1. The following statement is associated with the Buchholz relay is not true
(A) It is a gas actuated device(B) It is a current operative device
(C) It is placed between the transformer tank and the conservator
(D) It causes alarm for minor fault and tripping for major fault
Answer.: (B) It is a current operative device
Explanation:–
The Buchholz relay is a gas operated relay used for the protection of oil immersed transformers against all the types of internal faults. The slow developing faults called incipient faults in the transformer tank below oil level operate Buchholz relay which gives an alarm. If the faults are severe it disconnects the transformer from the supply.
It uses the principle that due to the faults, oil in the tank decomposes, generating the gases. The 70% component of such gases is hydrogen which is light and hence rises upwards towards conservator through the pipe. Buchholz relay is connected to the pipe, as shown in Fig. Due to the gas collected in the upper portion of the Buchholz relay, the relay operates and gives an alarm.
There are many types of internal faults such as insulation fault, core heating, bad switch contacts, faulty joints etc. which can occur. When the fault occurs the decomposition of oil in the main tank starts due to which the gases are generated. As mentioned earlier, the major component of such gases is hydrogen. The hydrogen tries to rise up towards conservator but in its path, it gets accumulated in the upper part of the Buchholz relay.
When gas gets accumulated in the upper part of the housing, the oil level inside the housing falls. Due to which the hollow float tilts and close the contacts of the mercury switch attached to it. This completes the alarm circuit to sound an alarm. Due to this operator knows that there is some incipient fault in the transformer. The transformer is disconnected and the gas sample is tested. The alarm circuit does not immediately disconnects the transformer but gives the only indication to the operator. This is because sometimes bubbles in the oil circulating system may operate the alarm circuit though there is no fault.
However, if a serious fault such as internal short circuit between phases, earth fault inside the tank etc. occurs then the considerable amount of gas gets generated. Thus due to fast reduce the level of oil, the pressure in the tank increases. This energizes the trip circuit which opens the circuit breaker. Thus the transformer is totally disconnected from the supply.
The connecting pipe between the tank and the conservator should be as straight as possible and should slope upwards conservator at a small angle from the horizontal This angle should be between 10 to 11°
For the economic considerations, Buchholz relays are not provided for the transformers having the rating below 500 kVA.
Advantages
The various advantages of the Buchholz relay are,
Normally a protective relay does not indicate the appearance of the fault. It operates when the fault occurs. But Buchholz relay gives an indication of the fault at the very early stage, by anticipating the fault and operating the alarm circuit. Thus the transformer can be taken out of service before any type of serious damage occurs.
It is the simplest protection in case of transformed.
Limitations
The various limitations of the Buchholz relay are,
Can be used only for oil immersed transformers ring conservator larks.
Only faults below oil level are detected.
The relay is slow to operate having the minimum operating time of 0.1 seconds and an average time of 0.2 seconds.
2. A fuse is connected
(A) In series with the circuit(B) In parallel with the circuit
(C) Either in series or in parallel with circuit
(D) None of the above
Answer: (A) In series with the circuit
Explanation:–
Fuse is the current interrupting device which breaks or open the circuit (in which it is inserted) by fusing the elements when the current in the circuit exceeds a certain value.
A fuse is a safety device having a short length of a thin, tin-plated copper wire having the low melting point, which melts and breaks the circuit if the current exceeds a safe value. The thickness and length of the fuse wire depend on the maximum current allowed through the circuit. An electric fuse works on the heating effect of current. The fuse for protecting our domestic wiring is fitted just above our main switch on the switchboard. A fuse wire is connected in series in the electric circuits so that current flowing through the conductor to any load must also pass through the fuse.
The main fuse in domestic wiring consists of a porcelain fuse holder H having two brass terminals T1 and T2 in it. This is connected in the live wire. The other part of the fuse is a removable fuse grip G which is also made of porcelain. The fuse grip has a fuse wire fixed in it. When fuse grip is inserted in the fuse holder as shown in Figure, then the circuit of our domestic wiring is completed. So, under normal circumstances when the current is within the limit, then the fuse wire is intact and electric current is available in our wiring.
When a short circuit takes place, or when overloading takes place, the current becomes large and her the fuse wire too much. Since the melting point of fuse wire is much lower than copper wires, the fuse wire melts and breaks the circuit as shown in Figure. When the fuse wire breaks, electricity supply automatically switched off before any damage can be done to the rest of the wiring (or the electric appliances being used).
3. H.R.C. fuse, as compared to a rewirable fuse, has
(A) No aging effect(B) High speed of operation
(C) High rupturing capacity
(D) All of the above
Answer: (D) All of the above
Explanation:-
A fuse is a device, which protects the electrical wires, cables and other electrical equipment against overloading and short circuit. It contains a fuse element, generally thin wire or strips mounted on an insulated base. It is heated and destroyed while passing excessive current through it. It breaks the circuit by melting the fuse element when the current flowing in the circuit exceeds a certain predetermined value. The fuse is placed in series with the equipment or its parts which are to be protected.
