MEANS TIME DELAY
MEANS DIRECTIONAL POWER
MEANS UNDERCURRENT OR UNDERPOWER
MEANS PHASE-SEQUENCE VOLTAGE
MEANS INSTANTANEOUS OVERCURRENT
MEANS AC TIME OVER CURRENT
MEANS OVER VOLTAGE
MEANS VOLTAGE BALANCE
MEANS PRESSURE (MECHANICAL PROTECTION)
MEANS APPARATUS GROUND
MEANS AC DIRECTIONAL OVER CURRENT
MEANS DC OVER CURRENT
MEANS AC RECLOSING
MEANS CARRIER OR PILOT-WIRE
MEANS LOCK OUT
What do you know about Earth Leakage Circuit Breakers ELCB?
Earth Leakage Circuit Breakers ELCB)
4.3 Earth Leakage Circuit Breakers (ELCB)
A human operator may come in a direct contact with a live wire or he may touch a ****l-case of a leaky instrument. In either case, a current flows through the body of the operator to the earth. The amount of this current depends on the touch voltage and the human body resistance, which is a variable. A current exceeding a few tens of mill amperes through the body may prove to be fatal if permitted to do so for a long time. The Earth Leakage Circuit Breaker (ELCB) when installed in the electrical wiring protects the operator from such hazards. The ELCB is a safety device, which operates on low earth leakage current flowing in an electrical system. At such small currents, fuses or circuit breakers do not provide any protection, as they cannot operate till very high current flows in the wiring .
In USA and Canada, the same device (ELCB) is known as a Ground Fault Circuit Interrupter (GFCI) and in Europe it is called a Residual Current Circuit Breaker (RCCB). All these devices (ELCB, GFCI and RCCB) operate on the same principle .
In a typical wiring arrangement of an ELCB installation shown below , the live and neutral wires are passed through a magnetic core. Under the normal conditions, because the current in both conductors is equal and opposite, there is no flux in the core and the relay coil is de-energized. The electrical supply to the load equipment is not interrupted.
under certain fault conditions (e. g. leakage due to a faulty insulation), the current in the live wire can exceed the neutral wire. This produces flux in the core. The secondary winding on the same core produces a voltage, which is amplified and applied to a relay coil. The relay operates and interrupts the live and neutral conductor. The earth leakage trip current can be pre-adjusted between 5 mA to 30 mA depending on the application in which it is used.
A typical ELCB is built into a 230-volt circuit breaker and is located in the panel board. A reset lever is provided on the ELCB to reset it after the cause of the leakage is detected and removed. A test button, if provided on an ELCB, can be pushed to check the functioning of the device as this action simulates an imbalance in the live and the neutral wire.
In the three-phase ELCB variety, the vector sum of the currents in the three lines and the neutral is continuously monitored. A nonzero current indicates a leakage in the wiring and the ELCB operates and disconnects all the supply conductors of the equipment .
The technical data of a typical ELCB device available in the Indian market is shown in the Table 4.3  below:
Table 4.3: Technical Data of an ELCB
BS 4293 – 1983/IS12640 – 1988
High (30 mA)
Medium (100 mA)
Low (300 mA) Optional
2Pole 240 VAC 50 Hz
4 Pole 240/415 VAC 50 Hz
25/40 A – 16 sq. mm
63/80 A – 25 sq. mm
100 A – 35 sq. mm
Ambient Temperature Range
50C to ȴ00 C
Degree of Protection
Recommended Back-up HRC fuses
for 25 A & 40 A
for 63 A, 80 A & 100 A
Installation of an ELCB for a domestic environment and that for a number of equipment that performs a variety of unrelated functions in a field station can differ considerably. A tripping of a single ELCB need not bring to a halt the operation of the entire field station till the cause of the trip is located and rectified. For this, the equipment in the field station is divided in clusters and a separate ELCB is installed for each of them. This allows a majority of operations in a field station to continue in case an ELCB trips the supply to a group of equipment in a cluster.
ELCBs are available from following suppliers:
• Siemens Ltd., Bombay
• MDS Switchgear Ltd., Bombay
• Protech Switchgears Ltd., Vadodara.
