41-973.5R
10
Type PM Line of Relays For Pilot Wire Monitoring
And Transferred Tripping
ping is used), to prevent the possibility of inad-
vertently tripping the associated circuit breaker
during testing. If the relays do not perform as
expected, and diode failure is suspected, the
diode tests described in the following section
may be performed.
8.3
RECTIFIER (DIODE) CHECK
(AC RELAYS ONLY)
If there is suspicion of a rectifier (diode) failure, apply
30 volts dc reverse voltage (positive on cathode)
through a 300 ohm resistance to the diode. Measure
the voltage across the diode. If this voltage is not
essentially 30 volts, the diode is short circulated.
Now apply 30 volts dc in the forward direction
through the 300 ohm resistor, and measure the volt-
age across the resistor. If the voltage is not essen-
tially 30 volts, the diode may have a high forward
resistance. If voltage is zero, the diode is open-cir-
cuited.
9.0 CALIBRATION
If the relay has been dismantled or the calibration
has been disturbed, use the following procedure for
calibration. (Relays must be tested in the case.)
With the permanent magnet removed, see that the
moving armature floats in the central area of the air-
gap between the poles of the polar unit frame. If nec-
essary, loosen the core screw in the center rear of
the unit and shift the core and contact assembly until
the armature floats. (This can best be done with the
polar unit removed from the relay.) Then retighten the
core screw and replace he permanent magnet with
the dimple (north pole) on the magnet to the left
when viewed from the front.
9.1
POLAR UNITS – GENERAL
The following mechanical adjustments are given as a
guide, and some deviation from them may be neces-
sary to obtain proper electrical calibration.
9.1.1
Magnetic Shunt Adjustment
The sensitivity of the polar unit is adjusted by means
of two magnetic, screw-type shunts at the rear of the
unit, as shown in figure 1. These shunt screws are
held in proper adjustment by a flat strip spring across
the back of the polar unit frame, so no locking screws
are required. Looking at the relay, front view, turning
out the right-hand shunt to open the right-hand air
gap decreases the amount of current required to
close the right-hand contact. Conversely, drawing out
the left-hand shunt increases the amount of current
required to close the right-hand contact, or
decreases the amount of current required to close
the left-hand contact (with the proper direction of cur-
rent flow). Also, if a relay trips to the right at the
proper current, the dropout current can be raised by
turning in the right-hand shunt. The two shunt-screw
adjustments are not independent, however, and a
certain amount of trimming adjustment of both shunt
screws is generally necessary to obtain the desired
pickup and dropout calibration.
In general, the farther out the two shunt screws are
turned, the greater the toggle action will be, and as a
result, the lower the dropout current. For the tripping
units (3) of the PM-3, PM-13, and PM-23 relays, tog-
gle action is desirable, with a dropout current around
75 percent of the pickup current. For the monitoring
alarm relay units, toggle action is not desired.
Instead, the armature is adjusted to float between the
pole faces at a given current (1 or 2 mA), and to
move gradually toward the high or low current alarm
as the coil current is increased or decreased. Simi-
larly, the floating adjustment of the armature of the
ground alarm unit (5) requires that both shunt screws
be turned in relatively far. Then the armature will
move gradually to the left or right as the current
through the two #5 coils is unbalanced.
The electrical calibration of the polar unit is also
affected by the contact adjustment as this changes
the position of the polar unit armature. Do not change
the contact adjustment without rechecking the electri-
cal calibration.
The chart on page 11 indicates the units present in
each relay.
9.1.2
Contact Adjustment – all relays
For all monitoring alarm units, designed (1) or (2),
turn in all the stationary contact and contact stop
screws until they just touch the moving contact.
Advance the screws to hold the armature in the cen-
tral portion of the magnetic air gap between the two-
pole faces. (The stationary contact screws have a
round silver contact face; the stop screws do not
have this silver facing.) Now back off all the contact
and contact stop screws one full turn. This will give a
total contact travel of 0.050 inch. When the relay is
properly calibrated, some touch-up adjustment may
be necessary so that double contacts will both close
at the same current value. The contact gap between
the floating moving contact and the right-hand or left-
hand stationary contacts or contact stops will be
approximately 0.025 inch when the relay is in opera-
tion.
For the tripping (3) units of the PM-3, PM-13, PMG-
