<P> Analog meters can measure voltage and current by using power from the test circuit, but require a supplementary internal voltage source for resistance testing, while electronic meters always require an internal power supply to run their internal circuitry . Hand - held meters use batteries, while bench meters usually use mains power; either arrangement allows the meter to test devices . Testing often requires that the component under test be isolated from the circuit in which they are mounted, as otherwise stray or leakage current paths may distort measurements . In some cases, the voltage from the multimeter may turn active devices on, distorting a measurement, or in extreme cases even damage an element in the circuit being investigated . </P> <P> It is safest (for both multimeter, circuit under test, and the operator) to disconnect a component from its circuit, and almost always, to remove power from the device being investigated . Removing all power connections from a piece of mains powered equipment before testing (and ensuring that all large capacitance devices are discharged safely) is the safest choice . Leaving equipment attached to the mains supply while making measurements should be only a very carefully considered alternative choice . Amongst other issues, there are interactions between the ground arrangements for wall powered test equipment, and the device under test, which are unsafe, and can damage test equipment and the device under test . This is especially so when there is a fault, suspected or not, in any of the interconnected devices . Battery powered test equipment may be the safest choice in such situations . </P> <P> Meters intended for testing in hazardous locations or for use on blasting circuits may require use of a manufacturer - specified battery to maintain their safety rating . </P>

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