Raco Verbatim, Owner'S Manual

Page: 107 / 246

Analog Signal Input
Verbatim Owner's Manual B-13
However, it is possible and even likely, that an observed AC signal is merely a
"softly" induced hum that holds no sway when it meets any clamping
introduced by our analog input. With this in mind, it may be best to defer even
taking AC reading until after the DC oriented methods have proven
unsuccessful.
With the main focus being the DC voltage readings, we are looking at some
point on the loop that is much less than eight volts DC away from ground, and
that is where the Verbatim input should be relocated in the loop. Chances are
good that the Verbatim had previously been placed at a point on the loop well
away from ground potential and that the relocation will end the problem.
An added step that may be useful in addition to the two sets of voltage readings
(AC and DC), especially if the voltage readings seem to be erratic, is to have the
customer use a jumper wire to temporarily connect some candidate point in the
loop to electrical ground, and observe whether the loop is disturbed by this
temporary grounding. If it is not, that is a good place to locate our input in the
loop. In fact, this approach can be used without taking voltage readings at all.
But if it does not work, then we do want the voltage readings in order to best
understand what is going on.
Occasionally, something in the loop will cause there to be no available point in
the loop that is close to ground potential. In such cases, if this cannot be
changed, then the customer will need to install an optical isolator between the
loop and our inputs. The customer may be referred to: Action Instruments,
San Diego, CA, (619) 279-5726. Isolators cost $300 per loop.