Negative Bus Auto-Grounding System
Function
The function of the Negative Bus Auto-Grounding System is to ground the negative bus in the rectifier
substation whenever a voltage potential develops between the rectifier negative bus (running rail) and
ground. This grounding of the negative bus (running rail) to station ground eliminates the voltage between the two, thus eliminating a potential personnel safety hazard in the traction power system.
The “Negative Bus Auto-Grounding System” is comprised of four basic elements: DC Contactor (PN:
1916) [GS], Switch Control Board (PN: 0304) [57NR], Voltage Sensor (PN: 0305) [VOSE], and Current Sensor (PN: 0306) [CUSE]. Each of these system elements has it’s own specific function and is
described separately in later pages. Other lesser components are Hall Effect Transducer (PN: 0176)
[HS], Time Delay Relay (PN: 6278) [TDO], Holding Relay (PN: 0372) [57LOX], and Reset Pushbutton
(PN: 3265) [PB].
System Description of Operation
With 125VDC control power available to all system elements and with no voltage potential sensed, the
Auto-Grounding System is in a passive state but is continuously monitoring voltage potential between
rectifier negative bus and equipment (or substation) ground. When a voltage potential is sensed between rectifier negative bus and equipment (or substation) ground that is greater than the voltage setpoint, then the voltage sensor VOSE contacts close. One voltage sensor contact is wired to the Switch
Control Board 57NR; another is wired to time delay relay, TDO. Once voltage sensor, VOSE contacts
close, the coil of TDO energizes and, in turn, the instantaneous contact of TDO closes to energize the
Grounding DC Contactor GS and also an amber colored timing indicating light (when provided). Another contact from voltage sensor, VOSE is inputted to the Switch Control Board, 57NR where it’s used
in the control board logic.
Once DC Contactor GS closes, current sensor CUSE looks for current flow between negative bus and
equipment (or substation) ground because the voltage potential between negative bus and equipment
(or substation) ground becomes zero. If a current exists that is greater than the current sensor setpoint then the current sensor will output a contact closure keeping the time delay relay TDO energized
thereby keeping DC Contactor SG closed. At the same time the counter circuit on the Switch Control
Board will make one count and retain this count. The count will be indicated by an amber colored light
on the Switch Control Board 57NR.
If current is flowing between the negative bus and ground but is below the setpoint of sensor CUSE,
then time delay relay TDO will time out and open DC contactor GS. If after opening of DC contactor
GS the voltage between negative bus and equipment (or substation) ground returns to level exceeding
the voltage sensor VOSE setpoint, the cycle repeats itself as described above.
Negative Bus Auto-Grounding System
After the fourth (4th) closure cycle of the DC contactor GS, the Switch Control Board 57NR sends a
contact closure to holding relay 57LOX and a contact closure from 57LOX acts as a seal-in to hold the
DC contactor GS closed. At this point, the DC contactor GS will keep the negative bus firmly grounded
through its connection to equipment (or substation) ground. Once the Auto-Grounding System seals
and holds the DC contactor GS in the Negative Drain Cabinet, the system must be manually reset by
depressing the Reset Pushbutton PB on the door of the cabinet. After the Reset Pushbutton PB is
depressed, the system returns to monitoring the voltage between negative bus and ground. If unsafe
voltage is present, the system functions as described above to mitigate the hazard.
References
See TAB “Negative Drain Cabinet” in Volume I of the Operation and Maintenance Manual for each
traction power substation.
See Drawings relating to the Negative Drain Cabinet for each traction power substation.
Denise
TPS (#7)
Glenbury
TPS (#8)
Washington
Junction
TSP (#9)
South Park
TPS (#10)
Drawing Title
00144S2C
00144S0C
00144S4C
00144S6C
One line diagram
00144M2T
00144M0T
00144M4T
00144M6T
Negative return, drainage & rail grounding
cabinet elevation & section views
00144S2P
00144S0P
00144S4P
00144S6P
Negative return, drainage & rail grounding
control schematic
00144W2M
0011W0M
00144W4M
00144W6M
Negative equalizing bus enclosure & drainage
panel wiring diagram