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grounding amp meter

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dougb
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grounding amp meter

Post by dougb » Mon May 30, 2016 11:19 pm

1940 17' B back,
Heard the steel strips in the wood dash board will cause problems by grounding to the amp meter trim. Any truth to this ?
Does Amp meter need to be grounded ? if so advise?
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jim g
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Re: grounding amp meter

Post by jim g » Tue May 31, 2016 5:57 am

No. The amp meter does not need to be grounded.

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Bilge Rat
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Re: grounding amp meter

Post by Bilge Rat » Tue May 31, 2016 8:13 am

I second the no ground needed. Amps are read by sensing positive current (amps) through a coil of wire that is in series with the electrical load of the generator. The current creates magnetism in the meter's movable armature that is mounted with the pointer. No connection to the battery negative is required. Not sure how gauge back lighting was done in 1940, maybe its like the 50's where its a separate window the gauge has the admits light from an external bulb?
1966 Lyman Cruisette 25 foot "Serenity Now!"
1953 Chris Craft Sportsman 22 foot "Summerwind"

dougb
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Re: grounding amp meter

Post by dougb » Thu Jun 02, 2016 12:36 pm

Thanks for response, I didn't think it needed grounding. yes again, the lights are a external separate bulbs, the concern is the amp meter trim will ground itself to the dash metal strips and then thru to the other gauges. Any way that is how it was explained.

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Bilge Rat
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Re: grounding amp meter

Post by Bilge Rat » Fri Jun 03, 2016 7:17 am

I think the real issue is that the ammeter is in an un-fused section of the generator/battery circuit. If the positive voltage in the ammeter is shorted through the case to the negative ground, there's no fuse to protect the wiring, generator and ammeter which equals a spectacular disaster. Those little fiber or plastic washer bushings that insulate the terminals on the ammeter case are all that prevents the case from coming in contact with the positive voltage. So keeping the case of the ammeter "floating" from negative helps prevent the positive voltage finding a path to negative. A fuse for this circuit would have to be large enough for the maximum charging current that the generator can produce, possibly as large as 40 amps.

In a modern ammeter, this high current from the generator/alternator does not appear at the meter and would be fused also.
1966 Lyman Cruisette 25 foot "Serenity Now!"
1953 Chris Craft Sportsman 22 foot "Summerwind"

Greg Wallace
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Re: grounding amp meter

Post by Greg Wallace » Mon Jun 06, 2016 8:23 am

We almost lost the only 23 barrel with twins to this very same problem when the ammeter shorted. We believe in fuse or breaker in this circuit. Also a battery switch for emergency shut off... and for when nobody's home.
Greg Wallace 23 Custom 22166 former Chris-Craft dealer Russells Point, Oh.

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