How to measure small current in high voltage environment?

Moving Coil Micro ammeter.

With the help of Potential Transformers

I have to use hall Sensor or current transformer

Current can be measure using a know I=V/R with know R,

BY USING  V=IR

where I = the current 

         R= the resistance 

Using a current transformer.

photo multiplier tubes and  avalacnhe photo diodes.

To check how the system is operating, current provides a valuable insight as it’s a signal.

For each task to be performed, the current consistent, therefore current is a useful indicator to check system faults and health.

A few measurement methods to measure the signal (current) and they are:

1)   In the return path to ground (low-side sensing)

Using an amplifier which is able to handle common mode signal reaching down to ground, in addition placing a sensing resistor (shunt resistor) which therefore, voltage develops across the resistor potential to the current.

HINT: All these amplifiers are able to perform this task (Current sensing amplifiers, difference amplifiers, standard operational and instrumentation amplifiers)

2)   In series with the power rail being monitored (High-side Sensing)

 This allows for measuring the entire system current and can easily detect the faults.

Always use CT (current transformer) that transforms or converts proportional small currents from high voltage sources usualy used for metering and protection.

How to measure small current in high voltage environment?

I would like to thank you, Salma Eid for referring me to answer that question.

I would just give an example of your question, when working in transmission substation of High voltages such as 220 KV or 66KV or even 11KV, the protection instrumentation of those substation takes a low current. When testing those Protection relays, you need such a small current to do so. Even those panels of protection relays have a very low current to be feed with.

You can measure small current in a circuit with very high DC voltage using a hall sensor.

Onother way to measure small current,if you know U -  high DC voltage,you can also measure the voltage drop across a known resistance - R. Then using Ohm's Law formula U=IR  you can calculate small current - I.