In addition to John's really good post you may want to give this a read to better understand high side current sensing. This will all go back to "common mode" which you need to understand. I would be asking whoever set this up as high side how they expected you to follow up?

Ron

Yes thankyou i will check it

 

It seems to me that this is a classic case for a "high side current monitor" (do web search for) IC. Just as an example (I am NOT making a recommendation of a specific device), take a look at the Microchip Technology (Supertex) HV7800 series devices. Very low cost. Easy to use.
Link to datasheet: Datasheet for HV7800

Thankyou

 

It will be around 100 amp

Which also begs the question, how much current do you want to sense?

 

Have you considered converting the analogue voltage to digital form and passing the digital signal through an optical isolator. (Or via an RF link using HC-12s or similar. ) Would would need well isolated power supply for the part of the circuit that was sitting at +300 volts. You could use an op amp to scale up the voltage across the shunt to a suitable level to feed the ADC input of a microcontroller which would convert it into a digital format to meet your needs. Being lazy if I was doing it I would use a slightly modified version of a device I use for remotely monitoring battery charging. This uses an INA219, a PIC12F1640 and an HC-12.

Les.

 

Have you considered converting the analogue voltage to digital form and passing the digital signal through an optical isolator. (Or via an RF link using HC-12s or similar. ) Would would need well isolated power supply for the part of the circuit that was sitting at +300 volts. You could use an op amp to scale up the voltage across the shunt to a suitable level to feed the ADC input of a microcontroller which would convert it into a digital format to meet your needs. Being lazy if I was doing it I would use a slightly modified version of a device I use for remotely monitoring battery charging. This uses an INA219, a PIC12F1640 and an HC-12.

Les.

thankyou no i have not converted the analog to digital and for ina219 i need to try it .