A resistance measurement must be performed with the power off. The meter measures resistance by putting a small current across the probes and measures the resulting voltage.
Ohmmeters always contain a battery or some other internal source of electrical power so that the component under test may be supplied with a small amount of current, in order to measure how hard it is for current to go through.
Unlike voltmeters or ammeters, ohmmeters must contain their own power sources. Therefore, they should not be used to measure the resistance of an energized or powered circuit or component.