What is meant by 100% (S/E/F)in the generator protection
(ANSI 64G-100%)? and give two methods of how to do that?
means 100% Stator Earth Fault, and this protection covers the entire widing by detecting the ground faults that may occur near the neutral end of the stator winding.
the two basic methods:
1- use of the third-harmonic voltage, or
2- the injection of a subharmonic voltage.
What are the P.T & C.T tests?
for the P.T *
types of P.T
core type & the capacitor type
THE TESTS ARE
1- insulation test
in this test we found on the p.t manual that it's error test =+3 or 3p
and that means that error = 3%
there is the turns ratio test&
types of P.T
core type & the capacitor type
*for the C.T
in this test we apply voltage in the primary & check the secondary current
& it's written on the c.t manual as
1500...is the primary applied voltage
5....the secondary current
it's written on the c.t manual as
20=20 * rated current
we measure the resistance to check the insulation
Q: WHAT IS THE DIFFERENCE BETWEEN PRIMARY INJECTION AND
SECONDARY INJECTION FOR PROTECTION RELAYS
primary injection is carried out with full rated current of the circuit by using a.c current source and injecting it in the primary side of the current transformer of circuit
secondary injection is carried out at secondary side of ct by injecting secondary current in to the relay to prove the function and operation and setting of the relay by special testing set.
what's the principle operated the main protection relays should be considered on the (ATS function between normal power supply and emergency power supply)
2.1 Automatic Transfer Operation
In general, the automatic transfer operation is initiated if a low voltage is detected on incoming Line A. When initiated, the ATS will simultaneously check for these four conditions: a) both incoming breakers are closed and in operating position; b) the bus tie breaker is open and in operating position; c) incoming breaker 52-A is not carrying fault currents; d) voltage on Line B is adequate and has been adequate for at least a set duration. If all four conditions are satisfied, the ATS will open incoming breaker 52-A. The ATS will then check to ensure that the residual voltage on the switchgear Bus A is at a safe level. If Bus A residual voltage is at a safe level, the ATS will close the bus tie breaker, allowing the whole switchgear to be supplied from incoming Line B. Otherwise, automatic transfer is blocked and operator intervention will be needed to reset and return the ATS to its normal position.
2.1.2 Detailed Sequence of Operation.
The under voltage relay device 27-A monitors the voltage on incoming Line A. This relay shall be of the induction type, equal to GE type IAV54EIA or ABB Type CV-2 short-time. It shall be set so that it does not pick up on ground faults in a high resistance grounded system. The recommended setting is 45% of nominal voltage, with a time delay of 20.
If a low voltage is detected, the under voltage relay device 27-A initiates the automatic transfer operation mode of the ATS. Low voltage on incoming Line A is due to various reasons. It may be due to a voltage depression at the upstream supplying bus. For the case where incoming Line A is without a transformer, it may be due to a fault on the line between the upstream supplying breaker and the incoming breaker 52-A. For the case where Line A is with a transformer, it may be due to: a fault on the line between the upstream supplying breaker and the transformer; a fault in the transformer primary winding; a fault in the transformer secondary winding; a fault in the transformer pressure; or a fault on the bus duct between the transformer and the incoming breaker 52-A. Or, it may be due to a fault downstream of incoming breaker 52-A.
When initiated, the ATS simultaneously performs the following verifications:
WHEN READ THIS INFORMATION YOU WILL GET THE ANSWER.
المحولات اعلاه تستخدم في شبكات توزيع الكهرباء
وضع لنك فيوز مع سلك مناسب للفيوز
موازنة الاحمال على اطوار المحوله
وضع مانعة صواعق
الحمل المطلوب لا يتجاوز 80%من حمل المحوله
صيانة المحوله بشكل متواصل
How do the circuit breaker and relay operate?
Circuit breaker and relays step by step operating procedure while isolating a fault (the time intervals between each event will be in the order of a few electrical cycles i e milliseconds.
The relay receives information which it analyses and determines that the circuit should be opened
Relay closes its contacts energizing the trip coil of the circuit breaker
The C.B is unlatched and opens its main contacts under the control of the tripping spring.
The trip coil is de energized by opening of the circuit breaker auxiliary contacts
Q: WHAT ARE THE MAIN GENERATOR PROTECTION RELAYS FOR GENERATOR >120 MW?
