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The Insulation Resistance test at 500V can cause an issue here. Most surge protected devices will have a threshold of approximately 300V before they activate.
As a result a 500V test voltage will see a very low resistance reading as the surge device will have shorted Live / Neutral to Earth. If your tester has the option, reduce the output test voltage to 250V.
Furthermore this will ensure the test does not activate the surge protection device. If you do not have this capability on your tester then we follow the IEE Code of Practice for in Service Inspection and Testing recommendation to carry out a Protective Current Conductor Test (Class I) or Touch Current (Class II) in place of the Insulation Resistance Test.