Van's Air Force
Don't miss anything! Register now for full access to the definitive RV support community.
What is the purpose of fuses and breakers ?
- Thread starter N787R
- Start date
tomcostanza
Well Known Member
Yes!
Seriously, it depends on the size of the fuse/cb and where it's located. The short answer is, the wiring. You would use a 7A fuse to protect a 20 gauge wire. Obviously this won't do diddly to keep a nav/com that draws a max of 3A from self-destructing. I expect those items to be internally protected, or alternately, to instruct me to fuse it for a certain current.
Clear skies,
Protect wiring
Thanks. I thought it was the wiring.
Well here is the main reason I asked the questiion:
I was thinking to put few RELAYs like (S704-1 form B&C) in the wingtip to control switching of nav, landing, strobe and even pitot. Then I would only run one power line into the wing with several thin wires to control switches.
1) Does this make any sense?
2) If the fuses are to protect the wiring, then I only need one fuse to protect the main wire that goes out to wing to supply those components. Right?
Ben
Thanks. I thought it was the wiring.
Well here is the main reason I asked the questiion:
I was thinking to put few RELAYs like (S704-1 form B&C) in the wingtip to control switching of nav, landing, strobe and even pitot. Then I would only run one power line into the wing with several thin wires to control switches.
1) Does this make any sense?
2) If the fuses are to protect the wiring, then I only need one fuse to protect the main wire that goes out to wing to supply those components. Right?
Ben
John Clark
Well Known Member
Maybe...
If the "control" wires, the thin ones to the relays, are setup to ground the relay to activate it you don't need to fuse them. My concern about your plan is that if, for instance, the pitot heat shorts and blows the fuse/breaker you are going to lose everything else on that power source. Kinda putting all your eggs in one basket. But then I'm a big fan of the KISS principle.
John Clark
RV8 N18U "Sunshine"
KSBA
If the "control" wires, the thin ones to the relays, are setup to ground the relay to activate it you don't need to fuse them. My concern about your plan is that if, for instance, the pitot heat shorts and blows the fuse/breaker you are going to lose everything else on that power source. Kinda putting all your eggs in one basket. But then I'm a big fan of the KISS principle.
John Clark
RV8 N18U "Sunshine"
KSBA
tomcostanza
Well Known Member
Ditto what John said. If you're day VFR that might be OK. But you should always keep redundancy in mind. Or, what will I do if X happens.
Geico266
Well Known Member
I always wondered how electricity works.
GAHco
Well Known Member
This might sound funny from a guy who works in the space program, but I agree with keeping things as simple as possible (and no simpler). Switches are pretty darn robust, and relays are finicky....why have two devices to do the task when one has been used in virtually every airplane around. If the idea is to experiment, then that's OK...if the idea is to have a reliable, robust system - I'd buy good switches and few relays.
Paul
Paul
vlittle
Well Known Member
Despite the comments about switches being more reliable than relays, I've had three switch failures in my airplane (Carling Switches). This was traced to loose rivets on the switches (bad manufacturing). There is no fundamental reason why a relay is less reliable than a switch, if properly designed.
Lets say you want a landing light, nav light, strobe unit and Pitot heat on one circuit. You could run one large wire (probably 12 gauge) to the wing tip, along with 4 control lines. In the wing tip you'd have 4 relays, along with 4 fuses or circuit breakers for the individual circuits. This would prevent any one circuit fault from taking out the others. You'll need access to the fuses. A good location would be under the wing tip lense for easy access and visibility. Now THAT will stop you from diagnosing a fault in flight!
Is this simpler and more reliable than the traditional method? Not sure.
Where it might work well is when using solid-state drivers with individual programmable current limits and current feedback, driven by a serial bus connected to a central controller (Vertical Power does this). I'm not crazy about the Vertical Power concept of complete automation, but they are on the right track. I think a properly designed distributed system could save several pounds of weight and provide excellent fault diagnosis.
http://www.verticalpower.com/
Vern
az_gila
Well Known Member
May actually weigh more...
