Insulation resistance testing

Aside from it being ludricrously dangerous to you and the ohmmeter?

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Well that wasn’t clear I meant loads connected but not switched on isolated back at the board.

That's not the way you do an insulation test.

Bill
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Ohm's law
Not just a good idea;
It's the LAW!

There are some ground detectors for ungrounded ac system which continuously or momentarily apply a dc voltage to ground, measure the leakage current and compute a composite resistance. That checks everything from the supply (delta or ungrounded wye) to the cable to the loads. It's not something you should try yourself unless you know what you're doing (or buy a detector for the application). It's also is not feasible for grounded systems.

> due to cables being connected through the load

There should not be any ground connections at the load (other than safety ground / protective earth)

> and neutral being tied to earth at the installation?

yes this would be a path to ground that would render the test infeasible. also any grounded wye transformer windings would be a low-resistance path to ground which would render the test infeasible and could disrupt the transformer if dc were applied.

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(2B)+(2B)' ?