Q:What is the difference between Isolator annd Circuit Breaker?

                 A:Isolator is a off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintanance on downstream circuits. it is manually operated and does not contain any solenoid unlike circuit breaker. it should not be operated while it is having load. first the load on it must be made zero and then it can safely operated. its specification only rated current is given.But circuit breaker is onload automatic device used for breaking the circuit incase of abnormal conditions like shortcircuit, overload etc., it is having three specification 1 is rated current and 2 is short circuit breaking capacity and 3 is instantaneous tripping current.

What is the difference between MCB & MCCB, Where it can be used?

                 MCB is miniature circuit breaker which is thermal oprated and use for short circuit protection in small current rating circuit. MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition.under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A.

Why star delta starter is preferred with induction motor?

Star delta starter is preferred with induction motor due to following reasons:
• Starting current is reduced 3-4 times of the direct current
due to which voltage drops and hence it causes less losses.
• Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.
• In addition, starting torque is increased and it prevents the damage of motor winding.

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 circuittofully 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 10¹⁸electrons, 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

But,

Why make up a new unit for battery capacity rating when an old one unit is doing just fine? L

Of course! To make your lives as technicians and students more difficult.   

As they do for electricity units… i.e. 1 Unit of Electricity = 1kWh = 1 board of Trade Unit…

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).