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When checking the mho unit at angles of more than 30 degrees off the maximum reach position, the error
becomes relatively large with phase-angle error.
at the zero-degree position that a two or three degree error in phase angle will cause a considerably
apparent error in reach.
Before placing a relay into service, the following mechanical adjustments should be checked,
The armature and contacts of the target and seal-in units should operate by hand.
There should be a screw in only one of the taps on the right-hand contact of the target and seal-in
units.
The target should reset promptly when the reset button at the bottom of the cover is operated, with the
cover on the relay.
MHO UNITS
There should be no noticeable friction in the rotating structure of the mho unit.
contact should just return to the backstop when the relay is de-energized, and in the vertical position.
There should be approximately .010-.015 inch end play in the shafts of the rotating structures.
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.
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.
by the relay, the tap value at which the relay will just operate can be calculated.
sary 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.
reach of the mho unit at low currents.
A shorter test which will check for most of the possible open circuits in the a-c portion of the relay
can be accomplished by
plug.
All units should have strong torque to the right when full voltage is applied.
Replace the lower plug and open the restraint taps.
are away from the station bus and into the protected line section.
is into the station bus, the resultant phase angle may be such that the units will not operate.
In view of the vital role of protective relays in the operation of a power system it is important that a
periodic test program be followed.
depending upon environment, type of relay and the user's experience with periodic testing.
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 at an interval of from one to two
years.
C_ O NTACT CLEANING
For cleaning fine silver contacts, a flexible burnishing tool should be used.
ble strip of metal with an etched-roughened surface resembling in effect a superfine file.
action is so delicate that no scratches are left, yet it will clean off any corrosion thoroughly and rapidly.
Its flexibility insures the cleaning of the actual points of contact.
paper or cloth of any kind to clean relay contacts.
When replacing a jewel, have the top pivot in the shaft while screwing
This effect is shown in Fig. 8.
disconnecting the current circuits.
PERIODIC CHECKS AND ROUTINE MAINTENANCE
It is recognized that the interval between periodic checks will vary
GEK-4 549 5
This is apparent from Fig.
INSPECTION
Knowing the impedance and phase angle seen
The calculated value should take into account the shorter
This can be done by removing the lower connection
All units should operate if power and reactive flow
If the direction of reactive power flow
14
where it is seen, for example,
12
The mho unit moving
It is then only neces
Until the user
This consists of a flexi
The polishing
Do not use knives, files, abrasive
The
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