SL30 power inputs?

I've got the installation manual for the UPS AT SL-30 and it appears that to power an SL-30, you use two fuses, or breakers. One for the Comm side and one for the Nav side. Can anyone confirm? I'm in the middle of panel design and if I go with breakers, I'd like to make sure I get this right. What's odd, is that I've seen some breaker based panel designs that *don't* have two breakers for this radio???

Any help appreciated.

Thanks,
Alan

Hi Alan,

Technically you're correct. That being said, it kind of depends on you mission for the plane. The reason you see a lot of planes (including certified ones) with only 1 breaker for both the Nav and Comm sides of radios (yep...lot of certified birds came that way) is because the though process is to treat it like one unit. If the Nav fries on a VFR bird it's not that detrimental and if the Comm side dies you're out of luck anyway.

I can't tell you which way to do it, we do a lot of both for customers. Some only want 1 breaker for the entire unit, some want one of each. Some radios wont work with 2, some require only 1. In the case of the SL-30 take your pick...either will work fine.

Hope my confustion statement helps!

Cheers,
Stein.

Thanks

Stein,

Thanks, yep, made sense. Hmmm... Well, what else is new with experimentals... Decisions, decisions, decisions... How do you determine which size of breaker or fuse if you are going to combine to one? It can't be the largest of the two, so it must be the combined values closest size. This later concept doesn't make me feel very good about wire size and smoke in the cockpit if one side or the other frys. The failure scenarios around combined fusing, just don't make me feel comfortable.... I have the space, so perhaps I'll just do two.

Alan

IIRC --- I didn't get to make the choice. Fast Stack Hubs made it for me; when the wiring harness showed up with only one hot wire. (and I had to promise Pacific Coast Avionics that I WOULDN'T do the avionics wiring.)

Never crossed my mind until now. 10 amp for my Nav/com.

very interesting. thanks guys.

My problem with one

Ok, so here is my basic problem with one breaker...

Now you are saying it takes 10 amps of current to trip the breaker, and yet that's 3 more amps than either the 7 amp or the 5 amp seperates would take( I made those numbers up, as I don't know actually what each individual value is, but each alone is less than the 10A). So, first of all, if you trip the breaker you take down both parts of the radio, regardless of which half tripped. Also you tripped the breaker with more current than either separate input causing something *worse* to happen to draw that 10 amps! Internal wiring or circuit board traces will be what goes with that additional current. In effect, something that might have been repairable, just cost you a whole radio! And IIRC, the SL-30's aren't cheap.

I think I've just talked myself into separate breakers. BTW, this isn't an SL-30 only thing, I believe the 430 also uses separate inputs (perhaps as many as 3 voltage sources to feed the nav, comm, and gps). So if you have one with one breaker or fuse, you have the same problem.

Thanks for the dialog, y'all just convinced me of my decision.

Alan

One or two?

One guide for reference is the FARs... while we do not need to meed the certification requirements, they do give a guideline...

FAR 23.1357 is the applicable one

http://www.flightsimaviation.com/data/FARS/part_23-1357.html

"(b) A protective device for a circuit essential to flight safety may not be used to protect any other circuit."

So decide if your SL-30 parts are independently needed to be essential to flight safety (maybe so if you fly IFR and it's the only Nav you have) and use one or two breakers/fuses...

gil in Tucson ... a much easier decision if you are VFR...

Power inputs

I also have the Approach Systems Pro G wiring hub. (Great piece of equipment.) The SL40 harness only had one "red" wire. The 430 had several. The two biggest ones were labeled 10amp. I was advised to wire everything to one breaker. The reason for two big red wires was to eliminate the need for ONE significantly large wire.

Don't know for a fact but I believe if you lose any part you lose all. Although the 430 has numerous functions, I don't believe they are stand alone in the event of failure.

Darwin N. Barrie
Chandler AZ

aadamson said:

Ok, so here is my basic problem with one breaker...

Now you are saying it takes 10 amps of current to trip the breaker, and yet that's 3 more amps than either the 7 amp or the 5 amp seperates would take( I made those numbers up, as I don't know actually what each individual value is, but each alone is less than the 10A).

Click to expand...

I'm kinda new here, so pardon me for the following somewhat tangential monologue. It doesn't really answer the one vs. two breaker question. But I think that has already essentially been answered. One breaker is fine, or two breakers are fine. Whichever you choose. But here's some basic info on how to size your breakers.

When sizing a "standard" rated breaker, the 80 percent rule applies for continuous load circuits. In other words, you shouldn't load the breaker with more than 80 percent of the expected continuous current. So for a 10 amp breaker, you should never connect more than 8 amps of "continuous" load. (Continuous is defined in the National Electric Code, or NEC, as "a load where the maximum current is expected to continue for three hours or more.")

It's important to remember that the breakers are there to protect the wiring and the power source, not the load. Most radios (i.e. expensive loads) have some sort of internal Over Current Protection Device (OCPD) such as a fuse or thermal cutoff switch. I haven't looked inside of an SL30 so I don't know what internal protection it has, if any. But as far as the aircraft wiring circuit breaker is concerned, the radio is expendable.

With a radio system, the maximum load is usually generated when transmitting. So using the 3-hour defintion, you should never see a "continuous load" from your SL30. Therefore you could look in the specs for the "nominal" operational load for your sizing.

There's a lot of science and math behind the sizing of breakers. You can study the breaker's characteristic trip curve, which takes into account time and thermal factors, in addition to overall load.... And then, some breakers are "100% rated" rather than standard rated, which means they can take 100 percent of the continuous load rather than the 80 percent... Which really means that they were tested at higher loads or under different thermal conditions than the standard breakers....

You can see that its easy to get wrapped around the axle on this topic. So I just keep it simple and use the 80 percent rule.

Happy wiring!