Mho Units; Periodic Checks And Routine Maintenance; Contact Cleaning - GE CEY51A Instructions Manual

GEK-1275
The target shou 1 d reset prompt ly when the reset button at the bottom of the
cover is operated, with the cover on the relay.
MHO UNITS
There shou 1 d be no not i ceab 1 e fri ct i on in the rotating structure of the mho
unit. The mho unit moving contact should just return to the backstop when the relay
is de-energized and in the vertical position.
There should be approximately
0.010 - 0.015
inch end play in the shafts of the
rotating structures. The lower jewel screw bearing should be screwed firmly into
place and the top pivot locked in place by its set screw.
If there is reason to believe that the jewel is cracked or dirty, the screw
assembly can be removed from the bottom of the unit and examined. When replacing a
jewel, have the top pivot engaged in the shaft while screwing in the jewel screw.
All nuts and screws should be tight, with particular attention paid to the tap
plugs.
The felt gasket on the cover should be securely cemented in place in order to
keep out dust.
Determine the impedance and phase angle seen by the relays. Knowing the
impedance and phase angle seen by the relay, the tap value at which the relay will
just operate can be calculated. It is then only necessary to reduce the tap setting
of the relay until the mho units operate, and see how close the actual tap value
found checks with the calculated value. The calculated value should take into
account the shorter reach of the mho unit at low currents. This effect is shown in
Figure 2.
A shorter test that will check for most of the possible open circuits in the AC
portion of the relay can be accomplished by disconnecting the current circuits. This
can be done by removing the lower connection plug. All units should have strong
torque to the right when full voltage is applied.
Replace the lower plug and open the restraint taps. All units should operate
if power and reactive flow are away from the station bus and into the protected line
section. If the direction of reactive power flow is into the station bus, the
resultant phase angle may be such that the units will not operate.
PERIODIC CHECKS AND ROUTINE MAINTENANCE
I n view of the v ita 1 ro 1 e of protect i ve re 1 ays in the operat i on of a power
system, it is important that a periodic test program be followed. It is recognized
that the interval between periodic checks will vary depending upon environment, type
of relay and the user's experience with periodic testing. Until the user has
accumulated enough experience to select the test interval best suited to his
individual requirements, it is suggested that the points listed under INSTALLATION
PROCEDURE be checked once a year.
CONTACT CLEANING
For cleaning fine silver contacts, a flexible burnishing tool should be used.
This consists of a flexible strip of metal with an etched-roughened surface,
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