Is there a reason for a fuse AND a c/B for instrument lighting?

[NicoN]

We have this little panel with 4 fuses in the copilots foot well. 

One is for instrument lighting.

On top there is a C/B for the same purpose.

 

Why?

[M20F-1968]

You will likely need to physically trace the wiring.

[GeeBee]

C/B protects wiring. Fuses protect the appliance.

[Yetti]

One is for the panel lighting. ie glareshield lighting and one is for the Instrument lights ie it goes to the radios that have a separate lead for lighting internal to the unit.

 

[DonMuncy]

5 hours ago, GeeBee said:

C/B protects wiring. Fuses protect the appliance.

I don't think I had ever thought of this. You are right.  I think it also may have to do where the fuse/CB is placed.  I think ideally, one would size the C/B very near the origin for the smallest (least current carrying) wire in the circuit, to make sure the wires don't over-heat. Then just in front of the appliance, put a fuse sized for that appliance. 

(Remember when, on very old houses, before C/Bs were invented, they used fuses for everything)

[GeeBee]

5 minutes ago, DonMuncy said:

I don't think I had ever thought of this. You are right.  I think it also may have to do where the fuse/CB is placed.  I think ideally, one would size the C/B very near the origin for the smallest (least current carrying) wire in the circuit, to make sure the wires don't over-heat. Then just in front of the appliance, put a fuse sized for that appliance. 

(Remember when, on very old houses, before C/Bs were invented, they used fuses for everything)

That is correct. The C/B protects the wiring starting at the power bus. The fuse should be close to the appliance. 

[Vance Harral]

1 hour ago, DonMuncy said:

I don't think I had ever thought of this. You are right.  I think it also may have to do where the fuse/CB is placed.

It only has to do with where the fuse/CB breaker is placed, through the mechanical size of those different devices may dictate where they wind up.  Otherwise, there is no truth to this idea that one type of circuit interruption device is designed to protect wiring and another to protect the appliance the wire connects to.  That's just not correct.  Both types of interruption devices can protect both wiring and appliances.  Both types of interruption devices provide more safety when placed on/near the bus bar than on/near the appliance being powered.  This is due to the nasty failure mode of a power wire which chafes and shorts to ground between the bus bar and the appliance, and winds up carrying more current than it's rated for.  This type of failure tends to melt the insulation, dribbling hot, waxy gobs of plastic-like material onto surfaces like clothing and carpet that are prone to catch fire.  It's nice to protect the appliance too, but the failure mode of an internal short in the appliance tends to simply burn out a component that's encased in a metal box.  It could still hurt you or start a fire, but that's much less likely than an over-current wire.

If you can only place one circuit interruption device in the electrical path to an appliance, putting it near the large, robust (cool) bus bar is good because it means most of the wire that might melt is "downstream" of the circuit interruption device.  If you instead put the interruption device near the appliance, most of the wire that might short is "upstream" of the interruptor.  That's true regardless of whether the poorly-placed interruption device is a breaker or a fuse.

[GeeBee]

The question was, why use both a C/B and a fuse. In most typical circuits, a thermal C/B is rated for the wire where as the appliance if it absorbs that much current would fry itself into oblivion before the C/B tripped. In that case, you would place a fuse close to the appliance to melt before the wire melts or the C/B tripped. This is often the case where two appliances are applied to one circuit where the possibility of one appliance would absorb the full load of the C/B is great. For instance, there may be a C/B protected circuit called "Panel lights" but it may be split into "Left" and "Right" panel lights which may be protected by a lower rated fuses respectively. Thus if the appliance developed a short, it would blow the fuse and leave the feed wire intact to power the remaining items on the circuit.

[MikeOH]

The only reason to put a separate circuit protection device (C/B or fuse) next to a load is to prevent a failing load from disabling other loads on the same bus.  Circuit protection devices are present to protect wiring.

Think about it, how does a circuit protection device at the load protect the load?  The only reason it would melt/trip would be if the load has a failure!  That is, the load is already damaged; there is nothing to 'protect'.

As others have said, the circuit protection device should be placed at the bus origin.  Otherwise, the downstream wiring would NOT be properly protected.

[PT20J]

Agree with @GeeBee. The circuit breakers protect the wiring between the circuit breaker and the load. (Think where the circuit breakers are in your home). You always want the circuit breaker as close to the power source as possible. 

The panel/glareshield lights are controlled by a couple of transistors. The fast blow fuses between the collector and the lamps for each transistor protect the transistors (and also the wire to the lamps) in case there is a short at the lamps or wire to the lamps.

[Vance Harral]

12 minutes ago, GeeBee said:

This is often the case where two appliances are applied to one circuit where the possibility of one appliance would absorb the full load of the C/B is great.

This makes sense, but I believe is considered poor practice.  I'm sure driving multiple devices from one circuit breaker is done all the time including by major manufacturers.  But I once called an avionics supplier about whether it would be OK to gang a voice annunicator that draws less than 100mA with an audio panel that specified a 3A breaker, and you would have thought I was Peter Graves being disavowed by the secretary of whatever.  We were out of circuit breaker holes, and wound up powering the voice annunciator through a fuse - as allowed by its installation manual - that tapped off a 12V source right at the bus bars.  We put it as close to the bus bar as possible, such that most of the wiring to the voice annunciator was downstream of the fuse.

[GeeBee]

Used to have a King Air that had a 50 amp C/B that protected the A/C system. The relay for the electric motor that drove the compressor was on the same circuit, but since the relay coil would never on its best day blow a 50 amp breaker it was protected by a 5 amp fuse, that would blow because there was no delay between the coil energizing and the compressor motor start up. The compressor motor would draw so much current on start up the relay would draw excessive amps to maintain its commanded latch. The fuse was in the nose and required the removal of screws and a panel to get to the fuse and relay box. Solution was to insert a slo-blo fuse. 

