very important

That is a very vague question. Voltage monitoring in what kind of application? Voltage monitoring for a PC board is entirely different from voltage monitoring on an AC motor, which is again entirely different from what you would want for an entire power grid. Please define your reference by stating the context.

Your question is too broad and general for any worthwhile answer. Please re-post your question with a bit more specificity, and maybe some useful answers will be offered. This is not a homework question, is it Ali?

Try www.time-mark.com This manufacturer has diagrams and application information that should answer most of your questions.

Good luck,

James

A calibrated multimeter would fit that description, as would a simple buzzer, as would a power metering unit at the inlet of a large installation. Please clarify the question with context, particularly as it is "very important"!

IF your application is to monitor quality of supply, then data loggers may be used for a period.

the data loggers or power quality analysers do voltage, harmonics, power factor etc.

best of luck

hi everybody in this forum

I'm asking about "voltage monitoring relay" ,,, so please the experts .............

best regards,,,,,,

Very good. You respond to our request for more information by doing what? You repeat the vague question...

Can't help you.