Vern, I think the weight savings in our little RVs would be quite negligible...
For your example, say 15 ft of wire to one tip
Your example... 60 ft of 22 g. and 15 ft of 12 g.
= 0.21 + 0.33 = 0.54 lbs.
Traditional wiring .... 4 x 18 g wires = 60 ft 18 g
= 0.46 lbs.
Actually 0.08 lbs LIGHTER for the traditional approach.
Other items...
The four 22 g. control wires may be more sensitive to damage.
The relays to switch 10 amps with reliability probably will weigh more than the 10 amp switches.
You still need 4 switches, but they will be smaller, so you need to compare a 10 amp switch weight vs. a small switch plus a 10 amp relay
I think that for our RV size and complexity, this added effort saves little... that's why small planes have used traditional methods sucessfully for a long time....
Now if you go digital with a single bus.....
gil A
MIL-W-22759/16 wire weights are here....
http://www.awcwire.com/ProductSpec.aspx?id=MIL-W-22759/16-Standard
Vern, I think the weight savings in our little RVs would be quite negligible...
For your example, say 15 ft of wire to one tip
Your example... 60 ft of 22 g. and 15 ft of 12 g.
= 0.21 + 0.33 = 0.54 lbs.
Traditional wiring .... 4 x 18 g wires = 60 ft 18 g
= 0.46 lbs.
Actually 0.08 lbs LIGHTER for the traditional approach.
Other items...
The four 22 g. control wires may be more sensitive to damage.
The relays to switch 10 amps with reliability probably will weigh more than the 10 amp switches.
You still need 4 switches, but they will be smaller, so you need to compare a 10 amp switch weight vs. a small switch plus a 10 amp relay
I think that for our RV size and complexity, this added effort saves little... that's why small planes have used traditional methods sucessfully for a long time....
Now if you go digital with a single bus.....
gil A
MIL-W-22759/16 wire weights are here....
http://www.awcwire.com/ProductSpec.aspx?id=MIL-W-22759/16-Standard
vlittle
Well Known Member
I agree, in general that the benefits are slim. KISS works, and failures are isolated.
Back in the 1980's IEEE Spectrum publish a graph on the probability of success for any particular solution to a problem. In summary it said this: Simple, limited function, high probability. Complex, comprehensive function, high probability. Everything else in the middle, lower probability.
It's a classic bathtub curve (Like a 'U').
This curve applies to everything from pens to space shuttles. I've seen many products fail because they fell somewhere in the middle. Switches, Breakers and wires = simple. Vertical Systems products = complex. Will Vertical Systems succeed? Yes, if the complexity is so great that it handles all reasonable failure modes, provides clear benefits and is reasonably priced. The big challenge is price. I can provide a protected circuit on my airplane for $7.50: $5.00 for the switch and $2.83 for the circuit breaker.
What! $2.83 for a circuit breaker? Go to Digikey and search for W28 series breakers. I use 22 of them in my airplane, and they work fine. They are cheaper than a fuse holder + a fuse.
V
Back in the 1980's IEEE Spectrum publish a graph on the probability of success for any particular solution to a problem. In summary it said this: Simple, limited function, high probability. Complex, comprehensive function, high probability. Everything else in the middle, lower probability.
It's a classic bathtub curve (Like a 'U').
This curve applies to everything from pens to space shuttles. I've seen many products fail because they fell somewhere in the middle. Switches, Breakers and wires = simple. Vertical Systems products = complex. Will Vertical Systems succeed? Yes, if the complexity is so great that it handles all reasonable failure modes, provides clear benefits and is reasonably priced. The big challenge is price. I can provide a protected circuit on my airplane for $7.50: $5.00 for the switch and $2.83 for the circuit breaker.
What! $2.83 for a circuit breaker? Go to Digikey and search for W28 series breakers. I use 22 of them in my airplane, and they work fine. They are cheaper than a fuse holder + a fuse.
