This cause deterioration of the insulation, as well as involving a waste of energy which would increase the running costs of the installation.
The resistance between Phase-Phase-Neutral-Earth must never be less than 0.5 M Ohms for the usual supply voltages.
Transformer windings are never left floating for insulation resistance measurements.
Short circuit each winding of the transformer at the bushing terminals.
The recommended practice in measuring insulation resistance is to always ground the tank (and the core).
In the case of a very large installation where there are many earth paths in parallel, the reading would be expected to be lower.
If this happens, the installation should be subdivided and retested, when each part must meet the minimum requirement.
Jignesh Parmarjiguparmar - Jignesh Parmar has completed M. He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad, India.
He has published numbers of Technical Articles in “Electrical Mirror”, “Electrical India”, “Lighting India”, “Industrial Electrix”(Australian Power Publications) Magazines.He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.Home remodeling projects can be huge undertakings, making it crucial for homeowners to entrust their projects to reputable and experienced contractors.To test the other, both two-way switches should be operated and the system retested.If desired, the installation can be tested as a whole, when a value of at least 0.5 M Ohms should be achieved. If the ground cannot be removed, as in the case of some windings with solidly grounded neutrals, the insulation resistance of the winding cannot be measured.