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Why is it important to establish an equipotential zone:
To eliminate step and touch voltage potential hazards
What are current Transformers used for? Range? Effect of opening circuit?
- For metering and protection
- 0-5 Amps
- Opening secondary side will cause dangerous situation to allow secondary to see dangerously high voltage(don’t open)
What are potential transformers used for? Range?
- For metering and Protection
- 0-120 volts
- OK to open secondary
Voltage is the electric pressure that pushes electrons through a conductor.
Resistance to current flow in a purely resistive circuit.
Define a Coulomb.
- A quantity measurement of electrons.
- 1 Coulomb = 6.25 * 10^18 electrons
How can the polarity of an electromagnet be determined if the direction of current flow is known?
The left hand rule… thumb = north magnetic pole and fingers = direction of current flow.
Define Flux Density.
A measurement of magnetic strength
When a magnet eventually reaches a point where its strength increases only slightly with an increase in current.
Define Residual Magnetism.
The amount of magnetism left in a material once the magnetizing force has stopped.
State Kirchoff’s voltage law.
The algebraic sum of voltage drops and sources in a closed circuit must = 0.
State Kirchoff’s current law.
The algebraic sum of currents entering and leaving a point must = 0.
Name 3 factors that determine the amount of induced voltage in a coil.
- -Number of turns of wire
- -Strength of the magnetic field (flux density)
- -Speed of the cutting action
Describe current/voltage relationship in a purely inductive circuit
Current lags voltage by 90 degrees
Describe current/voltage relationship in a purely capacitive circuit
Current leads voltage by 90 degrees
To what is inductive reactance proportional?
The inductance of the coil, and frequency of the line.
What is frequency?
The number of complete cycles per second.
What is the formula for calculating instantaneous voltage?
E(instant) = E(max) x sin angle theta
List 3 factors that determine capacitance of a capacitor.
- -Surface area of the plates
- -Distance between the plates
- -Type of di electric used
Name two factors that determine the capacitive reactance of a capacitor.
Frequency and capacitance.
What is a transformer? What is the operating efficiency
- A machine which operates magnetically and changes the voltage, current and impedance without changing frequency.
- Operating Efficiency is 90%-99%
What is the formula for Inductive Reactance(Xl)
What is the formula for Capacitive Reactance(Xc)
What is the formula for power in a 3 phase circuit
What are 3 ways to calculate Power Factor(PF)
- -PF=cos(angle theta)
What is a circuit breaker used for
To interrupt both load and fault current
What is capacitive reactance
The current limiting property of a capacitor in an AC circuit
What is inductive reactance
The current limiting property of an inductor in an AC circuit
Anytime a circuit breaker, disconnect switch or cutout is operated, one of the following six consequences occurs
- -pick up load
- -drop load
- -make parallel
- -break parallel
What is a ground fault
A current path to ground that was not the intended path
What range of electric current causes death? How?
- 50-200 milliamperes (mA)
- Through defibrillation of the heart
The measure of power in an electric circuit
What are 3 ways to calculate Power(single phase circuit)
The rate of flow of electrons
Define Ohm's Law
- The electric relationship between voltage, current, resistance
- E = I * R
What is a dielectric?
A material with an electric insulating quality
Define Power Factor
Comparison of true(real) power to apparent power in an AC circuit
What is the formula for calculating impedance in a Reactor
Rx = kV / (1.732 * I rating) * Z%