Types of Fuses
They are of following two types:
Rewirable fuse.
Totally enclosed or cartridge fuse.
H.R.C fuse
Rewirable fuse
In case of a rewirable fuse, the blown out fuse element can be replaced by a new element. The fuse element can be either open or semi-enclosed. An open type is rarely used. Semi-enclosed type is generally used for low-voltage applications in houses and installations for up to 440 V. This is the cheapest and simplest form of protection.
Rewirable fuse is a set of two units. One is known as base-unit and the other is known as fuse-carrier. Fuse element can be fixed, removed and refixed in the fuse-carrier. Fuse-carrier is also known as removable Unit. The base unit is the permanently fixed unit with the switchboard.
Both the units are fixed on the porcelain chassis. The incoming wire is connected with the metal clip on one side of the porcelain base. The outgoing wire is connected with the other terminal of the porcelain base. These two terminals situated inside the base of the fuse-carrier are interconnected by fuse wire. This fuse wire is the weakest part of the circuit which melts during overloading or short circuit. After that, a new fuse wire is connected between the terminals. A constructional diagram of the rewirable fuse is shown in Figure.
Advantages of rewireable fuses:
Cheap and simple to install.
Due to a rewireable feature, it can be used for a long period of time until the contacts are in good condition.
Low maintenance required.
Disadvantages of rewireable fuse:
Exact estimation of wire diameter is a challenge at times.
Less reliable operation, as it is affected by ambient temperature.
Due to low-current breaking capacity, it cannot be employed with the circuits having the probability of large currents.
Continuous heating of fuse element results in oxidation, which causes the fuse to blow at a current below its rated current.
H.R.C Fuse
HRC fuses are used for high current applications. The HRC fuse is a high-grade ceramic barrel containing the fuse element. The barrel is filled with silica powder (sand), which helps to quench the arc produced when the element melts. The barrel is able to withstand the shocks which occur when a high fault current is interrupted. The fuse elements are in parts connected in the middle by bridges which have a very precise melting point of about 230° C. With a specific current, the temperature rises and the bridge melts producing a break in the circuit. The metal vapor diffuses with silica powder. When the fuse is blown off, the entire fuse unit (fuse with fuse carrier) is replaced. The HRC fuses are expensive. They are manufactured with current ratings of up to 1250 A..
Advantages of HRC Fuses
The capability of clearing high values of fault currents
Fast operation
Non-deterioration for long periods
No maintenance needed
Reliable discrimination
Consistent in performance
Cheaper than other circuit interrupting devices
Current limitation by cut-off action
Inverse time-current characteristic
Disadvantages of HRC Fuses
It requires replacement after each operation.
Inter-locking is not possible.
It produces overheating of the adjacent contacts.
Applications of HRC Fuses
Protection of low voltage distribution systems against overloads and short-circuits.
Protection of cables
Protection of busbars
Protection of motors
Protection of semiconductor devices
Back up protection to the circuit breaker
4. The fuse rating is expressed in terms of
(A) Current(B) Voltage
(C) VAR
(D) KVA
Answer: (A) Current
Explanation:–
An electric fuse is a device used in the household electric wiring that protects appliances from high current. Fuse wire is connected between the mains and leads of the primary circuit of an electric appliance. When the excess amount of electricity passes through the wire, the fuse wire gets heated and the heat produced fuses or melts the wire breaking the power supply to the circuit of the appliance. Thus, the electric appliance is protected from the passage of a high electric current which destroys the appliance.
A fuse rating is a current need to blow the fuse. Fuses are rated in amps and will be placed as close to a power distribution point as possible to minimize runs of unprotected cable.
Generally, the lowest rated fuse is selected consistent with reliable operation, but for emergency equipment, the highest rating is used consistent with cable protection.
5. Which one of the given fuse is bigger?
(A) DC(B) AC
(C) DC or AC
(D) Both have same fuse size
Answer: (A) DC
Explanation:–
DC Fuses
The main difference between the DC Fuses and AC Fuses is the size of the fuse. In a DC circuit, when the current exceeds the limit, the metallic wire in the fuse melts and disconnects the rest of the circuit from the power supply. When the fuse blows due to over-current, an arc can form, re-energizing faulty load.
Since DC is the constant value and is always above 0V, there is a chance of an electric arc between the melted wires, which will be difficult to avoid and turn off. Hence, usually, the electrodes of the DC Fuses are placed at the larger distance when compared to the AC Fuses.
