Electrical Engg (EEE) - MCQ Practice Questions

The dielectric strength of air at standard conditions (1 atm, 25°C) is approximately $30\text{kV/cm}$. For a 1 cm gap, the breakdown voltage is $30\text{kV/cm}\times 1\text{cm}=30\text{kV}$. This is a fundamental reference value in high voltage engineering.

Paschen's law states that the breakdown voltage $V_b$ of a uniform-field gas gap is a function of the product $pd$, i.e., $V_b=f(pd)$. This is expressed as the Paschen curve, which shows a minimum breakdown voltage at an optimum $pd$ value. Both pressure and gap distance affect breakdown only through their product.

In a Marx generator, $n$ capacitors are charged in parallel (each to $V_0$) and then switched in series during discharge. The theoretical maximum output voltage is $n\cdot V_0$. In practice, the actual output is slightly less due to stray capacitances and resistive losses, but $n\cdot V_0$ is the ideal value.

According to IEC 60060-1, the standard lightning impulse waveform has a front time of $1.2\mu \text{s}$ and a time to half-value of $50\mu \text{s}$, denoted as $1.2/50\mu \text{s}$. The $8/20\mu \text{s}$ waveform is used for surge current testing, and $250/2500\mu \text{s}$ is the standard switching impulse.

At balance in the Schering bridge, the loss tangent (dissipation factor) of the test capacitor is given by $\tan \delta =\omega R_4{C}_4$, where $\omega =2\pi f$ is the angular frequency, $R_4$ is the variable resistance, and $C_4$ is the capacitance in the bridge arm. This relationship allows direct measurement of dielectric loss.

Corona onset occurs when the electric field at the surface of the conductor equals the critical ionization field of surrounding air, approximately $30\text{kV/cm}$ (or $3\text{MV/m}$) at standard atmospheric conditions (1 atm, 25°C). This is the point at which air molecules begin to ionize, causing partial discharge (corona). Temperature of the conductor or insulator rating are not the governing criteria.

An electrostatic voltmeter operates on the principle of electrostatic attraction between charged plates. Its input impedance is extremely high (essentially capacitive only), so it draws negligible current from the high-voltage source. This eliminates loading errors, which is critical when measuring high voltages where even small currents can disturb the source. It cannot practically measure up to 1 MV directly and is less sensitive at low voltages.

The divider ratio is $n=(R_1+R_2)/R_2=(9000+1000)/1000=10$. The actual high voltage is $V_{\text{HV}}=n\times V_{\text{out}}=10\times 10\text{kV}=100\text{kV}$. This principle of resistive voltage division is fundamental in high voltage measurement systems.

Townsend's second ionization coefficient $\gamma$ represents the number of secondary electrons emitted from the cathode surface per positive ion (or photon/metastable) bombarding it. Townsend's first coefficient $\alpha$ gives the number of ionizing collisions per unit path length by one electron. Together, $\alpha$ and $\gamma$ determine the condition for self-sustained discharge: $\gamma (e^{\alpha d}-1)=1$.

Sphere gaps are used as a standard measuring device in high voltage laboratories because the electric field between two equal spheres is nearly uniform (for small gap-to-diameter ratios), and the sparkover voltage is a well-defined, reproducible value that depends on sphere diameter and gap spacing. Standard tables (IEC 60052) provide sparkover voltages for different sphere sizes and gaps under specified atmospheric conditions, allowing accurate voltage measurement.

This is the standard definition of dielectric strength — the electric field intensity at which breakdown occurs.

For a coaxial cylindrical geometry, the electric field varies as $E(r)=\frac{V}{r\ln (r_2/r_1)}$. The maximum field occurs at $r=r_1$ (inner conductor surface):

Numerically, $\ln (4/1)=\ln 4\approx 1.386$, so the field is highest at the inner conductor.

To minimize $E_{\max }=\frac{V}{r_1\ln (r_2/r_1)}$ for fixed $V$ and $r_2$, we differentiate with respect to $r_1$ and set to zero. This yields the condition $\ln (r_2/r_1)=1$, i.e.,

This is the well-known optimum ratio for coaxial cable insulation design.

By definition of the standard impulse waveform $1.2/50\mu \text{s}$: the front time is $1.2\mu \text{s}$ (time to peak) and the tail time is $50\mu \text{s}$, which is the time from virtual origin to the point where the voltage decays to

The $50\mu \text{s}$ parameter is specifically defined as the time to half the peak value on the tail.

The Streamer (Kanal) criterion states that an electron avalanche transforms into a streamer when the number of ion pairs in the avalanche head reaches a critical value of approximately $10^8$. This requires:

Option C describes the Townsend (self-sustaining discharge) criterion, not the Streamer criterion.

Grading rings (also called corona rings or stress control rings) are metallic toroidal rings fitted at the high-voltage ends of insulators and bushings. Their primary function is to redistribute the non-uniform electric field along the insulator surface, reducing the peak electric stress concentration at the metal-insulator junction. This prevents premature corona inception and surface flashover. They do not primarily serve a mechanical, grounding, or leakage-distance function.

For healthy, high-quality cable insulation (such as XLPE or impregnated paper), the dissipation factor $\tan \delta$ is very small, typically in the range:

Values significantly higher than this indicate moisture ingress, aging, or contamination. A $\tan \delta$ approaching $0.1$ or higher would indicate severely deteriorated insulation.

The apparent charge is the charge that, if injected at the terminals of the test object, would give the same reading on the measuring instrument as the actual partial discharge.

Back-flashover is a specific phenomenon where lightning strikes the tower top or shield (ground) wire. The large lightning current $I$ flowing through the tower footing resistance $R_f$ raises the tower potential to $V_{\text{tower}}=I\cdot R_f$. If this potential exceeds the insulator string's impulse flashover voltage, a flashover occurs from the grounded tower structure back to the (relatively lower potential) line conductor — hence the term 'back-flashover'. This is distinct from a shielding failure (direct stroke to conductor) and is strongly influenced by the tower footing resistance.