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LRU Breakers

mfleming

Well Known Member
Patron
My drawing from SteinAir for my all Garmin panel shows all the LRUs with their own breakers.

That's a lot of breakers (or in my case fuses). With all the LRUs having their own breaker, my count is up to 25 :eek:

Is it considered good practice to gang up some of the LRUs?

AND if it is acceptable, do you put all the LRUs calling for a 2amp breaker on a single 2 amp one??
 
Short answer to your questions is a pretty flat NO.

If each device truly is an LRU, it needs to be individually powered. Sometimes this is simply so you can take one LRU off line for troubleshooting purposes while leaving others powered. Sometimes you need to power them up one at a time for software updates.

With respect to your question around ganging devices on a 2amp breaker, think of it this way. On Saturday you hop in your Cessna Caravan with one kid. Mid-flight that kid says he is hungry for a granola bar and you happen to have ONE granola bar in the airplane. You feed the kid, the kid is happy as he had an appetite for one granola bar.

Now it's Sunday and you have SIX kids in your Caravan. You've wisely re-stocked the galley with your one trusty granola bar. Mid-flight you have SIX kids, each with a one-granola-bar hunger yelling they're hungry. You have one granola bar. Exactly how many kids are going to be happy if you split that granola bar six ways? ZERO kids will be happy.

Same goes for LRU's.

Now for a MUCH BIGGER CONCEPT...

Circuit protection devices (breakers, fuses) protect the circuit. DOH! Sounds pretty obvious, doesn't it? Well, the CIRCUIT is the WIRE!!! It's NOT the LRU!!! If an LRU has a specific need for power input protection, that protection will be located INSIDE the LRU.

Spend some time with AC43.13 and get familiar with its wire sizing and circuit protection charts. Fuse or breaker the wires based on the WIRE SIZE. Avionics manufacturer's recommendations for circuit protection sizes are a guideline based on the power consumption of their device, NOT on airworthiness requirements of the total installed system including the long-term continuing airworthiness requirements of the aircraft. Take their recommendation for wire size, double-check it against 43.13 and choose the bigger of the two wires (manufacturer wire size will take into account things like inrush current, while 43.13 takes into account things like whether the wire is in a conduit or a bundle).

I know this sounds like I'm being a bit of a jerk, but the reality is that if we stick with this basic concept of fusing the wire to protect the wire and its ability to conduct current, we are future-proofing ourselves. Imagine Garmin or Dynon had a little box with only a few functions that consumed one amp and was fed by a 20ga wire (by their recommendation). A couple of years later they come out with a replacement device that does a zillion different things, has a far brighter display, a far faster processor, and takes 4 amps. If you installed a 1A circuit breaker you're going to be kicking yourself. If, however, you installed the 7.5A breaker that 43.13 would have recommended, you have no problems in changing the LRU to the new fancy box without ever having to worry about how well its power input wiring is protected.

See, when you look at it that way, protecting the wire based on the wire's needs makes a whole lot of sense. That's why it's the industry standard method for electrical system design!
 
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I agree with the above comments - circuit protection devices are there to protect the circuit, not the load. My only addition on that topic is that the manufacturer of a particular device might have reason to specify a fuse/breaker with a lower rating than would otherwise be appropriate for the wire gauge, on account of things like the size of PCB traces, connector pins, etc.

My RV has a complex glass panel and only two breakers, one for each alternator field. Everything else is fuse blocks and ATO blade fuses, ala the Bob Nuckolls approach. It's a personal preference, but I like it because it's cheaper, lighter, and consumes no panel space. My philosophy is that a properly designed electrical system shouldn't need in-flight access to reset the circuit protection for most devices (alternators are an exception due to the nature of crowbar overvoltage protection). Also my experience with Garmin LRUs is that there isn't a particular need to "cycle the breaker" for any in-flight condition.

Your mileage may vary - the above is not intended to be an argument for or against any particular approach, just how I chose to do it.