This will minimize the chance of arc and since the distance between the electrodes is increased, the size of the DC Fuses is comparatively large.
AC Fuses
We know that AC current (and voltage) oscillates as 50 or 60 times per second and in this, the amplitude of the signal varies from minimum to maximum. At one point of this oscillations, the AC Voltage touches the 0V and hence the arc between the melted electrodes can be easily terminated.
As a result, the size of the AC Fuses can be much smaller when compared to the size of DC Fuses.
6. MCB protects a circuit from
(A) Short circuit(B) Over Load only
(C) Both short circuit and overload
(D) None of the above
Answer: (C) Both short circuit and overload
Explanation:–
Miniature Circuit breaker (MCB)
This is an electro-mechanical device which breaks (that is, opens) the circuit when the current in it exceeds a given value. Unlike fuses, which melt at 50% to 100% overload, MCBs break the circuit at just 5% to 15% overload and thus provide better safety. Note that an MCB protects the circuit against both overloads and short-circuit.
An MCB incorporates a thermal and magnetic tripping device. The load current flows through the thermal and the electromagnetic mechanisms. In normal operation, the current is insufficient to operate either device, but when an overload occurs, the bimetal strip heats up, bends and trips the mechanism. The bimetallic strip is made up of copper and iron pasted together. A bimetallic strip is used to convert a temperature change into mechanical displacement. The time taken for this action to occur provides an MCB with the ability to discriminate between an overload which persists for a very short time, for example, the starting current of a motor, and overload due to a fault. The device only trips when a fault current occurs. This slow operating time is ideal for overloads but when a short circuit occurs it is important to break the faulty circuit very quickly. This is achieved by the coil electromagnetic device.
(b) To prevent high short-circuit current from damaging circuit elements, the MCB is also provided with a magnetic coil and plunger (known as the solenoid ). At normal operating condition, the electromagnet is not strong enough to release the contact.
When the short circuit occurs in the circuit, the current energizes the solenoid which attracts the plunger which strikes the trip lever which immediately releases the latch mechanism to open the contacts. The instantaneous opening of contacts is achieved due to the rapid operation of the solenoid.
7.In which of the following equipment, current rating is not necessary?
(A) Circuit breaker(B) Relay
(C) Isolator
(D) Load break switch
Answer: (C) Isolator
Explanation:–
In order to disconnect a part of the power system for maintenance and repair purposes, isolating switches are used. These are operated after switching off the load by means of a circuit breaker. The isolators are connected on both sides of circuit breakers Thus to open isolators, circuit breakers are to be opened first.
An isolator is a off-load device which is employed only for isolating the circuit when the current has already been interrupted. They ensure that the current is not switched into the circuit until everything is in order. Isolator operates under no load condition. They are not equipped with any arch-quenching device and don’t have any specified current breaking capacity or current making capacity.
8. A fuse in a motor circuit provides protection against
(A) Overload(B) Short-circuit
(C) Open circuit, short-circuit and overload
(D) None of the above
Answer: (B) Short-circuit
Explanation:–
PROTECTION OF AC MOTORS
Contractors, fuses and overload relays are used for control of motors upto 150 hp while for larger high voltage motors circuit breakers along with protective relaying are used. Moulded Case Circuit Breakers (MCCB) is used for LT (low tension i.e. low voltages motors of high ratings. MCCBs are available for range settings upto 800 A. In general two basic protections are always provided for every motor;
Over-load protection
Short-circuit protection.
Often a motor is loaded beyond its rated capacity due to incorrect operating conditions. This leads to a motor overload, an increase in current flowing through the winding and an increase in the temperature of the winding. This results in permanent damage to the motor winding and the cables.
In a motor circuit, an overload protective device or relay provides protection to a motor against carrying sustained overcurrent which may be due to mechanical over-loading or under voltage supply. Fuses, on the other hand, provide the required degree protection against any earth’ fault or short circuit in the motor winding or at its terminals. An induction motor draws 6 to 7 times the full load current during the starting period and this current approximately remains at this high value upto about 80% of the rated speed. The fuse characteristic and the staring current characteristic of the motor should be matched. A short-circuit protection is required to protect motor conductors, overload relays, and motors from the short-circuit condition. It is achieved by using the non-time delay fuse, instantaneous trip breaker, or the inverse time-breaker.
Usually, manufacturers give recommendations regarding the fuse ratings required to cope with the motor starting surges and indicate the minimum cable sizes required to achieve a short-circuit protection. In a well-designed combination, the starter itself interrupts all the overloads up to the stalled rotor condition. The fuses should only operate in the event of an electrical fault. The starter manufacturers indicate the maximum fuse rating, which may be used with a given starter to ensure satisfactory protection.
