What is a shunt?

konstantin.neo
konstantin.neo Registered Users Posts: 19
Hi all.
Could someone explain to me what does a shunt does?
I am looking at the outback power products and there is this shunt on the DC negative bus bar.

Comments

  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: What is a shunt?

    A "shunt" is a kind of very low value resistor, capable of passing relatively huge currents without burning up. As with all resistors, when current flows through the shunt, a voltage is developed across it. In the case of a shunt, due to it's very low resistance value, that developed voltage will be very low to reduce power losses to the system, yet it's enough to signal current meters, battery state of charge devices etc.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: What is a shunt?

    A shunt (in this case) is a high current/high power precision resistor:

    wind-sun_2203_27757083Deltec 500 amp, 50 millivolt current shunt

    The load/source cable connects between the left and right top terminals.

    And the meter connects to the left and right small screws on the side of the shunt.

    In the above case, the equation of (note: 50 mVolts = 0.050 volts):
    • V=I*R
    • R = V/I = 0.050 volts / 500 amps = 0.001 Ohms

    We can now take the above and re-write as:
    • I = V/R = 0.050 volts / 0.001 Ohm = 500 amps full current
    • Power = V*I = 0.050 volts * 500 amps = 25 watts of "heat" under full current for shunt

    So--A battery monitor or remote meter simply measures the voltage drop across the shunt/resistor and displays the current based on the above equation/calibration.

    Note the two screws are known as Kelvin Contacts or 4 wire measurement (two terminals carry the current, two others to sense the small signal current/voltage) -- If you are interested in the history/details. Basically, the location of the screws makes precision voltage readings a bit more accurate because it bypasses any variable resistance in the heavy current connections which can mess up the readings.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: What is a shunt?

    Note, I should add you normally will see these on the ground/negative side of the battery bank (for a negative ground system)... That way, if there is a short circuit in any of the wiring, there will not be a dangerous fault current caused by the short.

    In cases where the shunt has to be in the positive lead (or using to measure battery voltage), there should be a fuse or high value resistor to prevent high current flow in case there is a short in the measurement circuit.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • konstantin.neo
    konstantin.neo Registered Users Posts: 19
    Re: What is a shunt?

    Awesome. Thanks for the explanation.
  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    Re: What is a shunt?

    Using and understanding the uses of a shunt in a DC system is the best way to know how your system is performing.
    One of the best places to connect a shunt is between the battery/ies and the inverter. Measuring the current drawn from the battery /ies will show you better than any other way to measure the output devices power consumption . Example :You are then able to try different lights on your premises to see if you are getting a high or lower power use. Far better than just guestimating if one type of light is better than another.
    Also good to see if there are any mystery losses anywhere. A device not turning off correctly,a bad connection that is causing extra current to be drawn..

    With a shunt you have the choice of using a simple multimeter set to millivolts or a millivolt meter.. usually 1mv =1a

    Now what im going to write many will not agree with and I have said it before. I leave it up to each person as to weather they accept my ideas or not.
    I use and where I work we use shunts everwhere in DC circuits. There is no better way to seeing a DC powered devices power draw in normal conditions and then watch to see if and when its current draw increases. This will show you impending failure better than any other way.
    Its under such conditions as above that a shunt makes a better fuse than a fuse.. WHY? A fuse never shows impending failure of some electrical item its either working or not working.. A shunt will. and a shunt in most cases will give a more predictable failure than a fuse. Fuses by their design suffer from "power surge" it gradually weakens the fuse. Shunts are made more sturdy and while they do of couse suffer also from "power surge" they suffer far less. They have a larger surface of metal exposed to air. Remember a shunt is usually a high precision laser trimmed resistor. And for that reason its gives very predictable results .
    I n a post once I was told "A shunt is a precision resistor so wont give predictable results as a fuse " What a contradiction of terms,!!!!!!!,
    Shunts are usulally rated like this
    100% of rated current indefinately
    120% of current 30 mins
    150% of current 10 mins
    200% of current less than 5 seconds
    There is never any need to worry about the millivolt meter or multimeter across the shunt carrying and current as its not capable of doing so.