Edit to add: There is certainly validity to the idea that the pilot ought to have a cockpit-accessible means to interrupt power to the electric trim system in the event of a stuck switch or wire shorted to ground (not an impossible scenario given how these wires flex). In my case I have other devices that mitigate this scenario to my satisfaction, but it's something to keep in mind from a system design standpoint.
 
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When designing my panel, I was pulled between Van's design for the RV14 with all fuses, and the "standard" of an array of breakers. I felt a need to be able to switch off individual screens (all LRUs really) but didn't feel a need to reset in flight systems like lights (if a circuit blew, there is a reason best researched on the ground). After much reflection, I ended up with a hybrid set up - I have a switchable breaker on the panel for each major LRU (I ended up with about 10) and fuse panels for all the others, not accessible in flight for all practical purposes. The wiring is a little more complex (fuse buss and breaker buss for each of Main and Avionics circuits as well as an Essential buss), but I have been very happy with the functionality. In addition to having the ability to "reboot" a screen (and it has happened to me on a PFD in flight - full lockup), I use the breakers routinely during software updates to reduce the current draw.
 
Short answer to your questions is a pretty flat NO.

If each device truly is an LRU, it needs to be individually powered. Sometimes this is simply so you can take one LRU off line for troubleshooting purposes while leaving others powered. Sometimes you need to power them up one at a time for software updates.

With respect to your question around ganging devices on a 2amp breaker, think of it this way. On Saturday you hop in your Cessna Caravan with one kid. Mid-flight that kid says he is hungry for a granola bar and you happen to have ONE granola bar in the airplane. You feed the kid, the kid is happy as he had an appetite for one granola bar.

Now it's Sunday and you have SIX kids in your Caravan. You've wisely re-stocked the galley with your one trusty granola bar. Mid-flight you have SIX kids, each with a one-granola-bar hunger yelling they're hungry. You have one granola bar. Exactly how many kids are going to be happy if you split that granola bar six ways? ZERO kids will be happy.

Same goes for LRU's.

Now for a MUCH BIGGER CONCEPT...

Circuit protection devices (breakers, fuses) protect the circuit. DOH! Sounds pretty obvious, doesn't it? Well, the CIRCUIT is the WIRE!!! It's NOT the LRU!!! If an LRU has a specific need for power input protection, that protection will be located INSIDE the LRU.

Spend some time with AC43.13 and get familiar with its wire sizing and circuit protection charts. Fuse or breaker the wires based on the WIRE SIZE. Avionics manufacturer's recommendations for circuit protection sizes are a guideline based on the power consumption of their device, NOT on airworthiness requirements of the total installed system including the long-term continuing airworthiness requirements of the aircraft. Take their recommendation for wire size, double-check it against 43.13 and choose the bigger of the two wires (manufacturer wire size will take into account things like inrush current, while 43.13 takes into account things like whether the wire is in a conduit or a bundle).

I know this sounds like I'm being a bit of a jerk, but the reality is that if we stick with this basic concept of fusing the wire to protect the wire and its ability to conduct current, we are future-proofing ourselves. Imagine Garmin or Dynon had a little box with only a few functions that consumed one amp and was fed by a 20ga wire (by their recommendation). A couple of years later they come out with a replacement device that does a zillion different things, has a far brighter display, a far faster processor, and takes 4 amps. If you installed a 1A circuit breaker you're going to be kicking yourself. If, however, you installed the 7.5A breaker that 43.13 would have recommended, you have no problems in changing the LRU to the new fancy box without ever having to worry about how well its power input wiring is protected.

See, when you look at it that way, protecting the wire based on the wire's needs makes a whole lot of sense. That's why it's the industry standard method for electrical system design!

Yea....I'm an A&P.

I really should have prefaced my original post that I am aware that circuits are on their own fuse/breaker but was wondering if the LRUs are similar to for instance, NAV lights.

We don't have an individual breaker for each light. The NAV light circuit has multiple wires and multiple devices and is considered one circuit.

