AMMETER SHUNT

[calpilot]

Lately, I have seen a resurgance in Ammeter Shunts not being wired correctly. All of the alternator current can pass through the shunt (typically 60 amps). However, the small (usually 20-22 gauge wires) that read the current in the ammeter or EFIS can be subjected to the entire alternator load if grounded (insulation rubbed off, etc), and will definately melt, ruin an instrument or appliance, cause smoke, or start a fire. I recommend using a fuseable link (smaller wire for about 2 inches from the shunt to the ammeter wire, or a 1/2 or 1 amp fuse.) If those wires are going to melt, I want them to do it at the shunt, not inside the cockpit!
DAR Gary

 

[Scott Hersha]

How small does that section of wire have to be to protect a 60 amp load? Seems like a 22 awg ammeter wire would melt if there’s a short. I’m just asking because I can’t remember what I used back when I had that kind of shunt. I know I had the “fusible link” on my shunt, but don’t remember the gage. I use an ammeter donut now, which works fine.

 

[GalinHdz]

Yup, a wire will melt at the point of most resistance. A fuse is just a much smaller wire with a higher resistance. I used a 26ga wire in my installation as a fusible link. It will fail way before the 22ga sensor wire will.

 

[PaulvS]

I also used 26 AWG for the fusible link to protect the rest of the ammeter wire, which is 22 AWG. The shunt portion is inside a fiberglass sleeve to help contain any spark if it blows.

 

[Walt]

Same thing goes for any wire directly to the battery going inside the aircraft.
Lightspeed Engineering's drawing still has folks running unprotected wires into the cockpit.
Any 'HOT' wire from the battery needs to be protected/fused at the battery.
(at least with the shunt wires, turning off the master will kill power to these wires 'assuming' your alt is killed with the master)

 

[Tandem46]

Lightspeed Engineering's drawing still has folks running unprotected wires into the cockpit.

The LSE schematic shows a 5a pullable breaker.

 

[Walt]

The LSE schematic shows a 5a pullable breaker.

Yes, but that breaker is in the cockpit!
“• Power and ground (shield) connection must be directly to the battery terminals to achieve the best reliability, to avoid voltage spikes, and to minimize radio noise.”