Explain Ohm's law and how it applies to diagnosing an electrical fault in locomotive wiring.

• A. P = V × I; used to calculate power for troubleshooting.
• B. V = I × R; used to estimate voltage drop but not enough.
• C. I = V × R; not a valid relation.
• D. V = I × R; used to estimate current for a given resistance or predict voltage drop; helps determine whether observed current matches expected values.

Answer: (D)

Ohm's law describes how voltage, current, and resistance relate, and that link is what you use when diagnosing locomotive wiring faults. The form V = I × R is the practical workhorse here: it lets you figure out the current that should flow if you know the supply voltage and the resistance along the path, or it lets you predict how much voltage will drop across a section for a given current. In a fault investigation, you compare measured values to the predictions from this equation. If the current you observe differs from what V = IR says for the known resistance, there’s likely a problem such as a high-resistance connection, damaged insulation, or a short. This approach directly supports diagnosing where and why a fault is occurring. Power, given by P = V × I, is useful for understanding energy use, but it doesn’t by itself show the relationship between V, I, and R for diagnosing a fault. The incorrect I = V × R form isn’t a valid relation. The correct statement is that V = I × R is used to estimate current for a given resistance or predict voltage drop and to check whether observed values align with expectations.