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41-973.5R
the case of three-terminal lines, the monitoring
source relay output current is 2 mA in order to pro-
vide each receiving end relay with 1 mA. The alarm
unit of the monitoring current source relay is adjusted
to float between the high and low current contacts with
normal monitoring current. The PM-23, receiving end
alarm relay, is adjusted to float between the low-current
alarm contact and a contact stop with 1 mA flowing.
3.2
SHORT CIRCUITS
A complete or partial short circuit on the pilot-wires
increases the current in the current-source relay,
causing the high-current alarm contacts to close. The
resulting current decrease in the PM-23 relay closes
the alarm contact. Short circuits of 5000 ohms or less
will be detected.
3.3
OPEN CIRCUITS
Current decreases to zero in all relays. Low current
alarm contact of the current source relay closes.
Alarm contact of PM-23 relay closes.
3.4
REVERSE WIRES
On applications using the PM-23 relay, current
increases in the sending end relay to close the high
current alarm contacts. Current drops to zero in the
PM-23 relay monitoring coil to close the low-current
alarm contacts.
If the pilot-wire should be opened, the low-current
contacts of the (2) unit will close. If the pilot-wire is
then reconnected with reversed connections, the (2)
unit low-current contacts will remain closed. The
capacitor at the sending end discharges through the
pilot-wire and the trip unit (3) circuit when the pilot-
wire is connected reversed. This would momentarily
operate the trip unit (3) contact which would cause
an incorrect trip; however, this is prevented by the (2)
unit low-current contacts which connect a 2.25K
resistor across the trip unit (3) coil. The 2.2K resistor
remains connected across the trip-unit (3) coil until
the pilot-wire is connected properly.
The current decreases in both sending and receiving
end relays when the PM-2, or PM-4 relays are used.
Low-current alarm contacts close.
3.5
GROUNDS
The voltage-divider circuit of the PMA, PMD, and
PMG-13 source relays has its midpoint grounded
through a current-limiting resistor. Thus, a pilot-wire
ground will cause an increase in current in one coil
4
Type PM Line of Relays For Pilot Wire Monitoring
circuit, and a decrease in the other one. This unbal-
ance in the current flowing through the two windings
(5) of the ground alarm relay unit will cause it to close
one of its contacts (depending on which point wire is
grounded) to give an alarm. Grounds of 10,000 ohms
or less will be detected.
For adding the sensitive ground detection where
PMA-1, PMD-1, or PM-13 relays have been installed,
the PM-5 relay can be added to the circuitry, as
shown in figure 30. This relay also has a 10,000 ohm
ground sensitivity.
3.6
TRANSFERRED TRIPPING
Breakers located at the PMG-13 or PM-13 and PM-3
or PM-23 stations can be tripped by the application of
a dc voltage to the pilot-wires at remote locations, as
shown in figures 25 to 31. Transferred tripping can be
effected from any location by applying 48 volts dc
(through dropping resistors when required) to the
pilot-wire with polarity opposite to that of the monitor-
ing voltage. When tripping the PM-23, the current is
increased above 2.0 mA, in reverse direction, to
close the trip contact. When tripping the PMG-13 or
PM-13, the reversed dc voltage operates the trip unit
(3).
See Tables 2 and 3 for tripping resistor values. Nomi-
nal tripping currents is 5 mA at all rated voltages.
3.7
POLAR UNIT
Polar unit flux paths are shown in figure 1. With bal-
anced air gaps, permanent magnet flux flows in two
paths, one through the front, and one through the
rear gaps. This flux produces north and south poles,
as shown. By turning the left shunt in, some of the
flux is forced through the armature, making it a north
pole. Thus, reducing the left-hand rear gap will pro-
duce a force tending to pull the armature to the right.
Similarly reducing the right-hand gap will make the
armature a south pole and produce a force tending to
pull the armature to the left.
The alarm unit contacts of the sending and receiving
end relays are biased to move to the left when the
relay is deenergized. The PMG-13 or PM-13 and
PM-23 trip unit contact is biased to move to the left
when the relay is deenergized. The PM-5 is adjusted
so that the moving contact floats when the relay is
deenergized.
And Transferred Tripping
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