Understanding Insulation Resistance Testing Highest testing voltages

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Kiriakos GR:
Insulation Resistance Testing theory this is well documented due experts, all from this market sector.
Rule of thump this is double testing voltage than operating voltage.

But in this market of Insulation Resistance testers, there is product offerings which differentiate at Max testing voltage.
What any one interested to buy a new Insulation Resistance tester within 2017 must be aware, this is of which applications he will get evolved with?

Recently technology of solar energy (which this is still under development), it started using 1500V DC this be transformed to Grid energy.
Because of that need for 3000V testing voltages, manufacturers started to produce testers with such higher voltages.

Prior this new high voltage application, I am aware of one which is also rare but not unknown, and this is ventilation electric fans them operating at 1000V and serving in underground excavation sites.

One picture equals to 1000 words, therefore here is the picture.

liviu2004:
In my work, I witness a lot of insulation resistance in the form of high voltage ac 1 min and dc megger test. We usually test ac 2x the operating voltage + 1000V for rotating machinery. And indeed, for switchgear, busbar systems, usually its two times the operating voltage. However, the higher the voltage gets, this ratio gets smaller. Don't ask me why, probably commercial decision in the standard.

We 1st stress the insulation with the ac test, failure criteria is flash over and then within 2 minutes, we expose the insulation to a megger test dc. Minimum criteria is 1 Mohms for existing products.

High voltage cables, >1000V, we test them with dc 4.2 times U0 + 1000 for 15 minutes and then a megger test. Yes, test equipment is very specialized and very expensive. Not a lot of companies have it, including the big names struggle to get it.