V
the_other_dougreeves
Well Known Member
Along that line. switch-style breakers also work well in aircraft applications. Saves weight (wiring and components), saves $$ (wiring labor and components) and reduces the chance of part failure (fewer connections). Downside is that it's harder to tell if it's tripped.
TODR
Jeff R
Well Known Member
I had recently been thinking of something similiar - but rather than using mechanical relays, I was thinking of using solid state MOSFET transistors as solid state relays. You can get modern N channel MOSFETs which, when turned on, present resistances of less than .01 Ohms. These devices are far more reliable than any mechanical relay, though they may need a protective circuit to ensure voltage transients don't destroy them.
Hooking up a few small wires to a light-duty switch on the panel is certainly easier than hooking up 12 or 14 gage wires, and since MOSFETs draw essentially no power, you can use a low current (i.e. small and low cost) switch to turn them on and off. However, you still will need a switch, which will be the limiting factor on reliability, and you will still need to hook up those heavy wires at the wing tip, plus you have to figure out how to mount the electronics, be it a relay or MOSFET.
So, for those reasons, I dismissed the idea on the grounds of complexity.
Hooking up a few small wires to a light-duty switch on the panel is certainly easier than hooking up 12 or 14 gage wires, and since MOSFETs draw essentially no power, you can use a low current (i.e. small and low cost) switch to turn them on and off. However, you still will need a switch, which will be the limiting factor on reliability, and you will still need to hook up those heavy wires at the wing tip, plus you have to figure out how to mount the electronics, be it a relay or MOSFET.
So, for those reasons, I dismissed the idea on the grounds of complexity.
carguy614
Well Known Member
Relays
Just my 2 cents. My job is automotive electronics diagnostics and repair. I really feel that if at all possible you should consider NOT relaying the wingtip works. The added weight of the relays probably won't cancel the wiring weight, and the chances for failure multiply many times. Unless you use really high quality relays, expensive relays that is, your chances of relay trouble in the years to come are pretty high. The wingtip is a harsh environment. Look inside one thats a few years old.
I considered using a GM Rear Integration Module (RIM) to control the works digitally, but was concerned about the RF generated by the strobes and other interference issues, as I have VOR and GS antennas in my tips. Neat applicatin for sure, but much like killing a mouse with a machine gun!! Decided on straight runs of Tefzel to good quality toggles to be the reliable solution.
HTH...Chris
Just my 2 cents. My job is automotive electronics diagnostics and repair. I really feel that if at all possible you should consider NOT relaying the wingtip works. The added weight of the relays probably won't cancel the wiring weight, and the chances for failure multiply many times. Unless you use really high quality relays, expensive relays that is, your chances of relay trouble in the years to come are pretty high. The wingtip is a harsh environment. Look inside one thats a few years old.
I considered using a GM Rear Integration Module (RIM) to control the works digitally, but was concerned about the RF generated by the strobes and other interference issues, as I have VOR and GS antennas in my tips. Neat applicatin for sure, but much like killing a mouse with a machine gun!! Decided on straight runs of Tefzel to good quality toggles to be the reliable solution.
HTH...Chris
Piloto.Mendes
Active Member
I like Vertical Power's concept, buit it's still quite expensive. I'd buy it in 2.5-3.0 K range.
2) No. The fuse in the cockpit would protect the wire that feeds the relay, but you still need the appropriate protection for the wiring after the relay. Some of this wire will be integral to the device being powered, inside the pitot heat for example. Therefore, you cannot just simply up the size of the power and ground wires. For your design, you would need the fuse / breaker in the cockpit to handle the combined load of all components, then another fuse on each line out of the relay to each individual component.
rv7charlie
Well Known Member
back from the vault...