THE MAIN GENERATOR PROTECTION RELAYS ARE THE FOLLOWING
STATOR EARTH FAULT RELAY
ROTOR EARTH FAULT RELAY
GENERATOR DIFFERENTIAL RELAY
GENERATOR OVER VOLTAGE RELAY
GENERATOR UNDER VOLTAGE RELAY
GENERATOR UNDER FREQUENCY RELAY
NEGATIVE PHASE SEQUENCE RELAY
LOSS OF EXCITATION RELAY
GENERATOR OVERCURRENT RELAY
REVERSE POWER RELAY.
How do you carry out C.T ratio test?
1. Connect the C.T as indicated in figure 1.
2. Start the AC current source and inject 25%, 50%, 75%, and 100% of ct primary current rating
3. Read a1 and a2.
4. The ratio of a1 to a2 should approximate to ratio marked on the current transformer nameplate.
HOW DO YOU CARRY OUT POLARITY TEST FOR C.T?
1. CONNECT THE C.T IN THE CIRCUIT AS SHOWN IN FIG 1.THE AMMETER SHOULD A ROBUST MOVING COIL PERMANENT MAGNET CENTER ZERO TYPE
2. PRESS THE PUSH BUTTON AND SEE THE DEFLECTION IN THE AMMETER.
3. THE AMMETER SHOULD GIVE POSITIVE FLICK ON CLOSING THE CIRCUIT AND NEGATIVE FLICK ON OPENING THE CIRCUIT.
WHAT IS THE C.T AND P.T
CT IS CURRENT TRANSFORMER
PT IS VOLTAGE TRANSFORMER
What are the main tests carried out for current transformer before commissioning?
1. Polarity test
2. Magnetization curve
3. Ratio check
What are the main tests carried out on pt before commissioning?
1. Polarity check
2. Ratio check
3. Phasing check.
What is the main function of protective relay?
The protective relay senses the abnormal condition in a part of the power system and gives an alarm or isolate that part from the healthy system by opening the circuit breaker feeding that part.
What is the difference between primary and secondary protection?
Primary protection is the first protection to act and backup protection is the next in the line of defence. If the fault did not clear by primary protection back up protection comes in picture and clear the fault.
What are the types of circuit breaker based on arc quenching medium?
THE CLASSIFICATION OF CIRCUIT BREAKERS BASED ON THE MEDIUM OF ARC EXTINCTION IS AS FOLLOWS:-
1. AIR CIRCUIT BREAKER AND MINIATURE CIRCUIT BREAKER.
2. OIL CIRCUIT BREAKER.
3. MINIMUM OIL CIRCUIT BREAKER.
4. AIR BLAST CIRCUIT BREAKER.
5. SULPHUR HEXAFLUORIDE CIRCUIT BREAKER (SF 6).
6. VACCUM CIRCUIT BREAKER.
What is the meaning and function of HRC fuse?
HRC fuse is high rupturing capacity fuse and its function is to protect the electrical circuit from excessive current due to overload and or short circuit.
Why the secondary of the ct should never be left open.
The secondary of the ct should never be left open as the dangerous high voltage will be developed due to the absence of un opposed mmf. this is the reason why wherever secondary ct circuits are used there is always a facility provided for short circuiting the secondary.
Why the secondary of the ct should never be left open.
The secondary of the ct should never be left open as the dangerous high voltage will be developed due to the absence of un opposed mmf.this is the reason why wherever secondary ct circuits are used there is always a facility provided for short circuiting the secondary.
What are the tests to be performed on CT's ?
A) C.T ratio test: this test is important to conduct because the ct gives the true replica of primary current.
B) C.T polarity test: test of polarity is important otherwise wrong polarity may result in to spurious tripping.
C) C.T saturation test: if the ct is saturated it will not offer a correct information about the primary current. This situation normally occurs when ct has handled a large fault currents.
what will you predict about the transformer if the vector group mentioned is dy5? Explain thoroughly?
the vector group represent the manner in which the two windings are connected and phase position of the respective voltage vectors.
The capital letter indicates the connection of high voltage windings. Here high voltage winding is connected in delta and small letter indicates connection of LV winding which is star in this case.
The no 5 indicates the angle between HV and LV vectors on a clock.i.e,150 degree LV WDG vector lags behind that of HV winding.
WHAT ARE THE PROTECTIONS FOR THE FOR THE FOLLOWING ELECTRICAL EQUIPMENT’S.?
