New York State Master Electrician Practice Exam

What is the value in megohms of a resistor required to discharge a 30 Mfd capacitor energized to 650V?

1. 5 MΩ

2. 8 MΩ

3. 10 MΩ

4. 12 MΩ

The correct answer is: 10 MΩ

To find the value of the resistor needed to discharge a capacitor safely, we can use the formula that relates the time constant of an RC (resistor-capacitor) circuit, which is given by the product of resistance (R in ohms) and capacitance (C in farads). The time constant (τ) is also the time it takes for the voltage across the capacitor to drop to approximately 37% of its initial value. The equation for the resistor required can be derived from the relationship between voltage (V), capacitance (C), and the desired safe discharge time. In this case, the goal is to ensure the capacitor discharges to a safe level without posing a shock hazard. Given a capacitor value of 30 microfarads (Mfd) and an initial voltage of 650 volts, as well as the understanding that a typical discharge time is approximately five time constants (5τ) for a capacitor to be considered safe to work on, we can establish the formula: R = (Time constant in seconds) / Capacitance in farads If we assume a reasonable time constant value of 5 seconds for the capacitor to effectively discharge: 1. Convert capacitance from microfarads to farads: