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  #1  
Old 12-07-2021, 08:39 PM
riobison riobison is offline
 
Join Date: Jul 2011
Location: Oliver BC & Red Deer Alberta Canada
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Default Melted 60 Amp Shunt on RV 4

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Would anyone have any idea on what would cause a 60 amp shunt to fail? Melted or fractured and then melted.

8 years ago I replaced the Amp meter for a Volt meter due to preference and in 1000 hours on my RV 4 and never had an electrical issue.

Run up was good 14.3 volts and everything worked. 10 mins into the flight and total electrical failure.

Back on the ground this is all I could find. Battery still has 12.9 volts and no other signs of arcing.

Trying to insert the foto, so here goes...............



Thanks

Tim
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  #2  
Old 12-07-2021, 09:03 PM
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airguy airguy is online now
 
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About 80-90 amps will do it. Now you gotta figure out what caused THAT.
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  #3  
Old 12-07-2021, 09:17 PM
rapid_ascent rapid_ascent is offline
 
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A loose connection on a high current path might do it. It does look like there is a little discoloration on the lower part of the right hand screw terminal. Hard to tell if that is partially a shadow. Or it might have had a crack in the shunt resistor. There was some uneven current flow based on only part of the element remaining.
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  #4  
Old 12-07-2021, 09:27 PM
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fl-mike fl-mike is offline
 
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What is the shunt for? Alternator? Need more info on the circuit.
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  #5  
Old 12-07-2021, 09:31 PM
PilotjohnS PilotjohnS is offline
 
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Default Shunt short

If the shunt is mounted on the firewall and exposed, I would look for indication on the firewall of a particle that could have caused a short to ground. There should be a burn mark on the firewall (or wherever the short was) if 90A went thru it.

Also check to make sure the shunt sense wires to the instrument have fuses in line. I used a fusible link.

lastly, if these things do crack, then resistance will build up and then cause a failure. If you were seeing higher and higher current readings, that could be an indication of a pending shunt failure due to cracking.

I plane to make a simple fiberglass cover for my firewall shunt.
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Last edited by PilotjohnS : 12-07-2021 at 09:34 PM.
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  #6  
Old 12-07-2021, 10:14 PM
riobison riobison is offline
 
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Default Alternator side

Alternator side. I can turn the master on and activate the solenoid with 12.9 volts still there. So power from the battery to the one side of the shunt is ok.

All of the cables are tight and no signs of any arcing that I can see.

There is or was nothing floating around behind the panel that could have touched it. Its been 9 months and 40 hours since I've even been in behind the panel.

Never noticed any change on the voltage readings.

Is there a chance that the voltage regulator failed and sent a big spike to cause the failure of the shunt?

Thanks
Tim
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  #7  
Old 12-07-2021, 11:39 PM
PilotjohnS PilotjohnS is offline
 
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Default Lowmresistance

Shunts are very low resistance, so unlikely enough current can pass to cause damage. I suspect a cracked shunt that finally broke, or a short, but the shunt should take more current then the wire since it is low resistance by design. So it could be a crack propagation without any real resistance change until the very end. Is the whole break burnt and melted, or just the last little bit?
Sure looks like a short to the mounting screw.
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  #8  
Old 12-08-2021, 06:14 AM
Freemasm Freemasm is online now
 
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Default Question for my benefit

Does the shape of the "fracture" surface mean anything to those experienced in this art? I have no experience with such. The large amp fuses I've seen blow tended to melt/vaporize the conductor fairly uniformly but these were much higher/medium voltage devices. Would the non-uniform shape of the formerly single conductor make anyone think FOD was involved?
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  #9  
Old 12-08-2021, 06:56 AM
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DanH DanH is offline
 
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Only two possibilities, too many amps or material fault.

Given you find nothing to suggest a battery short to ground, I'll speculate the second, based on the picture. Note how the blocks have rotated slightly in the holder, and that the lower edges of the bridge are not aligned. The components rotated when the nuts were removed from the cable ends. It suggests they could have also rotated the other way when someone tightened those nuts without a holding wrench on the blocks, cracking the bridge.
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  #10  
Old 12-08-2021, 07:12 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by DanH View Post
Only two possibilities, too many amps or material fault.

Given you find nothing to suggest a battery short to ground, I'll speculate the second, based on the picture. Note how the blocks have rotated slightly in the holder, and that the lower edges of the bridge are not aligned. The components rotated when the nuts were removed from the cable ends. It suggests they could have also rotated the other way when someone tightened those nuts without a holding wrench on the blocks, cracking the bridge.
This would be my guess as well. Nut tightening or loosening put stress on the copper bar and over the years of vibration, a crack developed from the bottom up. Once the crack was large enough, there was not enough surface area to pass the current and the remaining material melted, just like a fuse.

Larry
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