Answer / dolphin

generally different conductors are having different
electron density.if current flows from high
electron density conductor to low density conductor
heat will be produced at the junction due to sudden
change in electron density.to avoid this the two
conductors to be junctioned should have minimum
difference in electron density.in some other cases
contact resistance causes heat if they are not
tightly held together.

Answer / kapil gehlot

when two conductor are joined then a contact resistance is
genrated due to that heat is generated and second reason
would be change of electron density and chnage of
resistivity.

Answer / amol

current passes from one conductor to another conductor, if
cross section area is less then it will take heating of
conductor

Answer / ayub

Because of losses due to the collission of electrons and
also due to the diffrent condutors the metal are diffrent
so losses will take place when passing through the one
conductor to the another diffrent conductor.

Answer / pradeep kumar rawat

at the junction of two conduntor some gap are formed at the
junction point, due to this gap some air take place and
ionised by other ions which is flowing in two conductor,air
between two conductor ionised and current posses through
one conductor to other conductor and heat produces, due to
this at the junction of two conductor heat produced.

Answer / ashwin

The two dissimilar metals behave or contribute a thermocouple. A thermocouple is a junction between two dissimilar metals when subjected to heat generates voltage. This is seebeck effect. The inverse of this effect is Peltier effect. When current is passed through a junction containing two dissimilar metals heat is generated.

For more details on the principle of thermocouple go to link below:

http://en.wikipedia.org/wiki/Thermocouple

From the above link if you can reverse the effects or its causative factors you can minimize this effect.

Answer / aziz_raghib2002

at the junction of the two conductors the conductors will
have different potential due to thier material
characteristics and thats why some P.D and so currentand
heat ie cu losses same material should be used and same
ch/hs material

Answer / naveen gupta

Two different metals have two different conductivity, so
the movements of electrons get obstructed during this
transition. Thats why the heat get generated.

Now to minimize it we need to increase the surface area
depending upon their conductivity levels.

Answer / vishnu p jariwala

Both conductors have differend impedance(Z),And Becose of
resister heat losses is accour That's why...

Answer / j.kesavan

we are use different conductors ,suppose we joint copper and
aluminium busbars using bimetalstrip reduce heat.

why we use capacitor in Fans not resistor?

4 Answers   Vardhman,

What is the role of power pack in substation protection system?

0 Answers   Facor,

Do you know how to measure the earth resistance? if so explain it

42 Answers   ABB, BEL, BridgeStone, College School Exams Tests, Dalkia, Dalmia Group, DEC, Electrical, Genpact, HEC, IOCL, JLL, L&T, MSEB, Saraca, Sical, Telecommunications, University Exams,

we specify current in case of feeder but in case of distributor voltage, why?

0 Answers  

What is the minimum underground tank size capacity considered for firefighting purpose?

0 Answers  

Please brief me about number of pole in alternator.In single phase alternator coils are connected in star or series. How to calculate frequency for the same.

0 Answers  

What safety followup during kWh meter isolation

0 Answers  

How to calculate voltage drop in copper and aluminium cable? I have laid 240Sq.mm Aluminium cable for the length of 600 Mtrs in 1 Mtrs underground. Normal load is 150A. Transformer is 630KVA, 6.KV/433V. But, I am getting only 395 Volts when load. How to calculate voltage drop and what is the permissible voltge drop for this circuit?

1 Answers  

hopkinson's test

5 Answers   Bhel, NTPC,

1.where in India the maximum transmission voltage is operated? 2.what the methods are used to cool down the transformer?give the specific ratings of the transformer & specific cooling process. 3.how zero & negative sequence reactances are formed in a rotating machine when fault occur in the system?

2 Answers   IES,

There is two transformers of 2MVA & 3MVA, which transformer is located at the sending end & which is at receiving end. why?

