Relay : Types of Relay

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Preliminary examination & checks

  1. Damage like dents, broken glass covers, loose parts etc.
  2. Iron filings in air gaps of magnets.
  3. Moving parts getting jammed etc.
  4. Confirming to rating, correct auxiliary supply voltage etc.

Types of Relay

1. Over current, Instantaneous / Time
2. Over/under voltage
3. Differential
4. Reverse power
5. Over/under frequency
6. Directional
7. Distance
8. Negative Sequence
9. Current balance

Lists of Electrical tests

a. Insulation Resistance f. Directional sensitivity.
b. Drop out value. g. Stray operation check.
c. Polarity checks h. Flag indication and auxiliary contacts.
d. Pick up value i. Slope characteristics.
e. Timing


1 . Pick up & Drop values (Instantaneous relays)

Current/voltage is applied suddenly in increasing steps till the relay operates. This gives pick up value. Then the
operating quantity is increased by 20% and reduced sharply till relay resets -this gives drop out values.
Near the pickup value, the relay operation should not be oscillatory.

2 .Time delay relays – Current/voltage applied in steps and the operating value noted. Reset value is also noted.
In each case sufficient time into be allowed to see if the relay operates. The values are compared with the
manufacture’s values.

3.Time Test – The Time of operation is noted by a time interval meter started at the same time as the operating quantity and stopped by the relay output circuit operation.

4. Polarity check & Directional Sensitivity 

For directional relays the correct relative polarity of current and voltage should give operating torque one of the quantities should be reversed to check non operation on restraint.
In phase values of current and voltage are applied and the minimum value to cause operation is checked.
Using a phase shifting transformer the characteristic angle (angle of Maximum torque) and the minimum value of current required for operation should be noted. This gives directional sensitivity on directional relays. During this test the operating and reset zones are also noted.

5 . Stray operation test

a. Current is applied upto 10-15 times tap value and voltage terminals shorted.
b. Rated voltage is then applied and current terminals shorted.
The relay should not operate in this cases.

6 .Slope characteristics
Restraint current of two to four times rated tap is applied suddenly and operating current applied is increasing steps till the relay operates. Two readings are taken:
1. Restraint terminals connected one way.
2. Interchanging restraint terminals.
The above two readings should be identical.

7 . Distance Relay testing
a. Verification of polar characteristics in the R-X plane with particular reference to distance setting along the relay impedance angle
b . Verification of setting for different zones.

8 . Stability Test : 
Performed on those relays which posses an electrical restraining /polarising feature. The purpose of the test is to determine whether the polarising feature is adequate enough to prevent maloperation of the relay even when both operating & polarising quantities are abnormally high.

9 . Dynamic Testing :
The primary and secondary injection are primarily steady state tests (voltage & current waves forms are purely sinusoidal) where as practical situations they may consists of DC offset and harmonies.

10 . Computer based testing
A digital computer has a tremendous amount of programming facility /flexibility. When its output is interfaced
with a high current, high power amplifier, one can generate any programmable wave form of voltage and current signals for testing of sensitivity and stability of the values.



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