My wonderings were, are LRU similar to the above example. If not, I'll move on with the individual fuse's/breakers. But the question is legitimate and not without precedence given the lighting circuit example listed above.

Your post will be helpful to those who read my OP and wonder if multiple circuits on a single breaker are OK. As a rule, having unrelated devices on a single fuse/breaker is not best practices.

But defining a circuit is NOT one breaker for one wire or one device.
Not that you said that but I thought it needed to be stated.
 
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I agree with the above comments - circuit protection devices are there to protect the circuit, not the load. My only addition on that topic is that the manufacturer of a particular device might have reason to specify a fuse/breaker with a lower rating than would otherwise be appropriate for the wire gauge, on account of things like the size of PCB traces, connector pins, etc.

My RV has a complex glass panel and only two breakers, one for each alternator field. Everything else is fuse blocks and ATO blade fuses, ala the Bob Nuckolls approach. It's a personal preference, but I like it because it's cheaper, lighter, and consumes no panel space. My philosophy is that a properly designed electrical system shouldn't need in-flight access to reset the circuit protection for most devices (alternators are an exception due to the nature of crowbar overvoltage protection). Also my experience with Garmin LRUs is that there isn't a particular need to "cycle the breaker" for any in-flight condition.

Your mileage may vary - the above is not intended to be an argument for or against any particular approach, just how I chose to do it.

Edit to add: There is certainly validity to the idea that the pilot ought to have a cockpit-accessible means to interrupt power to the electric trim system in the event of a stuck switch or wire shorted to ground (not an impossible scenario given how these wires flex). In my case I have other devices that mitigate this scenario to my satisfaction, but it's something to keep in mind from a system design standpoint.

I'm also going to go all (mostly) fuses for all the reasons you mention. I've read Paul Dye's PROs and CONs on the fuse/breaker debate. Mostly he championed breakers because they were more convenient during maintenance and software uploads. Paul also made it clear he was not against fuses.

I will have to talk with SteinAir about the fuse/breaker for the trim. My trim will run through the GAD 27. Maybe a breaker for it?

My OP was because I counted 25 fuses and thought that was a lot.

How did you mount your fuse blocks on your -7? I'm thinking of a drop down panel at the present time. Pictures if you got 'em :D
 
When designing my panel, I was pulled between Van's design for the RV14 with all fuses, and the "standard" of an array of breakers. I felt a need to be able to switch off individual screens (all LRUs really) but didn't feel a need to reset in flight systems like lights (if a circuit blew, there is a reason best researched on the ground). After much reflection, I ended up with a hybrid set up - I have a switchable breaker on the panel for each major LRU (I ended up with about 10) and fuse panels for all the others, not accessible in flight for all practical purposes. The wiring is a little more complex (fuse buss and breaker buss for each of Main and Avionics circuits as well as an Essential buss), but I have been very happy with the functionality. In addition to having the ability to "reboot" a screen (and it has happened to me on a PFD in flight - full lockup), I use the breakers routinely during software updates to reduce the current draw.

Having a combo of fuses and breakers sound appealing and I will have some breakers (ALT field, possibly trim). The main reason for not doing a combination fuse/breakers is the complexity as you describe. More power to you for doing it this way but I'm trying to keep it as simple as I can given I have a complex panel anyway.

Here's what I'm doing.
panel.jpg


I'll have a slightly deeper panel in the end but still, not a lot of room for extra switches.
 
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I'm also going to go all (mostly) fuses for all the reasons you mention. I've read Paul Dye's PROs and CONs on the fuse/breaker debate. Mostly he championed breakers because they were more convenient during maintenance and software uploads. Paul also made it clear he was not against fuses.

SNIP

I built my first RV with fuses (the new trend at the time) and found them to be a real PITA as they they end up getting mounted were access in not easy. The fuses in that first RV were replaced with breakers during the last panel update.

On the current RV-8 I have a total of 12 pull breakers plus two spares on the panel that support both left and right avionics (dual battery install). The non-avionics related loads have breakers that all fit on the side panel. The RV-10 was done the same way.

