MCQ ON CIRCUIT BREAKER

if we check dc circuit through a tester its glow. why ?

if indeed you do have one of the NEON type testers you will see two rods inside a glass bulb. With AC, both will tend to glow. Actually they are taking turns glowing at either 50 or 60 times per second, depending upon where in the world you live. For DC, only one electrode will glow. It will indicate the NEGATIVE lead. These units usually require somewhere around 55 to 60 volts to light up. Inside the housing, there is a resistor to limit the amount of current the bulb can draw.

What is inductance?

When alternating current passes through a conductor it will induce timely changing alternating magnetic field. According to lenz’s law this changing magnetic field induces an emf which is opposing it’s own cause (i.e. current through conductor). This opposition is called inductance. It is further segregated in to two parts self inductance and mutual inductance.

What is the relation between power factor and power consumption?

As we all know Total (Apparent) power = Real or Active power + Reactive power.

In a very simple language, power factor basically means, out of the total apparent power consumed by the load, how much useful or active or real power is present in it. In fact power factor is indicator of this above mentioned fact.

So, naturally power factor of unity (ideal case) indicates whole total power which is consumed by load is Active power or Real power, which means there is no reactive power consumed by load (case of Purely Resistive load).

As against this zero power factor indicates that no power will be transferred between the source and the load i.e whole power consumed by load will be just reactive power (just to get magnetized), no Active or real power is transferred which is actually used for doing useful work.

Consequences of low power factor and its effect on total power consumed by load:-

So, consider two loads, Load 1 and Load 2. Both have same power requirement for doing work (i.e Active power requirement). Now load one have low power factor, so as explained above out of total power which it draws from the source, Active power will have lesser share in it. load 2 has high power factor so the total power which it draws from source will contain more amount of Active power as compared to load 1 having lower power factor.

So, as the Active power requirement of both the loads is same, load 1 will have to draw more Apparent power (total power) as compared to load 2 which has high power factor, in order to have same amount of Active power as load 2.

Takeaway for you from this Answer:

A load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred.

Thus, a load with low power factor will draw more apparent power as compared to a load with high power factor for the same amount of useful power transferred.

The higher current increase the energy lost in the distribution system, and require larger wires and other equipment.

Because of the above fact, Electrical utilities usually charges higher cost to industrial or commercial customers, where there is a low power factor.

We all feel electrical shocks. Which is really responsible for the shock, voltage or current?

The ultimate cause is the amount of electricity flow in the body, so it can be said to be current.

However, there are other factors, one of which is voltage, to consider:

1) Path of flow: If the internal organs are in the path of current flow, much smaller currents can cause a fatality.

Hence, if the shock is applied between one arm and the other arm, such as when holding Live wire in one hand and Neutral in the other, the current can pass through the heart, causing death. The same source of electricity may perhaps not be fatal if the points of the shock delivery are an arm and a leg, for example.

This is probably also the reason that execution by electrocution is done by placing the electrodes across the head. So that the current path is through the brain.

2) Resistance of the body: This is the resistance offered by the human body to electric flow. This can vary depending on moisture level. It can also vary with voltage as mentioned in the next point.

3) Voltage: Important to note that the resistance of the body changes according to voltage applied! Hence, it is not just a simple linear increase of current with voltage (in accordance with ohm's law).

The skin is an insulator, and contributes greatly to the high resistance of the body (order of Mega-ohms) for low voltages. However, at high voltages, dielectric breakdown of skin occurs which drastically reduces resistance of body by almost 1000 times!

This is what makes voltages above a certain level fatal, since it can cause current flow to increase drastically.

4) Duration of shock: In general, higher the duration of shock - greater the damage.

As an interesting example, consider static electricity. We can easily build up static potentials in excess of 10000 Volts, which is enough to break down air at short distances! However, we don't see any deaths caused by handshakes. :)

This is because, while the potential generated by static can be huge, the total amount of source charge available is really small. So the duration of the current flow is also too small to have any effect.

So to answer the question, while it is the flow of electrons (current) that causes us problems, there are other factors (including voltage) which can significantly affect how much of this electron flow occurs.

A Step Up transformer _____________. how ?

Ans: Step Up the level of Voltage & Step down the level of current

A Step up transformer only step up the level of voltage and step down the level of current.

Because the input power is same.

So according to P=VI→ I = P/V…. We can see that, when Voltage increases, current decreases.

So in Step up transformer, input power is same, therefore, when voltage increases, then current decreases.

If the frequency of 3-phase supply to the stator of 3-phase induction motor is increased, then synchronous speed is ________? Why ?

Ans:------ Increased

Explanation:   As we know that;   f = NSP/ 120

It is clear that f ∝ NS i.e., frequency (f) is directly proportional to the Synchronous speed (NS).

