hot on the trail with an electronic bloodhound at my service.
Sure enough, a quick check at the terminal strip where we made
our final disconnection during the club meeting showed that the
current was flowing in the direction of the feeder that we had
disconnected. Clamping around the DCC bus wire past that point
showed zero current flowing in that direction.
The elegance of this method is that it does not require any
wires to be cut or disconnected, and each test takes only a
few seconds.
Second success
The next test of this
system came when we
tackled an on-again,
off-again short that
had plagued opera-
tions on Jack Parker's
P&W Railroad
, especially during
open houses. Using the
clamp-on ammeter, we
found a track feeder that
disappeared behind an
under-layout fluorescent
light fixture used to illu-
minate the lower level
town of Misty.
Further investigation
indicated that at some
time in the days of DC
and power routing, this
DCC Short detection - 3
4: Current limited by lamps
being measured at booster.
4
5: We see that the current in the main bus is zero past
this point. This means we have passed the branch
circuit with the short.
5
feeder was apparently added to the very end of an industrial sid-
ing rail, far from the other feeders in that section of the layout. It
was hidden from view beneath by the lamp, and virtually invisible
from above. (see the yellow arrow in fig 3.) Amazingly, it took us
less time to locate this longtime gremlin with the clamp-on cur-
rent meter than it has taken me to write about it.
The clamp-on ammeter I purchased also includes volt and ohm
functions, but the ranges are very limited, and of little use for
testing DCC. I doubt that the actual amp reading is accurate due
to the differences between the normal AC waveform and that
of DCC. However, for our troubleshooting purposes, precise val-
ues are unimportant. We just need to see which way that ornery
short-circuit current goes at each junction between wires.
MRH-Nov 2013
