Resistors and reactors, for use over 600 volts, shall not be installed in close enough proximity to combustible materials to constitute a fire hazard and shall have a clearance of not less than ___ from combustible materials.

Resistors and reactors, for use over 600 volts, shall not be installed in close enough proximity to combustible materials to constitute a fire hazard and shall have a clearance of not less than 300 mm from combustible materials.

Explanation:

The hazards associated with high power industrial resistors are primarily due to their open construction, which is necessary for cooling. The exposed conductors which make up the resistors can be not only a shock hazard but also a thermal burn hazard.When a resistor fails, it either goes open or the resistance increases. When the resistance increases, it can burn the board, or burn itself up.Avoid touching non-flammable resistors in operation; the surface temperature ranges from approximately 350 °C to 400°C when utilized at the full rated value. Maintaining a surface temperature of 200°C or less will extend resistors service life.Do not apply power to a circuit while measuring resistance. When you are finished using an ohmmeter, switch it to the OFF position if one is provided and remove the leads from the meter. Always adjust the ohmmeter for 0 (or in shunt ohmmeter) after you change ranges before making the resistance measurement.

The correct answer is  "c".  it is "c" because melting is an endothermic change, and endothermic get's energy from surroundings.