master contactor orientation

kens_cockpit · #1 02-09-2007, 01:11 AM

Can I put the master contactor in upside down? And if not how do I determine which is the right way up?

I have the Vans standard contactor. I've installed it on the back of the battery tray. The battery tray is aft mounted in my 8QB.

As mounted the "Bat" terminal is outboard and the other large terminal is inboard. That means I have to run the main power cable from right next to the ribs on the bottom of the fuselage. If I do that it rubs on them. The alternative is to run the main power cable up over the battery and then down again through the holes shown on drawing OP11.

OP11 shows the contactor with the "Bat" terminal inboard and the one for the power cable outboard. That makes the main power cable run neatly under the battery tray without interference. But to acheive that it looks like I need to install the contactor "upside down".

What's the story here?

Ken Stanton
RV8 QB

FredMagare · #2 02-09-2007, 07:19 AM

I'm no "avionics guru", but I think some people install it upside-down for a slight measure of security. Since the contactor is held apart by a spring, if the spring were to fail in the "upright" position the contactor is essentially on. Upside-down it will not make contact. Or at least that's the thinking.

I'll keep watching this thread to see if there is any hard evidence that it should NOT be installed upside-down.

Fred

tomcostanza · #3 02-09-2007, 08:12 AM

If what you say is true, isn't right-side-up the "fail safe" position for a spring failure (in flight that is)? If it fails on the ground, it seems the worst that will happen is that the battery will discharge. But if it fails in flight, depending on how you wire things, it could result in an electrical failure.

Or am I missing something?

FredMagare · #4 02-09-2007, 08:30 AM

Quote:

Originally Posted by tomcostanza

If what you say is true, isn't right-side-up the "fail safe" position for a spring failure (in flight that is)? If it fails on the ground, it seems the worst that will happen is that the battery will discharge. But if it fails in flight, depending on how you wire things, it could result in an electrical failure.

Or am I missing something?

Good point. But I think if the spring fails inflight but the contactor is otherwise functioning normally it will continue to function. No problem. My post was only to point out why some have told me they inverted the contactor orientation. Personally, I don't think it makes a difference either way. If it's rightside-up and I cut power to the contactor but nothing happens (everything remains powered-up) at least I'll know there's a problem which requires my attention. On the other hand, if upside-down, the contactor may continue to function without indication of a problem for years even with a broken spring. Then again, how often do contactors fail?

Hmm. I don't know. If there's something "solid" to indicate one way is obviously superior to the other - that's the way I'll go. Right now it just seems like it doesn't really matter.

Fred

captainron · #5 02-09-2007, 08:59 AM

Here is some good information from Skytec. Be sure to read the paragraph:"Wrong part-by design?"
http://www.skytecair.com/Cessna_Solenoids.htm

FredMagare · #6 02-09-2007, 11:17 AM

Quote:

Originally Posted by captainron

Here is some good information from Skytec. Be sure to read the paragraph:"Wrong part-by design?"
http://www.skytecair.com/Cessna_Solenoids.htm

It makes sense. The right part for the right job. Coincidentally, I can see a reason for mounting the starter solenoid upside-down. A broken spring would continue to energize the starter if it were rightside-up. (Well...I don't want THAT - in flight or on the ground.)

Fred