Page 7 - September 2021 Track N Times

OPERATIONS UPDATES

Short Cuts to Short Circuits-by Donald Dougherty, Senior Technician

There are occasions when a short circuit problem can be quite difficult to troubleshoot, especially when a single
breaker is used to protect the wiring of several devices and circuits.
Remember to use appropriate Lock-Out, Tag-Out procedures and PPE for High Voltage circuit trouble-
shooting and repairs!

First begin by referencing the schematic diagram to determine how many devices are connected to the tripped

breaker, and where the wiring for each of them go.
Fortunately, the schematics usually give a clear indica-
tion of where the wiring goes after the breaker. The
key is to look for a small number circling a connection
point close to the circuit breaker on the schematic.
This number represents a “TB” or terminal strip or
“block”. The legend for these terminal blocks is typical-
ly available at the back portion of the schematic and
are helpful in locating the wires you are looking for.

Schematic diagram with terminal block designator
numbers circle in red
Once you have found them, you will want to discon-
nect and isolate all these wires from each other on the
terminal strip. This method of troubleshooting a short
circuit is known as the “divide and conquer” method. It
allows you to find which wires have the short. Care
should be taken during this step making sure the wire
numbers do not fall off, especially if the wires do not
have ferrules on them

Using and Ohmmeter to find the short circuit:
Use an Ohmmeter to check between chassis ground
and each of those wires you removed from the TB.
Remember to use a good reference point for chassis ground. On most of our grind cars, power cars and control
cars this will be wire # 712. Support cars chassis grounds are usually wire # 1104. If in doubt, check the schematic
for the correct wire number for your machine. The use of a stable and reliable Chassis Ground is vital for successful
troubleshooting.
Typically, you will find at least one of these wires that has a low ohm reading to ground on your meter. Wires with a
reading below 5 Ohms should be traced down. Leave any wires with a reading below that disconnected.

At this point, you can reconnect the non-shorted wires (above 20 Ohms) to the terminal block and turn the tripped
breaker back on. If the breaker holds, use your voltmeter to check for the presence of power on the devices which

Page 5 SEPTEMBER 2021