Note - the only breakers on the center removable part of the panel go to the panel mounted radio and audio panel, plus a spare. This makes for a 15 minute job to remove the panel from the plane for work at the bench - and for the mods that always come along.

Carl
69-F130-FC-4-BBB-4292-B961-9-CA466-A67-D13.jpg
 
I used blade fuses in my RV-10, but the fuse block was behind the panel and was an issue for my IFR flight instructor, as it was not plainly visible for him to see. On my RV-9A, I am again using the blade fuses, but I managed to fit it on the panel below my custom map box. I bought 2 of these from WayTek as 20 spaces was not enough. If you look closely, you can see that they are modular units and after I took them apart drilling out the buss bars, I was able to modify one into a 24 position unit. I can't use the cover though. Perfect for my needs and still small enough to fit the panel space I had available. They will make them custom, but you need to order 100 at a time for that! I like that you order the blade fuse connectors separate and can then assemble them with the wire of your choice. I'll get a pic posted of my assembly in the panel next week.
49901983243_5e6e522778_o.jpg
[/url]15712-12_04-21A by David C, on Flickr[/IMG]
 
Having a combo of fuses and breakers sound appealing and I will have some breakers (ALT field, possibly trim). The main reason for not doing a combination fuse/breakers is the complexity as you describe. More power to you for doing it this way but I'm trying to keep it as simple as I can given I have a complex panel anyway.

Here's what I'm doing.
panel.jpg


I'll have a slightly deeper panel in the end but still, not a lot of room for extra switches.

I think you may have trouble fitting everything in this panel when you add some space for avionics rails for the radios, or you will have to move everything down and perhaps extend the panel if you want the switches on the bottom to fit.

Every panel I do has breakers for the primary LRU's along with a few for such things as field/trim/flaps, this usually is around 15 CB's

I always use a fuse panel for all the other 'stuff' that I consider non-essential which can easily add another 10-15 circuits.
No room for 30+ CB's on the average panel.

I only use Klixon breakers and Honeywell switches, no tyco switch/breakers allowed.
 
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Checking circuit protecting devices should be part of your preflight check.

While verifying CB's is easy and fast; checking blade fuses will be time consuming.

I agree with Walt, use blade fuses for non essential stuff only.
 
I think you may have trouble fitting everything in this panel when you add some space for avionics rails for the radios, or you will have to move everything down and perhaps extend the panel if you want the switches on the bottom to fit.

Every panel I do has breakers for the primary LRU's along with a few for such things as field/trim/flaps, this usually is around 15 CB's

I always use a fuse panel for all the other 'stuff' that I consider non-essential which can easily add another 10-15 circuits.
No room for 30+ CB's on the average panel.

I only use Klixon breakers and Honeywell switches, no tyco switch/breakers allowed.

Hopefully it will fit :D

SteinAir looked at it and said it would but using a slightly deeper panel. I'm using them to help design and cut out the panel, so we'll see soon enough.

I'm just getting started building the harness and have photos of some of your panel installs for inspiration.
 
Checking circuit protecting devices should be part of your preflight check....SNIP............

If you have circuit breakers to check ;)

There are airplanes with no fuses/circuit breakers available for the pilot to check.

I may be wrong but I don't think there are any breakers in this photo.
cockpit.jpg


You check that the system the fuse controls is operable.

  • Flaps go up and down - GAD27 fuse OK
  • GPS Talks to the GDU - GAD 29 fuse OK
  • COM comes on line - COM fuse OK
  • No ADHARS alarm - GSU-25 fuse OK
ETC....
 
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I'm a fuse guy for there simplicity and compactness.

Here is my fuse panel and its location. Note... It is for two buses so each LRU has two fuses. 32 fuse locations, several spares.

Also, like you commented, some things are ganged, but most are not.

picture-46.jpg

picture-68.jpg


Not pictured is a little fuse puller velcro'd just below it.
 