In more clear words, when frequency increases, Speed also increases.

Why Battery rated in Ah (Ampere hour) and not in VA. ?

Battery stores charge in the form of chemical energy and then converts it into electrical energy to utilize for a specific time. The amount of available charge is the capacity of a cell or battery which may be expressed in Ah (Ampere-hour). Moreover, in a charged battery, the numbers of molecules are limited to create a flow of electron in electric circuits, so, there must be a limited number of electrons in a cell/battery which they motivate through a circuit to fully discharge.

Now we have the option to rate the battery capacity in Number of flowing electrons for a specific time, but, it would be a headache, because there are a vast number of electrons in it.  So we have another option (1C (Coulomb) = 6.25 x 1018 electrons, or 6,250,000,000,000,000,000 electrons.

In addition, 1A (Ampere) = 1 coulomb of electrons per second and,

1h = 3600 Seconds

Therefore;

1Ah = (1A) x (3600s) = (C/s) x (3600s) = 3600 C.

∴ A (1 Ampere) = 1 Coulomb per second = C/s

Why Generator & Alternator rated in kVA. Not in kW?

The power √3 VL IL Cos φ delivered by the alternator for the same value of current, depends upon p.f. (Power Factor=Cos φ) of the load. But the alternator conductors are calculated for a definite current and the insulation at magnetic system are designed for a definite voltage independent of p.f. (Cos φ) of the load. For this reason apparent power measured in kVA is regarded as the rated power of the alternator.

Why Transformer Rating In kVA, Not in KW?

There are two type of losses in a transformer;

1. Copper Losses

2. Iron Losses or Core Losses or  Insulation Losses

Copper losses ( I²R)depends on Current which passing through transformer winding while Iron Losses or Core Losses or  Insulation Losses depends on Voltage.

That’s why the Transformer Rating may be expressed in kVA,Not in kW.

Why Motor rated in kW instead of kVA?

We know that Transformer rating may be expressed in kVA as well as Generator and Alternator rated in kVA Designer doesn’t know the actual consumer power factor while manufacturing transformers and generators i.e. the P.F (Power factor) of Transformer and Generator/Alternator depends on the nature of connected load such as resistive load, capacitive load, and inductive load as Motors, etc. But Motor has fixed Power factor, i.e. motor has defined power factor and the rating has been mentioned in KW on Motor nameplate data table. That’s why we are rated Motor in kW or HP (kilowatts/ Horsepower) instead of kVA.

In addition, Motor is a device which converts Electrical power into Mechanical power. In this case, the load is not electrical, but mechanical (Motor’s Output) and we take into the account only active power which has to be converted into mechanical load. Moreover, the motor power factor does not depend on the load and it works on any P.F because of its design.

Why AC rated in Tons, Not in kW or kVA? A Guide about Airconditioner and Refrigeration

Why AC rated in Tons, Not in kW?

AC (Air-conditions and Refrigeration are always rated in Tons.

Air conditioners are always rated in Tons capacityinstead of kW because Air conditioners are designed on the basis of quantity of heat removal from room, hall or specific area. Quantity of heat is termed in Tons means if an air conditioner is able to remove 1000 kilocalories of heat or 4120 kilojoules or 12000 BTU of heat in an hour that AC rated as 1 Ton of AC because 1000 Kilocalories or 4120 kilojoules or 12000 BTU equal to one Ton of heat. Also, this is the same case for freezer and refrigerator i.e. refrigeration system.

Good to know:

BTU = British thermal unit. A measurement of heat, specifically, the amount of heat needed to raise the temperature of a pound of water by 1°F.

Definition of Ton

A Ton of refrigeration (RT) is approximately equivalent to 12,000 BTU/h or 3,516.8528 W or 4.7142Hp.

A Ton of refrigeration (RT) is a unit of power used to describe the heat-extraction capacity of air conditioning and refrigeration equipments. It is defined as the heat of fusion absorbed by melting 1 short ton of pure ice at 0 °C (32 °F) in 24 hours.

How many kW and HP are there in 1 Ton?

1 Ton = 3.5168525 kW = 4.714Hp

Explanation

1 Ton = 12,000 BTU/h

1 Watt = 3.412141633 BTU/h

1 Ton = 12,000 / 3.412141633 = 3,516.8528 Watts = 3.5168528 kW.

1 Ton = 3,516.8528 Watts = 3.516 kW.

Also

1 Ton = 3,516.8528W / 746 = 4.7142798928 Hp →→→ (1 Hp = 746 Watts)

1 Ton = 4.714 Hp

How to convert Ton to Kw and vice versa?

