I went for an interview for scientist position in BARC India. They asked this problem

How will you measure a high voltage like 765kV AC ?

I replied that we can step down using potential transformer and then measure it using a voltmeter.

Then they said tell me some other method?

How will you measure HVDC line of 500kV?

I was not able to answer all this. Please help me with this problems.

  • 2
    \$\begingroup\$ I suppose the answer "very carefully" is not what they're looking for? \$\endgroup\$ Jun 10, 2018 at 1:55
  • \$\begingroup\$ I am not able to understand \$\endgroup\$ Jun 10, 2018 at 1:56
  • \$\begingroup\$ IIRC you can use polarisation rotation effects in optical fibre to measure voltage gradient in a fibre that just goes up to the line and down again. Was I dreaming, or is this a thing? \$\endgroup\$
    – Neil_UK
    Jun 10, 2018 at 6:25

2 Answers 2


HVAC is often measured with the cable interface insulators called bushings ( usually porcelain , glass or polycarbonate) that have built-in electrodes with an specified low capacitance value , supplier to large diameter coaxial jacks and have the same high BIL ratings Basic Insulation Level, to a low voltage higher capacitance load that acts as a capacitance voltage transformer of 10k:1 or cascaded to the desired end voltage range.

HVDC can be measured with E field sensors using a known impedance current loop as a voltage divider. These must be clean dry air gapped E Field sensors designed to withstand the HVDC breakdown in a very clean environment . Although I am not up to speed on laser E field tracking and other modern methods.


Caddock makes a line of ultra-stable resistors with a 20 KV rating and .025% accuracy. They can be soldered in series. The 40 KV Fluke test probe is just 2 of these at 500 M each for a 1 G ohm probe.

50 of them in series would give you 1 million volt isolation, if they are covered in heat shrink and have ample clearance to any conductor. The impedance would be so high you would need a CMOS op-amp to buffer the resistor divider before amplifying it to a useful level.

If it was AC you could use simple capacitive dividers.

  • \$\begingroup\$ And in case of HVDC ? \$\endgroup\$ Jun 10, 2018 at 2:12
  • \$\begingroup\$ I have never seen anyone in the transformer or power industry use this method , although it may work, it is not standard practice.being tested for your experience in this question. Due to creapage failures , this method is not suitable. \$\endgroup\$ Jun 10, 2018 at 3:48

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