3 Answers  

RELAY 1. REF RELAY : • OPERATION WITH CT CURRENT FLOW DIRECTION. • WHY REF RELAY WHEN DIFFERENTIAL RELAY IS THERE. • REF RELAY STABILITY CHECK BY PRIMARY INJECTION – WHICH POINT FOR CUR RENT INJECTION. • WHY REF RELAY NEED STABILISING RESISTOR. IF RESISTOR SHORTED OR BYPASSED, WHAT WILL HAPPEN. • WHICH TYPE OF RELAY FOR REF, HIGH IMPEDANCE OR LOW IMPEDANCE. • IF NEUTRAL CT IS NOT CONNECTED IN THE CIRCUIT, WHAT WILL HAPPEN. • REF PROTECTION ACTS ON WHICH OF FOLLOWING: SINGLE PH. TO E/F; PH-PH TO E/F; THREE PH. TO E/F. 2. DISTANCE RELAY : • BASIC PRINCIPLE AND TYPES • SWITCHED MODE PROTECTION – WHY IS IT CALLED SO. • SCHEMES USED • WHY ZONE-1 SET AT 80% • SETTING RANGE FOR ZONE-2 AND ZONE-3 • HOW SINGLE PHASE TO GROUND FAULT MEASUREMENT IS DONE. • 3. WHAT IS POLARITY. CT LOCATION IF INTERCHANGED WHAT WILL HAPPEN. 4. SOLKOR RELAY: 1. WHY SOLKOR IS USED. 2. CIRCUIT DIAGRAM OF SOLKOR RELAY 3. DIFFERENT PARTS OF RELAY 4. HOW IT WORKS. 5. EXPLAIN BY VECTOR DIGRAM. 6. WHAT ADVANTAGE FOR USING SUMMATION TR. 7. DIFFERENCE BETWEEN R & RF WITH CIRCUIT. 8. ADJUSTMENT OF PADDING RESISTANCE. 9. WHAT IS LEAKAGE CURRENT AND HOW TO MEASURE. 10. WHAT HAPPEN IF LINK IS LEFT OPEN. 11. 5KV/15KV INSULATION TR. ARE USED IN WHICH SYSTEM. 12. WHAT PRECAUTION TAKEN DURING TESTING. Relay Page 1 of 5 13. PROCEDURE BEFORE TESTING AND AFTER NORMALISATION. 14. HOW RELAY IS TESTED IN LIVE CONDITION. 15. RANGE OF SPILL CURRENT ANS SETTING OF RELAY 16. IF MEASURED CURRENT IS 5MILLI AMPS, CAN RELAY OPERATE. 17. EACH FAULT APPROXIMATE VALUE OF PICK UP. 18. WHAT IS THE REASON SOLKOR-R MODE PILOT SHORTED CONDITION RELAY CURRENT PRESENT. 19. AFTER SHUTDOWN, WHY NEED TO OPEN SOLKOR PILOT, IF NOT WHAT WILL HAPPEN. 20. AFTER NORMALISING WHAT CURRENT TO BE MEASURED, WHAT IS ACCEPTABLE LIMIT. 21. WHAT HAPPENS IF PILOT OPENS FROM RELAY, WHEN FEEDER IS LOADED. 22. CAN WE USE THIS RELAY AS O/C OR E/F RELAY. 23. ANY RELAY OTHER THAN PILOT CABLE TYPE. 24. IF FEEDER TRIPS ON SOLKOR, WHAT TO DO. 25. WHAT HAPPENS IF PILOT POLARITY REVERSE. 26. WHAT HAPPENS WHEN CT SHORTED AT ONE END. 27. WHAT HAPPENS IF PILOT SHORTED – STATUS OF RELAY 28. WHAT HAPPENS TO RELAY ,IF POWER CABLE GETS OPEN. 29. WHAT HAPPEN IF PADDING RESISTOR IS SHORTED AT ONE END. 5. TR. DIFFERENTIAL: 1. CIRCUIT DIAGRAM 2. DIRECTION OF FAULT CURRENT. 3. WHEN THE FAULT IS IN ZONE AND OUT OF ZONE. 4. CT CONNECTION IN PY. AND SY. SIDE. 5. BIAS SETTING AND BIAS CURRENT. 6. WHICH BIAS SETTING USED IN TRS. FOR HIGHER TAPS. 7. PURPOSE OF RESTRAINING BIAS COIL. 8. WHY BIASING REQUIRED. 9. EXPLAIN IPCT, FOR EXAMPLE, DYN10 VECTOR GROUP. WHY MATCHING TR. USED. 10. WHY NEED 2ND HARMONIC BLOCKING DURING INRUSH. 11. WHICH HARMONIC IS MORE 2ND OR 3RD. 12. HOW TO AVOID FALSE TRIPPING IN DIFF. PROTECTION WHEN INRUSH CURRENT FLOW DURING TR. IS ENERGISED. Relay Page2 of5 6. DIRECTIONAL RELAY: 1. CIRCUIT FOR USE OF DIRECTIONAL PROTECTION SYSTEM. 2. WORKING ZONE / NON-WORKING ZONE. 3. HOW THE CORRECT DIRECTION OF FLOW OF CURRENT IS DETERMINED. 