9. Which of the following fuse is very fast in operation?
(A) KitKat fuse(B) Semiconductor Fuse
(C) Cartridge fuse
(D) High rupturing capacity type
Answer: (B) Semiconductor Fuse
Explanation:–
Semiconductor Fuse
The semiconductor fuse is very fast acting and is commonly referred to as an I2T fuse. Besides the voltage and current ratings of the fuse, the I2T rating plays an important role. For the proper protection of the SCR coordination of the SCR’s and fuse’s I2T ratings are critical. The I2T of the fuse must be smaller than the SCR’s in order to protect the SCR.
The semiconductor fuse operates at a high temperature (over 100°C) in order to make it fast acting. As with all fuses, steady-state operation of the fuse should be at approximately 80% of its current rating.
Fig. shows the internal structure of the semiconductor fuse. The fusing element is made of silver with one or more notches. The cross-section of the fuse at the notch is less. When heavy current flows through the fuse, there is high current density at the notch. Because of high current density, temperature rises at the notch and arc is developed. This arc reduces the current flow. Therefore high voltage is formed across the notch. This further increases the temperature. Therefore the fuse element is vaporized and arc length is increased. Due to increased arc length, the current further reduces.
At some stage, the current is zero and the fuse becomes open. There is no arc and the complete voltage appears across the fuse. Note that the fuse has to withstand and the voltage in open condition. The fusing element is embedded in quartz sand. This helps to conduct heat and serves as a quenching medium for the arc at the time of fusing.
Note:-
KitKat fuse is a low voltage fuse.
Cartridge fuse is a high voltage fuse.
10. A fuse is never inserted in
(A) Neutral wire(B) Negative of D.C. circuit
(C) Positive of D.C. circuit
(D) Phase line
Answer: (A) Neutral wire
Explanation:–
The main function of a fuse is to protect the electrical appliances connected in a building from damage by blowing out under faulty or high voltage condition. The fuse is always provided on the phase wire which carries the line current.
If the fuse is provided on the neutral wire instead of phase wire, then during abnormal conditions, tough the fuse wire melts and blows out, the appliances are still connected to the live wire and in case of leakage of any sort, some trouble will arise and cause damage to the appliance.
If the fault is on the neutral wire between the lamp and the fuse provided in it, the fuse will burn off because the neutral wire is at slightly higher potential w.r.t earth and thus the fault current will flow through the neutral wire and the fuse wire blows out.
But even then, current will flow through the live wire. This will cause considerable damage to the wiring and the appliances. So, for these reasons, the fuse is always provided in phase and never on neutral.
The event that causes the fuse in the plug to blow is
A person touches the earth wire.
(The earth wire is considered protective wiring. In normal operations, the earth wire does not carry a current. Anyway, the fuse is also not connected to the earth wire. So this event will not cause the fuse to blow.)
A person touches the neutral wire.
(The neutral wire does not have a voltage relative to ground, so it does not have a current, even if connected. Further, the fuse is not connected to the neutral wire. So, even if it conducts a current, this event will not cause the fuse to blow.)
A person touches the live wire.
(Depending on the dampness of the touch (about 1000 Ω) or insulation of the person touching the live wire, the wire might conduct current through the person. The current is typically in the range of milliamperes. If the fuse is connected to the live wire, this event will not cause the fuse (rated at 3 A) to blow; though harm might occur to the person.)
The live wire touches the neutral wire.
(There is no resistive load between the live and the neutral wires. Once connected, a large current will flow. Since the fuse is connected to the live wire, this event will cause the fuse
10. A double squirrel-cage induction motor has
(A) Two series winding in stator(B) Two parallel windings in stator
(C) Two parallel winding in Rotor
(D) Two rotors moving in opposite direction
Answer: (C) Two parallel winding in Rotor
Explanation:
In squirrel cage motor there is no provision made for adding external resistance because the end rings are permanently shorted with the rotor conductor, therefore, the starting torque of squirrel cage induction motor is very poor due to low rotor resistance.
The starting torque could be increased by increasing starting resistance but due to high current in the rotor during starting will increase the copper loss which will decrease the motor efficiency.
As the name suggests the double squirrel-cage induction motor has two sets of parallel squirrel-cage winding in the same rotor.
The outer cage bar have smaller cross-section area as compared to inner cage bar
The outer cage bar is made up of high resistive material such as aluminum, brass etc.
The outer cage has less leakage flux linkage because of their relatively open slots hence it has lower reactance.
The Lower cage bar has high cross-section area and it is made up of low resistance material like copper.
The upper and lower cage bar is separated by narrow slit or construction.
The inner cage has high leakage flux linkage due to the presence of slit hence it has high self-inductance.
Therefore during starting period, it gave high resistance means high starting torque whereas it gives low resistance during running condition.