What is needed for proper functioning of a transformer being checked with an ohmmeter?

For proper functioning and accurate testing of a transformer with an ohmmeter, isolation from the circuit is essential. When a transformer is isolated, it ensures that the test is measuring only the winding resistance and not influenced by any other components or circuits connected to it. This isolation allows for a clear and accurate reading of the resistance values of the transformer's windings, which is crucial for diagnosing issues such as short circuits or open circuits within the transformer itself.

Using an ohmmeter on a transformer while it is connected to a live circuit could lead to misleading readings or even damage to the meter due to the presence of voltage. Other methods, such as visual inspections or temperature checks, do not provide the necessary measurement of winding resistance and therefore are not suitable to determine the transformer's condition in the same way an isolated ohmmeter test would.