Megohmmeters generate high voltage, low current signals for testing the breakdown strength of electrical insulation. They are called megohmmeters because the insulating resistance of a properly designed product is expressed in megohms (MO). Insulation-resistance (IR) is the DC resistance between two defined points at a specific voltage in a controlled environment. Typically, the applied voltage is 500 VDC and the environment is 25° C with less than 50% relative humidity. EN 60204-1, a standard for machinery safety, states that for test voltages at 500 VDC, the minimum insulation resistance is 1 MO. EN 60335-1, a safety standard for household and similar electrical appliances, states that for test voltages at 500 VDC, the minimum insulation resistance is 5 MO.