OVER CURRENT / EARTH FAULT / INSTANTANEOUS RELAY
REVERSE POWER RELAY
FIELD FAILURE RELAY
BEARING/WINDING /OIL TEMPERATURE RELAY
OVER CURRENT RELAY
WINDING /OIL TEMPERATURE RELAY
OIL LEVEL; RELAY
What are the main protection relays for ac motors?
the main protection relays for AC motors are
1. Overload relay
2. Earth fault relay
3. Over current relay
4. Thermal overload relay
5. Locked rotor relay
Explain how protective relays are primary injection tested?
The primary of the current transformer is connected to the primary injection test kit and the current is slowly increased.
The pick up /operating current of the relay is checked.
Describe the principle of directional over current relay?
Directional over current relay responds to over currents for a particular direction flow.
It comprises over current relay and power directional relay.
What is circuit breaker?
a device that is designed not only to open and close non automatically but also to open the circuit automatically at a predetermined current and overload values.
What is a feeder ?
the circuit conductors between the service entrance equipment or isolated generating plant and the branch circuit overload device
What does an isolating switch mean?
a switch that is intended for isolating an electric circuit from its source of power .it is intended to be operated only after the breaker has been opened by some another means
What is meant by 5p20. 1m5 ?
5p20 denotes the accuracy class of protection class ct 5% error and 20 times saturating current..
What precautions must be taken when working with current transformer? Why?
the secondary of the ct must never be opened when the primary circuit is energized. if it is necessary to disconnect an instrument while the circuit is energized the secondary of the c.t. must be shorted, otherwise the potential of the secondary may reach dangerous values causing damage of the c.t. but by short circuiting of the secondary damage is avoided.
What is the polarization index and how much it should be for a.c. machines ?
polarization index is the ratio of insulation reading for 10 minutes over insulation reading for 1 minute.
ir value for 10 minutes /ir value for 1 minute
reading should be > 1.5
How do you carry out polarity test for ct ? show by drawing?
when closing switch the meter gives + reading.when opening the switch meter gives - reading
What is the difference between primary and secondary injection test? draw circuit?
primary injection of the relay is carried out to prove the total circuit before start up is okay.we normally inject 2 to 3 times rated current to prove the circuit.
secondary injection is done from the secondary side of ct or pt to check the healthiness of the relay.
What is phase angle ?
it is the electrical angle degree between 2 vector i & v(30*)
Why dc system is not grounded. explain?
for reliable operation. if a circuit gets grounded it will go out of service immediately.
Why the ground fault is to be cleared quickly ?
circuit may energize without the knowledge of operators
What is meant by hi-pot test?
this is nothing but a high voltage test conducted on ht electrical equipments to check the quality of insulation.
What is the difference between core balance ct and zero sequence ct ?
both are same.
How a clamp meter or clip on meter is used for 3 phase load current measurement ?
the meter should be clamped one by one core and individual phase current should be measured.the meter should not be clamped in all three cores at the same time.
Why over current relays are not used to protect motors.
Over current relays does not follow the heating characteristic of motors. Only thermal o/l relays does this.
What an I.D.M.T. relay means?
Inverse definite minimum time lag relay.
What is meant by phase sequence ? How do you reverse it?
the phase sequence is the cyclic order in which the voltage wave changes their peak.correct phase sequence=r-y-b.reversed phase sequence=rby.this can be obtained by reversing b & y phases.
What are the basic protections in a moulded case circuit breaker?
1. thermal protection
2. short circuit protection
Where is a moulded case circuit breaker preferred and where is fuse protection preferred ?
fuse protection is mainly used only for short circuit protection and does not provide overload protection.
a moulded case circuit breaker gives both overload and short circuit protection. for heavy loads moulded case circuit breaker is more economical than fuses.
fuses need frequent replacement while moulded case circuit breaker does not need frequent replacement.
What is a single phasing preventer ?
this will protect the motor if any one of the three phases is open circuited.
Q: choose the correct answer? Three phase faults are generally faults
How a clamp meter or clip on meter is used for 3 phase load current measurement?
the meter should be clamped one by one core and individual phase current should be measured. The meter should not be clamped in all three cores at the same time.
