep
signal strength readings at midscale.

This case description is posted for its educational value. At the outset,
no assumptions were made as to what the source could be. No effort was
wasted listening to sound files to get some idea of what it could be.
Proper direction finding technique coupled with appropriate receiving
equipment lead the search to the source. Once the source was found, we knew
what it was.

Please direct any questions to the RFI list only so responses can be shared
with all readers.

73,

Frank N. Haas KB4T
Utility Interference Investigator
Florida
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