While prowling the archives for ideas on fuse panel locations (in general, I prefer fuses for weight, simplicity, reliability, & cost reasons), I stumbled across this thread and Vern's mention of W28 fuses. Thanks for the tip, Vern. Ever curious, I googled them & got an easy to browse Digikey catalog link to all their fuses & circuit breakers. Vern's breakers look good for a reset-able & easy to mount solution. After wandering around a bit, I found thiese:
http://www.digikey.com/catalog/en/partgroup/w51-series/4287
which look good if an economical switchable breaker is needed. Less than $4 ea, roughly the same current price as the W28 series. Only downside I see is the need to cut a rectangular hole (and the fact that you have to build your own bus to tie them together...).
Charlie
While prowling the archives for ideas on fuse panel locations (in general, I prefer fuses for weight, simplicity, reliability, & cost reasons), I stumbled across this thread and Vern's mention of W28 fuses. Thanks for the tip, Vern. Ever curious, I googled them & got an easy to browse Digikey catalog link to all their fuses & circuit breakers. Vern's breakers look good for a reset-able & easy to mount solution. After wandering around a bit, I found thiese:
http://www.digikey.com/catalog/en/partgroup/w51-series/4287
which look good if an economical switchable breaker is needed. Less than $4 ea, roughly the same current price as the W28 series. Only downside I see is the need to cut a rectangular hole (and the fact that you have to build your own bus to tie them together...).
Charlie
MauiLvrs
Well Known Member
It is!!!
az_gila
Well Known Member
Quote:
Originally Posted by N787R View Post
Thanks. I thought it was the wiring.
Not exactly, the "bible" (AC 34.13) says this -
11-47. GENERAL. All electrical wires
must be provided with some means of circuit
protection. Electrical wire should be protected
with circuit breakers or fuses located as close
as possible to the electrical power source bus.
Normally, the manufacturer of electrical
equipment will specify the fuse or breaker to
be used when installing the respective equipment,
or SAE publication, ARP 1199, may be
referred to for recommended practices.
As an example, the GTX-320 manual calls for 20 g wire and a 3 Amp fuse.
In this case, the fuse is sized to protect the device first, not the wire.
Originally Posted by N787R View Post
Thanks. I thought it was the wiring.
It is!!!
Not exactly, the "bible" (AC 34.13) says this -
11-47. GENERAL. All electrical wires
must be provided with some means of circuit
protection. Electrical wire should be protected
with circuit breakers or fuses located as close
as possible to the electrical power source bus.
Normally, the manufacturer of electrical
equipment will specify the fuse or breaker to
be used when installing the respective equipment,
or SAE publication, ARP 1199, may be
referred to for recommended practices.
As an example, the GTX-320 manual calls for 20 g wire and a 3 Amp fuse.
In this case, the fuse is sized to protect the device first, not the wire.
MauiLvrs
Well Known Member
Garmin can do what ever they want...
Fuses are to protect the wire
If it happens to protect a device ... that is a nice side effect.
az_gila
Well Known Member
Priority
NO, you have the order incorrect.
The fuse protects the device first - just like the FAA says - and in the process it protects the wire. In general, the device is more precious than the wire, and the fuse rating will be smaller than that needed for wire protection.
If the device is generic, like a nav light, then it is sized to protect the wire.
It's not a "one answer fits all" item.
Check the OP question.
Garmin can do what ever they want...
Fuses are to protect the wire
If it happens to protect a device ... that is a nice side effect.
NO, you have the order incorrect.
The fuse protects the device first - just like the FAA says - and in the process it protects the wire. In general, the device is more precious than the wire, and the fuse rating will be smaller than that needed for wire protection.
If the device is generic, like a nav light, then it is sized to protect the wire.
It's not a "one answer fits all" item.
Check the OP question.
Last edited:
MauiLvrs
Well Known Member
The question was " What is the purpose of fuses and breakers ?"
The question was not what does the fues do...
The purpose of the fuse is to protect the wire!
In you house you there is a 12ga wire with a 20 amp fuse...
It doesn't know or care what is plugged in...
The 20 amp fuse is to protect the 12 ga wire
The question was not what does the fues do...
The purpose of the fuse is to protect the wire!
In you house you there is a 12ga wire with a 20 amp fuse...