Q: how can the distance relay differentiate between a fault and a power swing
By the following criteria
1. Fault make the impedance measured by the relay to fall rapidly, power swing is somehow slow
2. in the case of power swing the impedance will not settle inside the trip c/c's of the relay it will get back to the outer zone or continue to the other side of the c/c's that means the system is out o f step
3. in the case of power swing , it's symmetrical, no negative or zero sequence exists in this case
in the case of a fault, zero sequence or negative sequence is present except for symmetrical faults
FOR GENERATOR TRANSFORMER(DELTA-STAR CONNECTION) DIFFERENTIAL RELAY.
WHY WE COONECT THE CT IN THE DELATA SIDE IN STAR &IN STAR SIDE WE CONNECT IN DELAT
من المعلوم ان التوصيل بهيئة دلتا استار للمحولات
-1 الدلتا تمنع مرور تيار التتابع الصفري ..
2- يتكون انزياح طوري بين الدلتا والنجمة
اذا لو افترضنا ان محول الطاقة موصل على هيئة دلتا نجمة 11،
فانه يجب ان توصل محولات التيار كالآتي :
جهة الدلتا لمحول الطاقة نجعل توصيل محولات التيار نجمة
وجهة الأستارلمحول الطاقة نجعل توصيل محولات التيار دلتا.
ونجعل التوصيل في ما بين محولات اليتار على هيئة نجمة دلتا 11
وبهذه الطريقة يعدل الأنزياح الطوري الناشىء بين الملف الأولي والملف الثانوي لمحول الطاقة،
ونمنع التيار الصفري من المرور في دائرة محولات التيار ..
2- BUS ISOLATOR 1, 2 OPEN
4-LINE ISOLATER CLOSED
5- FAULT BETWEEN LINE SIDE C.T &C.B
WHICH PROTECTION CLEAR THE FAULT
THE END FAULT PROTECTION WILL CLEAR THE FAULT BY SENDING
TRIP COMMAND TO REMOTE END C.B .
THIS FEATURE END FAULT IN THE BRF RELAY
Q- BOTH BUS ISOLATOR Q1& Q2 CLOSED THERE IS FAULT ON BUS BAR WHAT WILL HAPPEN
1- ALL SUBSTATION WILL TRIP )BLAKOUT)
IN CASE OF Q1&Q2 IT CALLED CROSSCOUPLED &THE BB 2 DIFF RELAY WILL BE THE MASTER .THROUGH CROSS COUPPLED TRIPBUS 1,2 WILL BE ONE BUS & ALL FEEDERS WILL TRIP
CONDTION REDAIL SUSBSTATION A FEEDING FROM
SUBSTATION B THROUGH OHL LINE
FAULT VERY CLOSE TO S/S B
HOW DISTANCE CLEAR THE FAULT IN S/S A
THE FAULT WILL BE CLEAR BY F21 IN S/S A BY WEAK INFEED FEATURE AND RECLOSE SUCCFULLY
WHAT IS THE CONDTION OF WEAK INFEED
1- CURRENT IS VERY LOW & RELAY CAN NOT START
2-THERE IS VOLTAGE DROT 30%
3-NO START IN REVERSE SIDE
4-THERE IS CARRIER RECIVE FROM STRONG END
5-C.B IS CLOSED
Q- What will happen in case of any phase voltage = o for distance relay
Distance relay will be blocked after 10 sec & it will run in emergency o/c mode
this feature is called fuse fail condition
there 2 conditions for this case
1- fault within 10 sec
the distance will operates as normal
2- fault after 10 sec passed
the fault will be cleared by O/C mode after 2.5 sec distance will be blocked
Q :What is the IP standard "Protection degrees of enclosures
كل جهاز او معدة يكون لها IP وهو درجة الحماية لهذه المعدة حيث يكون مكون من ثلاث ارقام ex. IP 543
الرقم الاول "5" وهي درجة حماية الجهاز من dust
الرقم الثاني "4" وهي درجة حماية الجهاز من "water or "liquid
الرقم الثالث "3" وهي درجة حماية الجهاز من mechanical impact
وهناك درجات حماية مختلفة لكل رقم
وناخذ في الاعتبار ان احيانان الرقم الثالث لا يكتب وذلك يوضح ان هذه الماكينة محققة mechanical impact
What is the breaker failure?