One RT(Refrigeration Ton) = 3.5168528 kW…

1 RT = 3.5168528 kW

1 kW = 0.284345 RT(Refrigeration Ton)

1 kW = 0.28434517 RT

So,

The power P in kW = Power P in RT (Refrigeration Ton) times 3.5168528….

P(kW) = P(RT) × 3.5168528

Example

Convert 3 Ton AC into kW i.e. Convert 3 RT to kW.

Solution:

P(kW) = 3 RT × 3.5168528

P(kW) = 10.55 kW

3 Ton AC = 10.55 kW

How much Current in Ampere will a 2 Tons AC draw in Single Phase & Three Phase System?

Suppose, There are 230V and Power factor = Cosθ = 0.95 in Single Phase AC system…

1 Ton = 3,516.8528 Watts = 3.516 kW.

2 Ton = 2 x 3.516 kW = 7.032kW = 7032W

Power in a Single Phase AC System

P = VxI Cosθ and current…

I = P / (V x Cosθ)….. Where Cosθ = Power factor

I = 7032W / (230V x .95)

I = 32.18 A

Therefore, a 2 Ton AC (Air-condition in Single Phase AC system will take 31.18 Ampere Current

And in Three Phase System

Suppose, There are 440V and Power factor = Cosθ = 0.85 in Three Phase AC system…

Power in a Three Phase AC System

P =√3 x VLxIL Cosθ and current….

I = P /( √3xVxCosθ)

I = 7032W / (1.732 x 440V x .85) Where Cosθ = Power factor and √3 = 1.732

I = 10.855 A

Therefore, a 2 Ton AC (Air-condition in Three Phase AC system will take 10.855 Ampere Current

Good to Know:This is just calculation based on Electrical formulas. In real, Air conditioner current depends a lot on operating conditions such as ambient temperature, refrigerant pressure, Energy Efficiency Ratio (EER) etc. for instance, if EER is 6, then input power for 2 Tons Air conditioner is 24000BTU/ 6 = 4000 watts..

If this is a 230 volt system, then air conditioner load current would be = 4000/(230x.95) = 18.5 A

For More detail…Check the Air conditioner Name plate rating.

Another similar rating is Coefficient of power (COP) which is the output power in watts divided by input power, so with a COP = 1.8, for instance, input power for 2 Tons Air conditioner  is 7032W / 1.8 = 3906 watts. Now you can find current by using the above method which is equal to 18A approx.

How many 2 Ton A.C (Air conditioner) can I run on a 25 kVA Generator?

2 Ton = 2 x 3.516 kW = 7.032kW = 7032W

The Efficiency of Utility Power Generator is 90% approximately.

Efficiency of Generator = 25kVA x (90/100) = 22.5kVA

Now the Number of 2 Ton AC (Air conditioners) which you can run on a 25 kVA Generator smoothly..

22.5kVA / 7032W

= 3

So you can run Three Air conditioners of 2 Tons each on a 25kVA Generator.

What is the suitable rating of MCB for 2 Ton and 1 Ton AC (Air conditioner) and why?

As we have calculated the load current for 2 Ton AC Air conditioner…

Calculated Current for 2 Ton A.C = I = 32.18 A

Now 40A Class “C” MCB (miniature circuit breaker) would be suitable for 2 Ton AC (air-condition) because in starting time it takes more current of the full load current

And 20 A Class “C” MCB would be better for 1 Ton AC (air-condition)

Good to Know:

Class “’C’ Type MCBs

Class “C” Type MCBs are suitable for installations with high inrush of current at the starting switching time. in other words, equipment and devices having inductive loads such as air-conditioners, induction motors, fluorescent lamps, transformers etc.

WHY POWER PLANT CAPACITY RATED IN MW AND NOT IN MVA ?

For the following reasons, a Power plant capacity rating may be expressed in MW instead of MVA.

In a Generating station, the prime mover (Turbine) generates only and onlyActive Power. That’s why we rated a power plant capacity in MW instead of MVA. Its mean no matter how large your generator is, but it depends on the capacity of the  engine (Prime mover/Turbine) I.e. a 50MW turbine connected to a 90MVA alternator in a power plant will generate only 50MW at full load. In short, a power plant rating is specified in terms of prime mover /Turbine (Turbine rating may be seen by nameplate rating which is in MW or Horsepower (HP) not in MVA) and not by the alternator set coupled to it.

Another thing is that, electric power company charges their consumer for kVA while they generate kW (or MW) at the power station (Power plant).They penalize their consumer for low Power factor because they are not responsible for low power factor and kVA but you. Moreover, in power plant, power factor is 1 therefore MW is equal to MVA …… (MW = MVA x P.f).

Another interesting & funny answer by one of our Facebook page fan…“Power House means, house of the Power, and we know that the unit or power is Watt. That’s why we rated power plant capacity in MW and not in MVA”