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Why did the multimeter leads arc?

  1. Nov 22, 2009 #1

    foo

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    I was about to go out to the car to check how many amps were being drawn from a parasitic component in the circuit. Battery has been dieing if I leave it connected for a few days. The 12VDC test light connected from the positive battery post to the positive cable(after pulling cable off) shows that there is a draw which causes the test light to come on. Pulled all the fuses one by one to see which one would cause the light to shut off. But none of the fuses being out stopped the draw.

    Anyways, what I did was I set the multimeter to 10 Amps. I know I've been able to get around 14 amps from the car battery when when the car is running which shows that the alternator is doing its job well. So, like a goofball I decided to see the reading on the a/c wall outlet thinking o ya, this circuit is 15 amps.
    Well, you know what happened, I had the multimeter set to 10ADC. A nice arc shot from one lead to the other as soon as I when from the hot to a grounded screw. I was surprised that the power didn't shoot through the meter and blow it up.
    Turns out there isn't even a 10A a/c setting for this multimeter.

    So, I have two questions:

    1) Why did the arc jump to the other lead instead of completing the path through the multimeter?

    2) Why is 10 amps of AC different from 10 amps of DC when it comes to taking a reading with a multimeter.

    I know I shouldnt have done that, but I don't understand why. Since I'm around to learn from my mistake I would really appreciate an explanation.

    Come to think of it. A car battery is advertised as having 300 cold cranking amps. Now I'm really confused as to why the ac amps were too much for the dc amps reading...

    Thanks in advance

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    Last edited: Nov 22, 2009
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  3. Nov 22, 2009 #2

    Integral

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    Essentially a ammeter is a direct short. So when you shorted the hot lead to ground it sparked, this is not surprising. You could have blown a fuse in your ammeter and/or potentially tripped a circuit breaker in your house breaker box.

    2. Because it is AC, and not DC?

    When measuring current you need a load. Any source can potentially be harmed by creating a short circuit as you did at the wall. The current rating of 15A means that at greater currents dangerous levels of heat could be produced, thus the circuit breakers open the circuit under high current conditions.

    The only thing limiting current is the load and the capabilities of the power supply. If you exceed the current limitations of a power source damage can be done.
     
  4. Nov 22, 2009 #3

    vk6kro

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    1) Why did the arc jump to the other lead instead of completing the path through the multimeter?

    2) Why is 10 amps of AC different from 10 amps of DC when it comes to taking a reading with a multimeter.

    I know I shouldnt have done that, but I don't understand why. Since I'm around to learn from my mistake I would really appreciate an explanation.

    Come to think of it. A car battery is advertised as having 300 cold cranking amps. Now I'm really confused as to why the ac amps were too much for the dc amps reading...


    Batteries and the mains deliver current according to the load put on them.

    So, if you put a 1000 ohm resistor across a 12 volt battery it will only draw 12 mA even though the battery is capable of 300 amps.

    You put a short circuit across the mains, so it tried to deliver infinite current. There is a shunt in those multimeters. It looks like a piece of wire between the 10 A terminals. This probably got melted and then the mains arced between the melted bits of wire. You may have seen this through the plastic case.

    If your house has an RCD it may not be working and it needs to be checked. It should have detected this as a fault and removed the power. An electrician should do this.... NOT YOU!

    If your house doesn't have an RCD it should have one. Next time you may not be so lucky. Better make sure there isn't a next time.
     
  5. Nov 22, 2009 #4

    mgb_phys

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    So the battery can provide 300amps through the resistance of a starter motor.
    Assuming the resistance of the meter is less than a starter motor (ideally the resistance of a meter on amps is zero) then more than 300amps would flow through the meter which can only handle 10A.

    Even then the 10A range on a cheap meter is a bit optimistic,it normally says somethign like 10A for 5-10seconds in the manual - look at the size of the cables in the car to handle 300amps and then look at the thickness of the leads on the meter.
     
  6. Nov 22, 2009 #5

    foo

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    What is an RDC?

    Thanks for the info all.
     
  7. Nov 22, 2009 #6

    mgb_phys

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    An RCD is a residual current detector (also known as Ground Fault indiciator or earth leakage trip) only applies to household AC wiring - it's nothing to do with cars.

    it basically monitors the current flowing out of the live wire and back through the neutral wire if there is any small differnce (normally 0.015A) then it turns the power off instantly.
    The reason is that the missing current might be flowing to ground through you!
     
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