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Avoid sharp corners that raise E-field (kV/mm) , and use a string of axial components to make a 10:1 divider using resistive elements.  Calibrate at low V. Good enuf for 50 Hz.

Use spark plug carbon wire as a probe to an R ladder to 10:1 probe for whatever you need. 1000:1 or 2000:1

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Partial Discharge (PD) may corrupt data but pass leakage test. Pulse rate can be 1/minute to 1/sec to >=1/cycle then it's called corona discharge if in air.   PD is only caused by contaminants in/on the dielectric insulator. Rise times are picoseconds unless filtered. Corona is a result of failure for dielectric withstanding. Since Corona is normally visible white/blue light, air temperatures are > 5000'K.

Normally porcelain bushings on grid poles are rated by impulse voltage. e.g. BIL200  = 200 kV impulse but can only handle 20 kVrms to 40 kVrms from creepage and might be 40 cm long.  ie. ionization time (microsec.) increases withstanding voltage if pulse is 1~10us.

To make a pre-scaler divider, use carbon spark plug wire to an R ladder with YAGEO HHV series resistors 1/4 W coated with silicone on a wood base with less than 1pF stray capacitance per each resistor (1pF=300Meg @ 50Hz).

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For a constant leakage current, the voltage drop is proportional to the impedance of every path in the loop until PD or ionization occurs.

So if one part of the path gets worse for leakage conductance, that puts more kV/mm stress on the rest of the insulation if that part did not change.

So do not overlook that.

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Pls confirm your test.

Is it the 1.25/10us crest voltage waveform with 8/20us current if arc'd?