P-Mag Test Switches, necessary?

apkp777 · #1 04-01-2010, 04:48 PM

Hey all,

Got my dual P-mags today. Currently I have two breakers (one for each) installed and no test-switches. The installation instructions call for test switches. However, are they really necessary? Why not just use the breakers? How often do you test your P-Mags for self generation?

Obviously no big deal to install a set of switches. Just can't think of a downside using breakers only.

RV8R999 · #2 04-01-2010, 04:57 PM

im not using any nor do I have breakers. I have mine wired through fuses only and will use the master switch to secure power to test the internal gen feature right after start up. My goal is to keep switches to an absolute minimum.

Scott Hersha · #3 04-01-2010, 05:17 PM

I don't know how frequently you check your E-mags, but breakers are not switches. They will wear out. Even the kind you can't manually pull wear out because they are mechanical, operated using a spring and a built in catch. Pullable CB's shouldn't be used as a switch (unless something is wrong). Any toggle, rocker, push-button, etc. switch rated for the load and voltage should work fine.

hydroguy2 · #4 04-01-2010, 05:32 PM

Quote:

Originally Posted by RV8R999

...... will use the master switch to secure power to test the internal gen.....

ME, too. I am using breakers for fault protection. But plan on just dumping the main buss to check self powering of the Pmag. in reality, I plan on testing the self power feature multiple times during phase 1 and then only occasionally after that(like VOR check). My other ign is an Emag off the BATT buss.

apkp777 · #5 04-01-2010, 05:40 PM

Are there any downsides to using the master? I can't think of any.

RVG8tor · #6 04-01-2010, 05:45 PM

I went through the same thought process a few months back. We use CBs as switches on many items on the B-767, Data link, TCAS, voice recorder, these are pulled between every overnight or long sit, so I was thinking along he same lines as you. But in the end I installed N.C. pushbuttons, just push and this removes the power release and it springs back.

apkp777 · #7 04-01-2010, 06:04 PM

The airlines use breakers for switches all the time. I am trying to imagine how often I will test the p-mags. If only once a week or month then I don't see the need for switches. If every flight then maybe.

RVG8tor · #8 04-01-2010, 06:45 PM

Tony,

I know what you are saying, I am not flying but my guess is most do this test once in a while not every flight. I put in the switch because I did not have faith the Klixon breakers I am using would be as robust as we have in the big jets. In the end, I actually think using the CB for the test would be fine. The switches were a convenience for me as they are spring loaded you have to hold them on, so I skipped the idea of using the breakers as switches.

The push button switches I bought were about $9 each and fit the same hole as the other toggles.

Cheers

apkp777 · #9 04-01-2010, 07:52 PM

Quote:

Originally Posted by RVG8tor

Tony,

I know what you are saying, I am not flying but my guess is most do this test once in a while not every flight. I put in the switch because I did not have faith the Klixon breakers I am using would be as robust as we have in the big jets. In the end, I actually think using the CB for the test would be fine. The switches were a convenience for me as they are spring loaded you have to hold them on, so I skipped the idea of using the breakers as switches.

The push button switches I bought were about $9 each and fit the same hole as the other toggles.

Cheers

I hear you. 767's use Klixon breakers if my memory is correct. Probably higher quality though than the $20 Stein version.

RV7ator · #10 04-02-2010, 07:41 PM

Here's your downside.

There's no way to shut off a P-mag if it's generating its own juice, which the later versions do above 900rpm, as they remain on internal supply even if ship's power is disconnected. So if one looses timing, it could very well keep making errant sparks and shake your engine/airplane to pieces and/or drastically reduce power output, bringing about a forced landing even though you have a second good ignition.

Those test switches ground the P-mag, so even if the electronics are still running (properly or not), it won't make a spark. Simply removing ship's power doesn't necessarily shut it down.

Been there, done that. I'm certainly glad I could shut down the bad P-mag.

John Siebold