RRB ALP Electrician Trade — Solved Practice Question Bank

Q: In a star-connected 3-phase supply with phase voltage 230V, what is the line voltage? Sol: Line voltage = √3 × phase voltage = 1.732 × 230 ≈ 398V (commonly rounded to 400V in practice). Q: Why is a star-delta starter used for large induction motors? Sol: It reduces the starting current to roughly 1/3 of direct-on-line starting current by connecting the motor windings in star (lower voltage per winding) during start-up, then switching to delta for normal running. Q: Why must a DC series motor never be run without a mechanical load? Sol: Because its speed rises inversely with load, and with no load at all it can accelerate to a dangerously high, potentially destructive speed.

Q: What is the standard color code for the earth wire in Indian domestic wiring? Sol: Green. Q: What does an ELCB/RCCB protect against that a standard MCB does not? Sol: Electric shock from earth leakage current (e.g. current passing through a person's body to ground) — an MCB only protects against overload and short-circuit current, not leakage current. Q: Why can a transformer not operate on pure DC supply? Sol: A transformer works by mutual induction, which requires a continuously CHANGING magnetic flux; a steady DC current produces a constant (non-changing) flux, inducing no EMF in the secondary winding after the initial transient.