The breaker failure relay is used when the fault happened on transmission line or power transformer and the breaker is not open to clear fault even if the protection relays sent the trip signal to the circuit breaker but the circuit breaker fail to open.
The breaker failure sense two factors the fault current and receiving trip signal from protection relays of the transmission line or power transformer the breaker failure have delay time after receiving the trip signal from protection devices about 150 ms to ensure that the circuit breaker is open if it is not open and sense also there is still fault current the breaker failure make retrip to the same circuit breaker if not open the breaker failure have 200 ms delay time then make trip for all feeders and the power transforms that sharing this transmission line or power transformer that circuit breaker fail to open at the fault case the trip go through the bus bar replica to clear all feeders that share the faulty cb feeder.
Q1. How deep must I bury USE cable?
A1. When cables or raceways are run underground, they must have a minimum “cover” in accordance with Table 300.5. Note 1 to Table 300.5 defines “Cover” as the distance from the top of the underground cable or raceway to the surface of finish grade. According to this table, Type USE cable must have 24” of cover, unless the installation is a residential 15 or 20A branch circuit rated 120V or less with GFCI protection.
Q2. Can low-voltage Class 2 thermostat wire be installed in the same raceway with power conductors for the a/c compressor?
A2. No, 725.55 specifies that Class 2 cables are not permitted to be placed in any enclosure, raceway, or cable with conductors of electric light or power.
However, Exception No. 2 to 725.52 allows Class 2 circuits to be reclassified as a Class 1 circuit if the Class 2 equipment markings are eliminated and the circuit is installed in a Chapter 3 wiring method [725.55(D)(2)(b)]. Reclassifying the circuit allows the Class 1 circuit to be installed with functionally associated power circuits in accordance with 725.26(B)(1). Figure 725-30
Q3. Can Type MC cable be installed in a cable tray?
A3. Yes. See Table 392.3(A).
Q4. Can I use a cable tray in a commercial occupancy?
Q4. Yes, cable tray installations aren’t limited to industrial establishments [392.3].
Q5. How do I size the circuit conductors and protection device for a 25 hp, 208V three-phase fire pump motor?
The fire pump motor circuit overcurrent protective device must be sized to carry indefinitely the sum of the locked-rotor current of the fire pump [695.4(B)(1)]. According to Table 430.251(B), the locked-rotor current of a 25 hp, 208V, three-phase motor is 404A. This requires a 450A protection device, in accordance with 240.6(A).
Conductors supplying a fire pump motor must must have an ampere rating of not less than 125 percent of the fire pump motor full-load current (FLC), as listed in Table 430.248 or 430.250. The FLC of a 25 hp, 208V, three-phase motor is 74.8A, so the conductors must be sized no smaller than 93.5A (74.8A x 1.25). According to Table 310.16, we can use a 3 AWG rated 100A at 75°C [110.14(C)(1)].
Careful, the branch-circuit conductors for a fire pump motor must be sized so that the voltage at the line terminals of the controller, when the motor starts (locked-rotor current), does not drop more than 15 percent below the controller’s rated voltage. In addition, the voltage at the motor terminals must not drop more than 5 percent below the voltage rating of the motor when the motor operates at 115 percent of the fire pump full-load current rating.
Q6. Can liquid tight flexible ****l conduit be installed under the raised floor of a computer room and if yes, must it be secured?
Branch-circuit supply conductors for information technology equipment can be installed in liquid tight flexible ****l conduit located under a raised floor. And yes, the raceway must be securely fastened in place [645.5(D)(2)].
Q7. Can I ground a satellite system to the ****l parts of electrical equipment?
A7. No. The antenna mast [810.15] and antenna discharge unit [810.20(C)] must be grounded to one of the following:
Building or structure grounding electrode system [250.50]
Interior ****l water piping system, within 5 ft from its point of entrance [250.52(A)(1)]
Accessible means external to the building, as covered in 250.94
****llic service raceway
Service equipment enclosure
Grounding electrode conductor or the grounding electrode conductor ****l enclosure
Grounding the lead-in antenna cables and the mast helps prevent voltage surges caused by static discharge or nearby lightning strikes from reaching the center conductor of the lead-in coaxial cable. Nothing can prevent damage from a direct lightning strike. But grounding with proper surge protection can help reduce damage to satellite and other equipment from nearby lightning strikes