I'm a fuse guy for there simplicity and compactness.

Here is my fuse panel and its location. Note... It is for two buses so each LRU has two fuses. 32 fuse locations, several spares.....SNIP............

Tell me more about the two fuses for each LRU?
My drawings only show one fuse/breaker per LRU. My drawing show some of the LRUs get power from the back-up battery but unfused.

PS. what model Bussmann panel is that?
 
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My fuse panel is made up of Bussmann Series 1571X fuse blocks. They come in various sizes. I bought mine from an RV builder who was selling different sizes of them. I'll see if I can find my notes on where I got them. Don't know if he still resells them.

Here's a link to a 20 slot 2 bus block like one of the ones I have...

https://www.waytekwire.com/item/46074/EATON-s-Bussmann-Series-15712-14-06-21A-ATC-Fuse/

It does take special pins so you need to buy them as well.

I wanted two 16 fuse position blocks but he had a 12 and a 20 so I bought those. The 20 was able to be split into two buses - 14/6 - so I combined them so that I had 18 slots for my main bus and 14 for my aux bus.

I used the Z-14 electrical system from the aero electric drawings as my architecture. Two alternators, two batteries on two separate busses that can be crossfed if needed.

Almost all of the G3x LRU's come with two power inputs with diodes built in to prevent crossfeed from one bus to the other. My SOP is to power up on the aux bus as soon as I get in the cockpit. Then when ready to start up turn on the main bus. After engine start, engage both alternators and I have two power paths to all critical equipment.

I had Stein make me an interconnect cable for all the equipment but with only unterminated leads for all the power/ground inputs and I custom wired all of that.
 
Here are a few photos of my customized Bussmann blade fuse module. I built this out of two 20 position blocks. They are modular in design so it was not hard to do. I have the option of three busses, two busses, or one buss with a jumper. 14 position, 6 position, and 8 position, or a combination of any.
49913435078_1346b41854_z.jpg
[/url]3 by David C, on Flickr[/IMG]
49913435103_9fa97d87cd_z.jpg
[/url]2 by David C, on Flickr[/IMG]
49913435123_e11d911f0d_z.jpg
[/url]1 by David C, on Flickr[/IMG]
49913949036_43b3fbd9ed_z.jpg
[/url]4 by David C, on Flickr[/IMG]
 
Our pre-loved RV9A has a hinged drop-down fuse panel for nearly every system above the right pilot's knees under the instrument panel, and three resettable breakers for the AP, trim and alternator, on the front of the instrument panel near the same location.

I like the idea of having a breaker for those three at least, especially if trim runaway occurs. Personal preference would be to have then right in front of the left pilot though.
 
Our pre-loved RV9A has a hinged drop-down fuse panel for nearly every system above the right pilot's knees under the instrument panel, and three resettable breakers for the AP, trim and alternator, on the front of the instrument panel near the same location.

I like the idea of having a breaker for those three at least, especially if trim runaway occurs. Personal preference would be to have then right in front of the left pilot though.

Having a hinged drop down panel on the right side is what I'm thinking of doing. Glad to hear its works for you.

Since my trim will go through the GAD-27 electronic adapter unit (magic box:p) I'm not sure if I need a breaker on the trim or not.
 
Here are a few photos of my customized Bussmann blade fuse module. I built this out of two 20 position blocks. They are modular in design so it was not hard to do. I have the option of three busses, two busses, or one buss with a jumper. 14 position, 6 position, and 8 position, or a combination of any.
49913435078_1346b41854_z.jpg
[/url]3 by David C, on Flickr[/IMG]
49913435103_9fa97d87cd_z.jpg
[/url]2 by David C, on Flickr[/IMG]
49913435123_e11d911f0d_z.jpg
[/url]1 by David C, on Flickr[/IMG]
49913949036_43b3fbd9ed_z.jpg
[/url]4 by David C, on Flickr[/IMG]

Very cool
icon14.gif
 
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