Hi, my name is Andrés, and need some help or a thing to solve a problem, i´m developing a system or embedded system to test a circuit breaker, and the problem starts when i need to read the voltage drop in his contacts, I don't have idea (and information about the contacts) the resistance, but i need to select a sensor to read 1.5 VDC with 3000 A, thanks a lot and sorry if my English is poor.
Amps or milliamps?
If there are high voltages involved (greater than 5V) you'll need over-voltage protection for the Arduino, and possibly electrical isolation.
The Arduino's ADC can read 0-5VDC, relative to its ground. When reading across a breaker you'd need to "float" the Arduino's ground.
Or, with an op-amp you can easily make a comparator that "triggers" when the voltage is above a preset threshold.
There are low voltage (1.5VDC) and high current (3000 amps)
That would be a good reason to post links, diagrams, data sheets, etc. instead of trying to describe it in words.
I can give you a picture of a simplification of the circuit, the switch (really a circuit breaker) is opened when circulates 1900 or 2000 amps on his contacts,his voltage is constant (I use a programmable current source with constant voltage, and can give until 4000 amps), I solved the current read (using a hall effect sensor) but I need read the voltage drop in the contacts, again sorry if my English is poor 
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You only need a current limiting resistor in series with an analog input, to make it safe (e.g. about 10k ohms). You can then make an analog reading just like any other reading, using the ADC.
Honestly, I'm a little surprised to hear this question from someone who is dealing with such extreme power levels. For safety reasons, it should be a qualified electronics engineer.
You posted in "Sensors". Are you not aware of the built in analog inputs on most Arduino? Which board do you have?
i need a sensor for the low voltage, I'm using a custom board and fit this project to this other, thinking i use all of the ADC of the board, maybe an ina219?
Why? The AVR ADC can perform a 1.1V full scale measurement. What accuracy do you need? A resistive voltage divider can give you full 10 bit resolution over any input range you want, that is more than 1.1V
in order of mVolts, the contacts have very low resistance (can´t make a read with a multimeter fluke).
You said 1.5V. I'm not imagining it -" a sensor to read 1.5 VDC". Please start over from scratch with real requirements.
The "order of x" is not a spec. How many millivolts full scale, and what precision?
Also, when the breaker contacts open, there will be 1.5V.
If you can't measure the voltage with a Fluke, how do you even know what to look for? Edit - I meant resistance. But you should try to use the Fluke to measure the voltage because a milliohm meter requires a heavy current to obtain readings.
The standard Arduino has 10-bit (0-1023) analog to digital converter. With the optional 1.1V reference you have resolution of about 1mV. Below that, you'll need amplification. And if your power supply is limited to 1.5V ("worst case" with the breaker open and no current) the Arduino's input is safe up-to 5V. (The readings will simply saturate/max-out at 1023.)
The contacts better would better have an extremely low resistance. Otherwise they get welded together...
Are you really sure about 4000A?
At 1.5 V that is 6000W!!!
That is way more than enough to start a fire...
It will melt a screw driver if you put it from plus to minus...
In our department I have seen melt fuses for 128A. They are just a strip of metal. What are your wires looking like? Do you have proper education to mess with this?
Right. Nobody wants themselves or an innocent bystander to get a face full of hot plasma and metal debris. What are your qualifications?
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I am electronics engineer, i understand the power that i use, i ever use my safety equipment when i am working, but i have the doubt because ina219 dont specify if have isolation in their vin inputs or high impedance/resistance... Also, thanks a lot!
Good. We didn't have a good chance to discuss isolation because you didn't show a complete diagram, just a simplified one.
It would be helpful to know a bit about the project and what it doing .
I wonder how you generate such current at 1.5v. - the slightest resistance will drop the 1.5v ( 0.0005 ohms ).
I’m guessing g your conductors must be something like 2” aluminium bars .
Something not right here ?
Did you ever encounter "metal migration" in regards to DC through relay/switch/circuit breaker contacts? My personal experience is the contact resistance will be zero after a few weeks continuous contact with DC current continuously flowing. in regards to relay contacts, they will not open when the relay is told to open. Happened many time in different equipment.
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