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Author Topic:   Electrical Short?

Tuna Thumper posted 03-06-2010 03:27 PM ET (US)
I have a 12v system fed by two new batteries. When I turn the main power switch (Perko) to "ALL" and begin to check amps on the bus bars something goes wrong. If I place my multimeter negative probe onto a ground lug, then take the (pos) probe to one of the positive lugs on the positive bus bar, I see a very low reading, e.g., 0.04 or 0.02. It never used to be like that. However if I place the multimeter negative probe to the negative terminal post on the firts battery and then place the positive probe to one lug on the positive bus bar I have a 12.60 reading. Each of the 10 positive lugs will then display something close to the 12.60. I sense this must mean I have a direct short somewhere on one of the positive lugs on the positive bus bar? Is that correct or or do I have a failed ground on the ground bus bar? Any assistance would be appreciated
Bella con23 posted 03-06-2010 05:02 PM ET (US)
Try testing from the positive buss bar to the negative post of the battery and note the voltage.
Then test from the negative bus to the positive battery post.
Zero voltage on one of the reading means an open in the circuit.
All hell would have let lose if you had a short circuit anywhere in this level of the battery circuit.
Joe
jimh posted 03-06-2010 05:08 PM ET (US)
Your narrative has me a bit confused. You mention that you are checking "amps" by which I take your meaning as you are measuring the flow of electrical current in amperes. If you connect an ammeter from the positive terminal of a battery terminal to the negative terminal, you will create a very high current flow--indeed a so-called "short circuit"--and most likely will open the fuse in the ammeter that is protecting the meter from self-destruction due to over-current and rapid heating. This would account for the observation of a reading of 0.0-amperes when the ammeter is connected between the positive bus and negative bus of your electrical distribution system.
If you are trying to measure the voltage but have misused the term "amp" please let me know and I will reassess your narrative with that change in mind.
Jerry Townsend posted 03-06-2010 11:58 PM ET (US)
You are misusing your meter - to measure amps, the meter must be inserted in the circuit. For example, to measure the current (amps) flow from a battery, disconnect one of the battery post cables - and then have one meter probe on the battery terminal and the other meter probe on the cable just removed. Frequently takes more than a couple of hands or some vise-grips. Be aware that meters are fused and typically to around 10 amps.
Measuring voltage is really an indication of the voltage drop across some load or source as for example putting one probe on the positive battery terminal and the other on the negative terminal. ---- Jerry/Idaho
castaiccraig posted 03-07-2010 09:40 PM ET (US)
Sounds like you lost your DC negative bus connection. I am assuming, based upon your submittal that you have at least two 12v [batteries] that you connect using a 1-2-selector switch If so, both [battery] negative poles would typically be tied together. Sounds like you lost this connection.

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Author	Topic: Electrical Short?
Tuna Thumper	posted 03-06-2010 03:27 PM ET (US) I have a 12v system fed by two new batteries. When I turn the main power switch (Perko) to "ALL" and begin to check amps on the bus bars something goes wrong. If I place my multimeter negative probe onto a ground lug, then take the (pos) probe to one of the positive lugs on the positive bus bar, I see a very low reading, e.g., 0.04 or 0.02. It never used to be like that. However if I place the multimeter negative probe to the negative terminal post on the firts battery and then place the positive probe to one lug on the positive bus bar I have a 12.60 reading. Each of the 10 positive lugs will then display something close to the 12.60. I sense this must mean I have a direct short somewhere on one of the positive lugs on the positive bus bar? Is that correct or or do I have a failed ground on the ground bus bar? Any assistance would be appreciated
Bella con23	posted 03-06-2010 05:02 PM ET (US) Try testing from the positive buss bar to the negative post of the battery and note the voltage. Then test from the negative bus to the positive battery post. Zero voltage on one of the reading means an open in the circuit. All hell would have let lose if you had a short circuit anywhere in this level of the battery circuit. Joe
jimh	posted 03-06-2010 05:08 PM ET (US) Your narrative has me a bit confused. You mention that you are checking "amps" by which I take your meaning as you are measuring the flow of electrical current in amperes. If you connect an ammeter from the positive terminal of a battery terminal to the negative terminal, you will create a very high current flow--indeed a so-called "short circuit"--and most likely will open the fuse in the ammeter that is protecting the meter from self-destruction due to over-current and rapid heating. This would account for the observation of a reading of 0.0-amperes when the ammeter is connected between the positive bus and negative bus of your electrical distribution system. If you are trying to measure the voltage but have misused the term "amp" please let me know and I will reassess your narrative with that change in mind.
Jerry Townsend	posted 03-06-2010 11:58 PM ET (US) You are misusing your meter - to measure amps, the meter must be inserted in the circuit. For example, to measure the current (amps) flow from a battery, disconnect one of the battery post cables - and then have one meter probe on the battery terminal and the other meter probe on the cable just removed. Frequently takes more than a couple of hands or some vise-grips. Be aware that meters are fused and typically to around 10 amps. Measuring voltage is really an indication of the voltage drop across some load or source as for example putting one probe on the positive battery terminal and the other on the negative terminal. ---- Jerry/Idaho
castaiccraig	posted 03-07-2010 09:40 PM ET (US) Sounds like you lost your DC negative bus connection. I am assuming, based upon your submittal that you have at least two 12v [batteries] that you connect using a 1-2-selector switch If so, both [battery] negative poles would typically be tied together. Sounds like you lost this connection.