Since a state-adopted industry standard would be preferred to borrowing from European standards for insulation tests during domestic commissioning @ 500 V d.c for their 220 V systems (United Kingdom BS7671-2008 Appendix 13), my unpublished claims that list empirical efficacy with 100-volt insulation tests for 120 V systems, when GFCI devices are in use, can also be ignored without state adoption.
Most modern insulation testers, including the Megger brand are sold with 50, 100, 250, and 500 volt selections for lower-voltage wiring, which empirically shows faulted HVAC compressors, carbonized or faulted connections & appliance cords <=1 Mega Ohm, and end-of-life appliances <= 5 Mega Ohms, which in my experience trip GFCI's more frequently under loading stress. However continuity testers are useless for checking faults, since they only detect bolted faults with much lower resistance, several orders of magnitude below 1 Mega Ohm.
Even though visual inspection may be more appropriate for rodent chewed cables on existing wiring, that may hold GFCI breakers on every circuit, some electricians on this forum insist the industry standard "Megger test" should blast the 120 Volt systems in your house with 1000+ Megger voltage to get an arc across the air gaps in damaged cables. The higher voltages for Megger testing air gaps is not disputed, but perhaps not required.
Industrial maintenance electricians working with equipment rated for several thousand volts may have created the industry standard "Megger test" procedure with 1000+ voltages for several minutes. This histogram record for medium voltage cables (2k-to-35k volts) helps determine serviceable life.
If the 600 volt building wire won't fail during such high-pot Megger tests, it will likely force complete-remodel wiring projects by burning thru the cheep wirenuts, some UL 486 listed as low as 300 volts.
Several pics of soft-plastic wirenuts are shown that typically accommodate more wire than listed, fault under load in contact with metallic-device yolks or raceways, and Megger testing may expedite that process.
Further, without incentive contractors have no reason to apply Megger settings below 1kv. To preserve existing wiring leaves work on the table, much less repairing soft wirenuts, bad connections, or damaged parts for peanuts. Complete building-remodel wiring is the standard-industry practice in the US states, especially for insurance purposes with Knob & Tube & aluminum-wire abatement, rodent damage, or lighting strikes.
If joe homeowner wants to keep existing GFCI devices and building wiring intact, the best way is to make sure no contractors touch your building with a "Megger tester".
