This allows me to use a DPST switch to control reversing polarity for the flap actuator.

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Comment by Brian A Manlove on January 7, 2014 at 1:58pm

For anyone with the question "why" for this setup - I realize that I could have done either a DPDT switch as outlined in the RAC documentation, or a single DPDT relay... the problem I had with those options were 1.) I didn't have the physical room for a big fat DPDT switch in the location where I wanted to put the switch - too wide to fit in-between the TCO and VSI gauges... and 2.) a single DPDT relay is not fail-safe should the relay short out.  Using two DPDT relays gives me added circuit protection in that if either relay shorts out, nothing bad can happen.  Theoretically, either relay shorting out won't even blow the fuse.  

My brother recommended this setup to me.  He is a Master Electrician and specializes in control wiring for making his living.  He says that mission critical systems do it this way because they have had numerous problems with DPDT switches and single relay layouts when the switch or the relay shorts out.  This is also the recommended wiring method for polarity reversing in super-critical systems, like underwater ROV's, drones, etc. 

So - it may be a little overkill - but the 2nd relay is only about 1.5 oz. so, that's good enough for me.

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