I finally decided on an approach for Master Power cut-off for the S550. To save this post from being over-long, I’ll post in this thread on how I selected the approach and second on the install. I wanted a solution that: 1) Uses low current switching to avoid the need to run thick (1 Gauge) wire 2) Enables both in cockpit and exterior cowl switch locations 3) Meets NASA Tech testing 4) Eliminates the risks of Alternator current spikes causing damage on Engine Kill 5) Is Fail Safe I looked at 3 approaches: a) A Solid State Device like this Cartek device: http://www.cartekmotorsport.com/Battery Isolator GT EN.pdf b) An “all in one” 6-pole like this: https://www.northstarmotorsports.co...l+Switches/manufacturer/Sparco/productID/7594 c) A solenoid based solution, like @captdistraction describes here: https://trackmustangsonline.com/thr...ant-s197-mustang-engine-cutoff-circuit.12908/ Solid State Pros: Lightweight and Small. Provides Alternator protection Cons: Uses Chassis ground for isolation—would require rework to support S550 smart charge current sensor. Fail Safe? 6-Pole: Pros: FIA approved, provides Alternator protection, Fail Safe Cons: Requires 1 gauge cable runs, 2 switch install complexity Solenoid: Pros: Simple multi switch install. Avoids 1 gauge wire runs. Relatively good re Fail Safe Cons: Need to deal with Alternator spikes and Starter current draw Based on this I decided to pursue the Solenoid option, as there was limited durability data on the Cartek solution and I wanted to avoid 1 Gauge cable runs associated with the 6-pole switch. The solution @captdistraction describes works well for the S197 and deals with the cons above as both the Starter and Alternator cables remain hot as the Battery Junction Box (BJB) cable is separate to both the Starter and Alternator. However the S550 the starter cable is crimped to the BJB cable at the Battery cable. Separating these cables would address the risk of Solenoid durability issues due to the repeated starter high current draw, but still leaves the issue of Alternator spikes to deal with as the Alternator terminates at the BJB. The solution is to repurpose the stock Battery cable to feed the Starter and take the current draw from the Alternator.