[A64Pilot]

On 1/16/2025 at 6:41 PM, GeeBee said:

Used to have a King Air that had a 50 amp C/B that protected the A/C system. The relay for the electric motor that drove the compressor was on the same circuit, but since the relay coil would never on its best day blow a 50 amp breaker it was protected by a 5 amp fuse, that would blow because there was no delay between the coil energizing and the compressor motor start up. The compressor motor would draw so much current on start up the relay would draw excessive amps to maintain its commanded latch. The fuse was in the nose and required the removal of screws and a panel to get to the fuse and relay box. Solution was to insert a slo-blo fuse. 

Better solution would have been to install a breaker. But fuses are much cheaper and are easily installed inline. I despise hidden fuses

A fuse does nothing that a CB can’t do, difference is of course a CB is resettable a fuse means you have to carry spares which was required back before breakers, probably a Commercial rule like the 2D cell flashlight was. Where the CB is has nothing to do protecting the device it’s connected to, but where the CB or fuse is important as the wire up to the device is unprotected, but it’s not prior to the device

For this reason any device connected to a battery for continuous power when the aircraft is “off” like say a clock requires for example that the protection device be within 1 ft of the battery, either way the clock is “protected” but being close to the battery means only 1 ft of wire that could cause a fire isn’t.

But I am confused how a CB or fuse protects a device, protects it from what? Internal shorts? It’s broken then, Anything else that I can think of that will cause excessive current to blow a fuse or trip a breaker means a wire is shorted, the device is broken and drawing excessive current of the protective is incorrectly sized.

[redbaron1982]

On 1/16/2025 at 4:38 PM, MikeOH said:

The only reason to put a separate circuit protection device (C/B or fuse) next to a load is to prevent a failing load from disabling other loads on the same bus.  Circuit protection devices are present to protect wiring.

Think about it, how does a circuit protection device at the load protect the load?  The only reason it would melt/trip would be if the load has a failure!  That is, the load is already damaged; there is nothing to 'protect'.

As others have said, the circuit protection device should be placed at the bus origin.  Otherwise, the downstream wiring would NOT be properly protected.

Yeah, this only makes sense if you have a C/B shared by multiple loads, and then each load has its own fuse. I don't get why we have a C/B for Elev Trim and then a fuse as well for Elev Trim. That seems to be a bus with only one load, why two protections?

[GeeBee]

Because fuses except for those with regulated blow times such as slo-blow have higher response times. Other reason is unless the C/B is trip free it can be mismanaged by the user (i.e. resetting it too many times or holding it in to prevent tripping) But the biggest reason is fast response. Most C/Bs are thermal and reaction time is often based upon ambient temps the C/B operates in. I am willing to bet the C/B for that trim is in the cabin where as the fuse is in an area of high variable temp, especially to the low side. 

[MikeOH]

1 hour ago, redbaron1982 said:

Yeah, this only makes sense if you have a C/B shared by multiple loads, and then each load has its own fuse. I don't get why we have a C/B for Elev Trim and then a fuse as well for Elev Trim. That seems to be a bus with only one load, why two protections?

In general, a point of load circuit protection device (e. g. a fuse in/at the load itself) is there because the manufacturer has no way of knowing whether it will be the ONLY load on a buss whose source circuit protection device may be of higher current to protect the wiring.

[A64Pilot]

9 hours ago, GeeBee said:

Because fuses except for those with regulated blow times such as slo-blow have higher response times. Other reason is unless the C/B is trip free it can be mismanaged by the user (i.e. resetting it too many times or holding it in to prevent tripping) But the biggest reason is fast response. Most C/Bs are thermal and reaction time is often based upon ambient temps the C/B operates in. I am willing to bet the C/B for that trim is in the cabin where as the fuse is in an area of high variable temp, especially to the low side. 

Fuses are also thermal. The fuse portion opens the circuit when it melts.

Edited January 21 by A64Pilot

[GeeBee]

1 hour ago, A64Pilot said:

Fuses are also thermal. The fuse portion opens the circuit when it melts.

That is true, but the thermal space is very limited. Thus it heats quickly, and the material is more heat sensitive.  A C/B unless it is explosion proof has a lot of thermal space. Sometimes like on big jets, the C/B panel has air circulation behind it. All you have to do is look at the "trip" times for similar rated C/Bs and fuses.

[A64Pilot]

We can continue to disagree, You don’t see any fuses on Military aircraft and I’d bet Commercial Jets for a reason. You do see them on old aircraft, just as old houses had fuses. Fuses are like vacuum tubes

Fuses are cheaper, easier to implement and take less room

[jlunseth]

Generally, pretty simple. Let's say you have five things on one circuit at the main panel and if they are all switched on at the same time and drawing their maximum you need a 10 amp switch/breaker to protect that circuit. You are protecting the wiring from the power source from melting or worse. But now let's say that each device is rated at a max 2 amp draw. You could expect five two amp fuses, one for each device, wired between the device and the 10 amp breaker and usually near the breaker. Now you switch the entire circuit on with the switch/breaker and each of the devices is appropriately protected.  Increasingly common with electronic devices that have very low draws compared to what was originally installed. It is preferable to give each device its own breaker rated to protect the circuit and the device, but as modifications are made to older craft and more and more low draw electronics are installed, sometimes multiple devices are installed to a single breaker. Rewired a fairly large sailboat last summer and ran into the issue quite a bit.

[GeeBee]

Uh, no. Down in the E&E lots of fuses. Especially on hot bus DC systems.  And if you want to see what fuses are on the main deck, just open the bulb kit. Lots of TR5 fuses in the avionics. You would not recognize them as fuses if you did not know what you were looking for.