RE: Power Cart Failure - What Caused It?

["Michael Graziano" <mgraziano@austin.rr.com>]

Do you think an additional posting to the OGR forum or this detailed email
to Scott's Odds and Ends might be of any additional help?

-----Original Message-----
From: ttat-members-owner@aoot.com [mailto:ttat-members-owner@aoot.com] On
Behalf Of Ira Schneider
Sent: Monday, January 24, 2011 12:43 AM
To: TTAT Members
Subject: Power Cart Failure - What Caused It?

Hi,

As some of you know, a couple of weeks ago we had a catastrophic failure
during our downtown run which burned out several components in the power
cart.  Some of the damaged components have been replaced and the others are
on order.  The layout is completely operational - we just can't read the
current flow on track 2.
Here is the scenario at the time of the failure:

1. Tracks 1, 2, and 4 had trains running on them.
2. Allan tried to run his passenger train on track 3 and kept tripping the
circuit
    breaker.  We decided to try the high power transformer.
2. We put the high power transformer on track 3.
3. Allan turned up the power.  As the voltage increased, the current flow
also
    increased.  When the current reached about 6 amps, the circuit breaker
    in the power monitor tripped.  We flipped the toggle switch in the power
    monitor to bypass the circuit breaker (relying on the 15 amp circuit
breaker
    with the transformer and the 15 amp circuit breaker in the TPC to shut
down
    the power in case of a derailment) and tried again.  We then saw the
same
    behavior - as Allan turned up the voltage, the current increased.  Allan
called
    my attention to the fact that the ammeter was reading 16 amps.
4. Out of the corner of my eye, I thought I saw the LEDs for tracks 1 and 2
flash white.
    We then smelled something burning.
5. The operators of the trains on tracks 1, 2, and 4 started reporting that
they had
    lost control of their trains.
6. We shut down the system and started to investigate.  The TVS-4 surge
protector
    was hot to the touch and the case appeared to be melted.  We
disconnected
    the TVS-4 and fashioned jumper cables to reconnect the TPCs to the track
feed wires.
7. The following week, I took the Power Monitor home and verified that the
red/green LEDs
    for tracks 1 and 2 were burned out.  I also determined that the ammeter
for track 2
    was not reading properly (i.e. it was reading 35 amps) and, whenever it
was plugged
    into the Power Monitor, tracks 1 and 2 appeared to be shorted together.
I replaced the
    burned out LEDs and left the track 2 ammeter unplugged pending acquiring
a replacement
    meter.
8. I contacted Scott's Odds-N-Ends about the TVS-4.  Their engineer sent me
the schematic
   diagram for the TVS-4, explained how it worked, and implied that the
ground terminal
   shouldn't be connected to earth ground.  I will get back to him for a
better explanation.

I attached two drawings to this note.  The first is the configuration of the
power cart at the time of the failure.  The second diagram is the same
configuration with the damaged components circled in red.  I also attached
the schematic for the TVS-4.

I would like anyone with any knowledge about electrical circuits, and
especially model train wiring, to look at the diagrams and come up with a
plausible theory as to what happened.  I am completely stumped.  My initial
thought was that the TVS-4 was overloaded by the high current on track 3 and
burned out.  However, my analysis of the TVS-4 shows that all four of the
transorbs (the components which are supposed to detect voltage spikes and
short them to ground) which were connected to the hot lines (one for each
track) are burned out.  The circuit board shows more scorching around the
transorb for track 1, less for track 2, and little scorching for tracks 3
and 4.
The transorbs connected to the neutral lines are undamaged.

If the transorb for track 3 burned out due to the high current flowing to
track 3, how did the other transorbs get burned out?  And, what could
possibly cause the LEDs for tracks 1 and 2, and the ammeter for track 2, to
fail?  Even if the TVS-4 caused a power surge when it failed, it couldn't
get a high voltage spike through the TPCs back to the LEDs and ammeters.
This just doesn't make any sense.

The only explanation I can come up with is that coincidentally with our use
of the new transformer there was a power surge in the building's power
system.  Perhaps the A/C system kicked on or off and caused a surge.  This
could have caused a high voltage spike at the imput to the transformers,
which could have also caused a corresponding voltage spike at the outputs,
which eventually got to the TVS-4 and burned it out.  This theory also has
some problems.  If there really was a high voltage spike, and a high current
flow, why didn't any of the circuit breakers trip?  There are circuit
breakers in each transformer, in the power monitor (except for track 3), and
in the TPCs.  Why did only the LEDs for tracks 1 and 2 fail?
(Perhaps because there are two transformers for each track?)  Why did a
35 amp AC ammeter on track 2 fail?  There is no way any of our transformers
could possibly supply 35 amps.

I am completely mystified.  If anyone can come up with a plausible theory as
to what happened, please share it with us.  If we can figure out what
happened, perhaps we can protect the layout from a similar occurance in the
future.

Thanks.





Ira