It doesn't know or care what is plugged in...
The 20 amp fuse is to protect the 12 ga wire
az_gila
Well Known Member
The question was " What is the purpose of fuses and breakers ?"
The question was not what does the fues do...
The purpose of the fuse is to protect the wire!
In you house you there is a 12ga wire with a 20 amp fuse...
It doesn't know or care what is plugged in...
The 20 amp fuse is to protect the 12 ga wire
Bad comparison to aircraft.
In my RV a 3 amp fuse is going to protect the transponder, and a 1 amp fuse for the intercom, even though they are both 20 g wire. A lot of the wiring installed is one size, but several different fuse values exist depending on the application.
My house doesn't know what is at the end of each 12 g wire, but just about every wire in my RV is dedicated to a particular device with a known load.
Last edited:
MauiLvrs
Well Known Member
An how exactly does the wire know that it is in an aircraft?
BillL
Well Known Member
So let's see, in the big picture the worst result of a short is death, start there.
Death is prevented (in this case) by fire,
Fire is prevented by not overheating the wire,
Overheating the wire is prevented by a fuse, or breaker.
A wire be able to carry a larger current than the fuse. = safe
Following FAA recommendations for wire sizing /fusing = safe
There may be internal modes of failure for an electronic device that would result in a short. Also, a fuse may be recommended so that that (awkward wording isn't it) mode of failure does not cascade into an internal hazard or more serious non-repairable event. Regardless, the fuse still protects the wire, the plane, the occupants, thus eliminating any cascading damage from the internal "short".
There are lots of details and paths to each big step, but does this cover it?
Death is prevented (in this case) by fire,
Fire is prevented by not overheating the wire,
Overheating the wire is prevented by a fuse, or breaker.
A wire be able to carry a larger current than the fuse. = safe
Following FAA recommendations for wire sizing /fusing = safe
There may be internal modes of failure for an electronic device that would result in a short. Also, a fuse may be recommended so that that (awkward wording isn't it) mode of failure does not cascade into an internal hazard or more serious non-repairable event. Regardless, the fuse still protects the wire, the plane, the occupants, thus eliminating any cascading damage from the internal "short".
There are lots of details and paths to each big step, but does this cover it?
vlittle
Well Known Member
This is a poor practice by the manufacturer. If the unit requires a fuse to protect the interior of the box, this fuse should be provided as part of the electrical design of the unit. The avionics devices that I design either have some form of current limiting or fusing bulit in to protect against internal component failure and potential fire.
To demand an external fuse to protect the device is poor practice. Oddly, I've seen it most with older Garmin products, before they purchased UPSAT, which brought more aviation 'best practices' to Garmin.
az_gila
Well Known Member
Vern, you are correct on good practices, but the FAA certification rules may have something to do with aircraft standard practices.
§23.1357 Circuit protective devices.
(a) Protective devices, such as fuses or circuit breakers, must be installed in all electrical circuits other than—
(1) Main circuits of starter motors used during starting only; and
(2) Circuits in which no hazard is presented by their omission.
(b) A protective device for a circuit essential to flight safety may not be used to protect any other circuit.
(c) Each resettable circuit protective device (“trip free” device in which the tripping mechanism cannot be overridden by the operating control) must be designed so that—
(1) A manual operation is required to restore service after tripping; and
(2) If an overload or circuit fault exists, the device will open the circuit regardless of the position of the operating control.
(d) If the ability to reset a circuit breaker or replace a fuse is essential to safety in flight, that circuit breaker or fuse must be so located and identified that it can be readily reset or replaced in flight.
(e) For fuses identified as replaceable in flight—
(1) There must be one spare of each rating or 50 percent spare fuses of each rating, whichever is greater; and
(2) The spare fuse(s) must be readily accessible to any required pilot.
If a transponder is "critical for safety in flight" (IFR?), then the fuse/breaker must protect the "circuit", which I read as both the wire and the device, with the device being more critical.
