System One Disaster Story

A Wangrow "System One" (SYS1) DCC system consist of TWO boxes.  One is the command station and the other is a booster. 

A user followed the drawing for power hookup as shown in the manual which involved having the booster power supply also power the command station.  Unfortunately this created an "AC common" between the command station and the booster.

By default, the command station always ties the B- of it's power supply to the control bus ground pin.  It it supposed to since this is the ground wire to power all the cabs.  What is NOT known by most people is that the SYS-1 booster automatically create a very WEAK "Booster Common" using the same control bus ground wire.   The Control bus cable uses uses a small flat gray telco cable that has very small 26AWG stranded wires inside.  

The combination of the "Control Bus common" and "Transformer Common" wiring electrically put the two power "bridge rectifiers" (negative side) between the two boxes in parallel with each other.  

The system worked fine until one day the rectifier in the booster died for an unknown reason and failed in the "Open" condition.  All of a sudden the bridge rectifier for the command station was being used to power the booster!  That means it had to support track power.  The high current required by the booster to run the trains overloaded the control bus cable ground wire and turned the small control cable into a blackened melted smelly mess.  Furthermore,  and as a secondary consequence  this same high current "took out" the command station rectifier which was not designed to take that level of current.  In other words, his SYS1 booster's failure took out his SYS1 command station too!  From bad to worse!

None of this would have happened if SYS1:

1) had not use the control bus ground to make a booster common connection between the two boxes  
2) recommended using separate power supplies for each box.

Note: NCE Boosters do NOT use the control bus ground for a Booster Common.