4. CT & PT CONNECTION DIAGRAM. 5. SETTING OF PHASE & EARTH FAULT ANGLE AND OPERATING REGION. 6. WHICH DIRECTION IT IS PROTECTING. 7. WHAT PROTECTION AVAILABLE FOR POWER TR. 8. WHY CT CONNECTION IS STAR, IF TR. WINDING IS DELTA. 9. DIFFERENCE BETWEEN REF AND DIFF. RELAY 10. WHERE IS O/C & E/F RELAY CONNECTED EITHER HV SIDE OR LV SIDE AND HOW E/F RELAY WORKS IF TR. WINDING IS DELTA CONNECTED. 11. IN DELTA WINDING OF TR., IF ONE WINDING IS CUT AND NOT EARTHED, WHICH PROTECTION WILL ACT. 12. IF DIFF. & REF RELAY ARE NOT WORKING, WHAT IS BACK UP PROTECTION IN TR. 13. WHAT IS PURPOSE OF NGR AND HOW STANDBY E/F RELAY WORKING, TESTING PRECAUTIONS IF COMMON NGR IS USED. 14. WHAT IS PROTECTIVE ZONE OF STANDBY E/F RELAY. 15. WHAT ARE PROTECTION AVAILABLE FOR FEEDER. 16. AFTER TR. IS NORMALISED, WHAT CURRENT TO BE MEASURED. 17. BUSBAR PROTECTION: a. BASIC PRINCIPLE AND DIAGRAM WITH CURRENT DIRECTION. b. WHY OVER LAPPING CT’S REQUIRED, IF NOT WHAT WILL BE THE EFFECT. c. WHY CT SUPERVISION REQUIRED, HOW TRIPPING IS AVOIDED, WHAT IS CT SUPERVISION SETTING VALUE. d. HIGH IMPEDANCE & LOW IMPEDANCE TYPE e. TEST PROCEDURE(STEP BY STEP) f. AFTER NORMALISING, SPILL CURRENT VALUE AND LIMIT. Relay Page3of5 18. CIRCUIT BREAKER FAILURE – FUNCTION, WHY NEEDED, HOW TRIPPING ARRANGEMENT. 19. WHY INSTANTANEOUS ELEMENT IS NOT USED IN THE NETWORK. 20. WHEN FEEDER IS LIVE AND BY MISTAKE LINE EARTH SWITCH IS CLOSED, WHICH RELAY OPERATES – O/C OR E/F. 21. DURING TR. SWITCHING ON, IF DIFF. RELAY IS BLOCKED BY 2ND HARMONICS, WHAT ARE THE OTHER PROTECTION IN SERVICE. 22. WHICH OF FOLLOWING PROTECTION OPERATES FAST, WHEN 11KV CABLE COMPARTMENT GET FLASH OVER: CABLE DIFFERENTIAL; OVERCURRENT & EARTH FAULT; ARC PROTECTION. 23. WHICH PROTECTION OPERATES WHEN ANY ONE OF THE OHL PHASE IS CUT AND HANGING, NOT TOUCHED WITH EARTH. 24. WHAT ARE DIFFERENT TTB ALARMS – HOW YOU KNOW THERE IS ANY ALARM. 25. WHAT IS CIRCULATING CURRENT AND HOW IT WILL COME. 26. IF ONE TR. TRIPPED ON DIFF. RELAY WHAT YOU WILL DO. 27. IF SUB-STATION TRIP ON BUSBAR PROTECTION, HOW YOU WILL RESTORE. 28. WHAT IS CT SATURATION. HOW IT IS CARRIED OUT. 29. D.C EARTH FAULT – HOW YOU WILL FIND OUT. Relay Page 4 of5 30. PILOT WIRE SUPERVISION – HOW IT WORKS 31. IN 11KV SWITCHGEAR DBB SCHEME, ONE E/F RELAY IS FAULTY IN ONE FEEDER AND THERE IS NO SPARE FEEDER – HOW YOU WILL NORMALISE THE FEEDER. 32. WHICH PROTECTION IS USED FOR LONG CABLE FEEDER 33. DIFFERENCE BETWEEN TRAFO. DIFF. AND CABLE DIFF. PROTECTION. 34. WHAT TYPE CHARACTERISTIC AVAILABLE FOR O/C AND E/F RELAY. 35. WHAT IS THE DIFFERENCE BETWEEN STATIC AND ELECTRO- MECHANICAL O/C RELAYS. 36. WHAT IS DEAD ZONE 37. HOW TRIPPING OF OTHER ZONE AVOIDED WHEN BUS SECTION CB IN OPEN CONDITION FOR DEAD ZONE FAULT. 38. IF OUT OF STEP ALARM COMES WHILE ALL TR. IN SAME TAP POSITION, WHAT YOU WILL DO. 39. WHAT YOU WILL DO IF AN 11KV FEEDER FAILED TO TRIP ON: O/C or E/F or SOLKOR. HOW YOU RESTORE THE SUPPLY. Relay Page 5 of5

2 Answers   KSEB, Siemens,