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Where are fuses required?

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donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Sun Nov 01, 2015 5:00 am Post subject: Where are fuses required?

I'm putting in a small battery to keep the EFIS & AHRS up prior to and during engine starts. It will also be able to power the EFIS & AHRS in the unlikely event that that primary electrical system should have some type catastrophic failure.



But my question is about fuses.  Since the backup battery has a switch that would remove the fuses in the fuse block from the circuit, I'm thinking that another fuse would be required between the battery and the switch (see attached).



Is this a common (or best) practice?



Thanks,

Don

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yellowduckduo(at)gmail.co
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Posted: Sun Nov 01, 2015 6:01 am Post subject: Where are fuses required?

In the event of a short in the wiring or the device under power you want 

to prevent the battery from burning something.  If the battery is large 

enough that a short will flow enough current to burn the wiring then yes 

a fuse would be appropriate. If it was a battery that might itself 

overheat with a shorted wire (lithium?) then a fuse would also be 

appropriate.

Ken



On 01/11/2015 8:00 AM, donjohnston wrote:

 	  Quote: 	
		   



 I'm putting in a small battery to keep the EFIS & AHRS up prior to and during engine starts. It will also be able to power the EFIS & AHRS in the unlikely event that that primary electrical system should have some type catastrophic failure.



 But my question is about fuses.  Since the backup battery has a switch that would remove the fuses in the fuse block from the circuit, I'm thinking that another fuse would be required between the battery and the switch (see attached).



 Is this a common (or best) practice?



 Thanks,

 Don





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p=448517#448517





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user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Sun Nov 01, 2015 7:23 am Post subject: Re: Where are fuses required?

That switch does not remove the fuses from the circuit.  The fuses will protect downstream wires and loads from excessive current no matter if the switch is open or closed.  As for needing additional short circuit protection near the battery, it all depends on how long the wires are between the battery and existing fuses and what danger those wires are in from potential damage.  Usually it is not a good idea to have fuses in series because it is unpredictable which fuse will blow first, even if they are different sizes.  If you think that the current path between the battery and fuse needs protection, then a fusible link should be considered.  For a circuit with 20awg wire, a fusible link made of 24awg wire will work.  If you really want to use another fuse at the battery, then use a much larger fuse.  It will protect against short circuits between the battery and 2 amp fuse.  The 2 amp fuse will protect against downstream shorts and overloads.

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donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Sun Nov 01, 2015 7:43 am Post subject: Re: Where are fuses required?

 	  user9253 wrote: 	
		  That switch does not remove the fuses from the circuit.  The fuses will protect downstream wires and loads from excessive current no matter if the switch is open or closed. 	




"It will protect downstream wires."  But not upstream wires. 



I am asking about the wire from the battery to the switch. With the switch open, there would be no circuit protection between the battery and the switch.  Is that something to be concerned about? 



The distance from the battery to the switch is about 6'. From the switch to the fuse block is another 3'. 



Obviously, there are many places were there is no circuit protection (the link from the battery to the master contactor, master contactor to the starter contactor, master contactor to the main bus bar, etc.)



I'm trying to understand what the criteria is for when fuses (or CB's) are called for.



Thanks,

Don

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billp(at)wwpc.com
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Posted: Sun Nov 01, 2015 8:58 am Post subject: Where are fuses required?

The criteria is; wherever you would not like an electrical fire.



The battery to the master contactor should be a few inches. That's your 

cut off if everything goes sideways. Of course once you open the master, 

you have no power for anything. The wire from the main contactor to the 

starter contactor and on to the starter is really big wire and it's not 

likely to get hot enough fast enough to cause a fire. The unlucky 

exception is when some part of the airframe cuts through the starter 

wire's insulation. In that case you'll get a pretty good flash while the 

electric arc is eating airframe and the wire at the point of contact. If 

it happens to be in a spot where there is something flammable there's a 

fire. If not, the arc will eat airframe and wire until the gap gets so 

wide the arc can't be sustained.



I think the usual thing to do is to put the fuse block(s)/circuit 

breakers really close to the main power bus and protect the main power 

bus really well. Then switches that control loads come after the 

protective devices. Switches are mechanical assemblies and I've seen a 

fair number of switches that have disassembled themselves. Then you have 

a power source flopping around with the switch actuator still firmly 

mounted to the panel.



If you have a 6' piece of #18 wire from the battery to the switch and 

then another 3' to the fuse, that is a potential 9' long hot knife or 

igniter. You really need to physically protect every one of those as if 

it was a main bus wire. That means, don't run it in bundles with other 

wires (or plumbing), make sure it's well protected and stood off from 

any grounded conductive surface.



For most designers, the pain of having to treat a bunch of wires as is 

they were main bus wires is such a pain in the neck that they just 

protect things as close to a single main bus as possible.



Bill



On 11/1/15 07:43, donjohnston wrote:

 	  Quote: 	
		   

 user9253 wrote:

> That switch does not remove the fuses from the circuit.  The fuses will protect downstream wires and loads from excessive current no matter if the switch is open or closed.



 "It will protect downstream wires."  But not upstream wires.



 I am asking about the wire from the battery to the switch. With the switch open, there would be no circuit protection between the battery and the switch.  Is that something to be concerned about?



 The distance from the battery to the switch is about 6'. From the switch to the fuse block is another 3'.



 Obviously, there are many places were there is no circuit protection (the link from the battery to the master contactor, master contactor to the starter contactor, master contactor to the main bus bar, etc.)



 I'm trying to understand what the criteria is for when fuses (or CB's) are called for.



 Thanks,

 Don





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p=448526#448526

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user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Sun Nov 01, 2015 11:16 am Post subject: Re: Where are fuses required?

 	  Quote: 	
		   With the switch open, there would be no circuit protection between the battery and the switch. 	


There is no circuit protection between the battery and the switch regardless of whether the switch is open or closed.  If that wire shorts to ground, it doesn't matter if the switch is open or closed.  The 2 amp fuse will not blow because it will not be conducting the ground fault current.

  It is difficult to give advice without seeing the physical location of components.   How about replacing the wire from the battery to the 2 amp fuse with 18 AWG wire and putting a fusible link or large value fuse (15 Amp ?) right at the battery?  I assume that a fuse located at the battery would not be accessible during flight.  So it should be sized so that the 2 amp fuse would always blow first if the load shorts out.

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Joe Gores

user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Sun Nov 01, 2015 11:19 am Post subject: Re: Where are fuses required?

Of course it is not a good idea to replace a blown fuse in flight.  It is safer to wait until on the ground before troubleshooting.

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