Explanation: When making "power-on" measurements on a switching power supply, it's essential to understand its internal design, particularly regarding isolation from the AC mains. Many switching power supplies (SMPS) are designed without a line isolation transformer, meaning parts of their internal circuitry, especially on the primary side, can be directly connected to the "hot" or "neutral" wires of the AC line. Connecting the ground lead of standard mains-powered test equipment (like an oscilloscope or some multimeters), which is typically earth-grounded, to a point in a non-isolated SMPS that is not at earth ground potential creates an immediate short circuit to ground. This destructive short can cause component failure within the power supply (e.g., rectifiers, switching transistors), damage the test equipment, or pose a serious electrical shock hazard to the operator. Using a line isolation transformer for the supply under test prevents this danger by floating the entire circuit relative to earth ground. Why other options are incorrect: A) Discharging filter capacitors is a critical safety step to perform *before* servicing or making measurements on a powered-off supply, not during "power-on" measurements. C) It is absolutely possible to cause component failure with ungrounded test equipment if used improperly (e.g., applying incorrect voltages or connecting to wrong points). D) While familiarity helps, a schematic is almost always crucial for accurate and safe measurements on complex circuits like switching power supplies, not something to be avoided.