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FUSES or CIRCUIT BREAKERS IN SERIES

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user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Mon Jun 01, 2020 4:59 am Post subject: FUSES or CIRCUIT BREAKERS IN SERIES

My microwave oven blew its ABC 20 fuse and also tripped the 20 amp 

circuit breaker in the service entrance panel.  I tested the 4 door switches 

and the transformer and large capacitor, finding nothing wrong.  So I 

replaced the fuse.  The microwave worked for a couple of days before the 

same thing happened again, blowing the 20 amp  internal fuse and tripping the 

20 amp circuit breaker.  Looking at the schematic

https://www.appliancefactoryparts.com/content/pdfs/170873-1.pdf

one of the door switches provides a short circuit path between the hot and 

neutral supply.  Evidently if plastic parts wear out, that can cause a timing 

issue between the operation of the door switches.  When that happens, the 

electrical supply to the microwave is shorted out.  That is a poor design in 

my opinion.  I pulled a wire off from the door switch, the one which is capable of 

shorting out the supply voltage.  The microwave has been working fine since.

  The purpose of this post is to warn against designing aircraft electrical 

systems with two circuit protection devices in series, for instance a main 

fuse and a load fuse.  Without extensive testing, it is not known if a short 

circuit in a load circuit will blow the main fuse in addition to the load fuse, 

even if the fuses are different sizes.  There is a reason why most aircraft 

electrical systems do not have a fuse protecting the main power bus.

If that fuse blows, the whole electrical system goes down.

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_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Mon Jun 01, 2020 6:34 pm Post subject: FUSES or CIRCUIT BREAKERS IN SERIES

 	  Quote: 	
		  Without extensive testing, it is not known if a short 

 circuit in a load circuit will blow the main fuse in addition to the load fuse, 

 even if the fuses are different sizes. 	


   It's easily predicted. Circuit protective devices of

   all stripes have plots of trip response times vs.

   current draw.



   Hair dryers have fuses built in. The wall socket

   used to supply the hair dryer is protected with 

   a breaker at the box. The box feeders may have

   a main breaker that supplies both buses via

   feeders that come into the house. The pole-transformer

   is probably fitted with a fuse at the HI-side

   of the primary tap to the neighborhood distribution

   feeder . . . and so it goes.



   From hair dryer to gas-fired turbine, there are

   perhaps dozens of circuit protective devices all

   of which are in series. But to be sure, they are

   tailored to anticipated loads vs. trip times

   under fault conditions.



   Careful crafting of this protective 'ladder'

   components prevents a short in your microwave

   from throwing the whole neighborhood into darkness.



  	  Quote: 	
		  There is a reason why most aircraft 

 electrical systems do not have a fuse protecting the main power bus.

 If that fuse blows, the whole electrical system goes down. 	


   It's true that feeder protection is rare in light aircraft.

   Not so rare in heavier singles and twins (See attached

   picture of A36 firewall circa 1995) and quite common

   in aircraft like the B787.



   But to be sure, any circuit protective device

   situated upstream of another CPD will be an

   order of magnitude more robust than its downstream

   brother.



 

   Bob . . .

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A36_Firewall.jpg

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millner(at)me.com
Guest

Posted: Mon Jun 01, 2020 7:27 pm Post subject: FUSES or CIRCUIT BREAKERS IN SERIES

In electric power, it’s called relay coordination. Sometimes they get it wrong, and the northeast is plunged into darkness for days... 9 months later, birth rate jumps!



Paul



Sent from my iPhone



 	  Quote: 	
		  On Jun 1, 2020, at 7:46 PM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com> wrote:



   	  Quote: 	
		  Without extensive testing, it is not known if a short 

 circuit in a load circuit will blow the main fuse in addition to the load fuse, 

 even if the fuses are different sizes. 	


   It's easily predicted. Circuit protective devices of

   all stripes have plots of trip response times vs.

   current draw.



   Hair dryers have fuses built in. The wall socket

   used to supply the hair dryer is protected with 

   a breaker at the box. The box feeders may have

   a main breaker that supplies both buses via

   feeders that come into the house. The pole-transformer

   is probably fitted with a fuse at the HI-side

   of the primary tap to the neighborhood distribution

   feeder . . . and so it goes.



   From hair dryer to gas-fired turbine, there are

   perhaps dozens of circuit protective devices all

   of which are in series. But to be sure, they are

   tailored to anticipated loads vs. trip times

   under fault conditions.



   Careful crafting of this protective 'ladder'

   components prevents a short in your microwave

   from throwing the whole neighborhood into darkness.



  	  Quote: 	
		  There is a reason why most aircraft 

 electrical systems do not have a fuse protecting the main power bus.

 If that fuse blows, the whole electrical system goes down. 	


   It's true that feeder protection is rare in light aircraft.

   Not so rare in heavier singles and twins (See attached

   picture of A36 firewall circa 1995) and quite common

   in aircraft like the B787.



   But to be sure, any circuit protective device

   situated upstream of another CPD will be an

   order of magnitude more robust than its downstream

   brother.



 

   Bob . . .   <A36_Firewall.jpg>

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Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

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