A) Protective relays are used in conjunction with medium voltage circuit breaker (above 600 volts) to sense an
abnormality and cause the trouble to be isolated with minimum disturbance to the electrical system and with the least
damage to the equipment at fault. They have the accuracy and sophistication demanded by the protective
requirements of the primary feeder circuits and larger electrical equipment. Protective relays are designed to be
responsive to an abnormal excursion in current or power flow, etc., and with varying operating characters are
commercially available. Each relay application requires custom engineering to satisfy the parameters of its particular
intended function on the system.
B) The more common protective relay is of the electromechanical type. That is some mechanical element such as
an induction-disc and induction cylinder of magnetic plunger is caused to move in response to an abnormal change
in a parameter of the electrical system. The movement can cause on contact in the control circuit to operate tripping
the related circuit breaker. Protective relays should be acceptance tested prior to being placed in service and
periodically thereafter to insure reliable performance. In a normal industrial application, periodic testing should be
done at least once a year.
C) The various facts involved in testing protective relays can be listed as follows:
1. The technicians must understand the construction operation and testing of the particular relay.
2. The technicians manufacturers instruction bulletin, as identified on the name plate of the relay should be
available to him. (It should be noted that Southern Substation, Inc. technicians carry on their test truck every
bulletin printed on all relays).
3. The technicians should be given the settings to be applied to each particular relay and the test points. This
data is often furnished on a time current curve of the coordination study displaying the characteristics of the relay.
4. A test instrument, suitable to accurately accommodate the various acceptance and periodic maintenance tests
described in the manufacturers instruction manual, should be available. (It should be noted that Southern
Substation, Inc., technicians use only the SSI SR-95 relay test set. This is the most modern unit on the market and
is capable of firing an SCR Solid State Circuit).
5. Most protective relays can be isolated for testing while the electrical system is in normal operation. However,
an operation of the relays contacts will trigger the intended reaction, such as to trip the associated circuit breaker.
It is recommended that only one relay be removed from a circuit at a time if the circuit is still energized. Testing
and cleaning of relays are covered in each of the manufacturers instruction manuals. We have taken the best of
each of these manuals, plus data from two testing publications and formed the following procedure to be the best to
A) Visual check and inspection of relay:
1. Remove the cover from the relay
a) inspect the cover gasket
b) check the glass for tightness in the frame, cracks, etc.
c) clean glass and covers inside and out.
2. Remove relay from case
a) short CT terminals for safety reasons
b) open all trip circuits
3. All foreign material such as dust or metal particles are removed from the relay and relay case. The
foreign material can cause trouble, particularly the air gap between the disc and magnet.
4. Dust particles are removed by gently blowing air from a small hand syringe.
5. Any rust or metal particles are removed from the disc or magnet poles with a magnet cleaner or brush.
6. Relay is put up to the light to make sure disc does not rub, and a good clearance exists between it and
the magnetic poles.
7. Relay is inspected for the presence of moisture. If free moisture exits, or rust spots are noted this
condition is pointed out to the owner. This condition may indicate that the relay is in the improper atmosphere
and could present a design problem.
8. All connections, especially taps, are checked for tightness. All screws, nuts and bolts that are not
pivotal joints are checked for tightness.
9. Sluggish bearings are detected by noting the smoothness of the relay reset. This is checked by rotating
the disc manually to close the contacts and observing that the operation is smooth, allowing the action of the spiral
spring to return the disc to its normal de-energized position. If the bearings appear to be bad or the operation of
the relay is questionable, this condition is then reported to the owner. If simple cleaning and oiling does not
remedy the problem, the relay should be removed from service and replaced by a spare. If the relay is new and
this problem is found on an acceptance test, this relay should be returned to the manufacturers. If this condition is
found during a maintenance test, the relay can be reconditioned by Southern Substation, Inc., technicians.
On plunger type relays, occasionally a burr a groove may develop causing the relay to hang up. All plungers are
given a very thorough inspection to assure that this has not occurred.
10. Mechanical operation of the targets are checked manually by fitting the armatures and observing a
showing of the target.
11. Relay coil are checked to assure that they have not been subjected to high currents for prolonged
period of time.
12. All relay components that touch when the relay is in a "normal" position and part as the relay
"operates" are cleaned. If these parts become dirty, they can cause the relay to stick and not operate properly
on relatively low current faults.
13. All pitted and/or burned contacts are cleaned and burnished with an approved contact burnishing tool.
B) Electrical Testing
a) Testing with the relay disconnected from the power and trip circuits. This method is standard yearly
b) Testing across the secondary of the current transformer with the primary de-energized. This is called
1a) This method is used
2b) This test includes checking the operation of the circuit breaker, presence of energy to trip breaker.
3c) This method is done by introducing the current voltage at the secondary terminals of instrument
transformer. This test, checks the relay connections as well as the relay.
C) Primary Circuit Test -High Current / Low Current (Primary injection)
This test is recommended on new or rebuilt equipment. This also requires total shut-down of both primary and
1. Complete system is checked, including the current transformer.
2. This test simultaneously checks current transformers, ratio, secondary wiring, polarity, relay operation and
identity of each phase on the switchboard.
3. This test is also valuable for initially checking buss differentials where many current transformers are
parallel at the relay.
D) Types of Testing
1. Insulation Resistance
2. Zero Check
3. Pick-Up (Induction disc. and/or instantaneous unit)
4. Time characteristics (when applicable)
5. Target and seal-in operation
6. Special test that are peculiar to the specific type of relay such as through fault, reach, dropout, etc.
Continue to Page two of Relay testing for more Information.