Your internal fuse/breaker would not qualify unless it was on the front panel of the Garmin device. I'm not sure, but I guess if Garmin uses an internal 5 amp fuse for safety (ie, not bursting into flames) then they specify a 3 amp fuse for the "circuit", irrespective of wire gauge, so that the external device is the active protection element and can meet the Part 23 requirements.
And yes, I know it isn't a requirement for Experimental planes, but it does define "standard" aircraft practices.
Last edited:
rv7charlie
Well Known Member
Looking back, what drove the requirement to carry spare fuses for circuits needed for 'safety of flight'? We're not required to carry spare wings, or carburetors, or rudders. All can fail, but failures are rare enough for us to effectively ignore the idea of 'spares'. When the rule was written, weren't glass fuses the most common type? On the other hand, I can't remember the last time I replaced an ATC type fuse in a car, unless the wire or device it protected had failed shorted to ground.
If a device has a hard enough failure to fail a properly sized ATC fuse (breakers are another story, even if you pay $50 aviation prices for them), what good is replacing the fuse in flight?
Charlie
If a device has a hard enough failure to fail a properly sized ATC fuse (breakers are another story, even if you pay $50 aviation prices for them), what good is replacing the fuse in flight?
Charlie
ifixf15
Active Member
Fun with Relays
Paul is absolutely correct on in his assessment of switches, relays and the KISS principle. However, if one feels that the use of a few relays is desirable we can glean a lesson or two from McDonnell Douglas and use the relay?s predominate failure mode to our benefit. One of the things McD learned from the F-4 was that when relays fail inflight they have unpredictable results and can be difficult to troubleshoot on the ground. For that reason, when they were designing the F-15 they made a design decision to have relays in their de-energized position during in-flight conditions as much as possible.
A typical circuit, let?s say position lights, would be wired so that when the switch in placed to the ON position the relay is energized and power is supplied to the position lights. The alternative would be to wire the circuit so that with the MASTER switch ON and the POSITION switch OFF the relay is energized and power is removed from the lights. Placing the POSITION switch to the ON position de-energizes the relay, allowing current to flow in the position light circuit. There are a couple of benefits to this set up. First, when the circuit fails, and it will, you will know because you cannot turn the lights off. This means that you can go ahead and fly that night sortie and fix the issue when you return, assuming that you have access to a fuse or circuit breaker if you really need to turn the lights off. Second, since the duty cycle of the relay is greatly reduced, only a few minutes per flight instead of being energized for the entire flight, the relay?s life span should be increased.
Nothing beats a switch, a load, and some wire for simplicity and reliability, but if we want to employ a relay or two, there is more than one way to skin that cat.
Paul is absolutely correct on in his assessment of switches, relays and the KISS principle. However, if one feels that the use of a few relays is desirable we can glean a lesson or two from McDonnell Douglas and use the relay?s predominate failure mode to our benefit. One of the things McD learned from the F-4 was that when relays fail inflight they have unpredictable results and can be difficult to troubleshoot on the ground. For that reason, when they were designing the F-15 they made a design decision to have relays in their de-energized position during in-flight conditions as much as possible.
A typical circuit, let?s say position lights, would be wired so that when the switch in placed to the ON position the relay is energized and power is supplied to the position lights. The alternative would be to wire the circuit so that with the MASTER switch ON and the POSITION switch OFF the relay is energized and power is removed from the lights. Placing the POSITION switch to the ON position de-energizes the relay, allowing current to flow in the position light circuit. There are a couple of benefits to this set up. First, when the circuit fails, and it will, you will know because you cannot turn the lights off. This means that you can go ahead and fly that night sortie and fix the issue when you return, assuming that you have access to a fuse or circuit breaker if you really need to turn the lights off. Second, since the duty cycle of the relay is greatly reduced, only a few minutes per flight instead of being energized for the entire flight, the relay?s life span should be increased.
Nothing beats a switch, a load, and some wire for simplicity and reliability, but if we want to employ a relay or two, there is more than one way to skin that cat.
