AeroElectric-List Digest Archive

Fri 01/06/12


Total Messages Posted: 3



Today's Message Index:
----------------------
 
     1. 12:31 AM - Re: Re: First Engine Start Problem (Jason Beaver)
     2. 10:03 AM - Re: Re: First Engine Start Problem (Richard Tasker)
     3. 06:55 PM - Re: Re: First Engine Start Problem (Robert L. Nuckolls, III)
 
 
 


Message 1


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    Time: 12:31:05 AM PST US
    Subject: Re: First Engine Start Problem
    From: Jason Beaver <jason@jasonbeaver.com>
    On Dec 30, 2011, at 7:14 PM, Robert L. Nuckolls, III wrote: > If you wanted to put a 20A, in-line fuse holder in its > place, that would be fine too as long as you don't have > BREAKERS downstream. Bob, can you explain what is bad about having circuit breakers downstream of fuses? I wired my Lightspeed Plasma II ignition through a fuse on the battery bus to the breaker on the panel because I didn't want such a long, always-hot wire from the battery directly to the panel. Should I rewire this? thanks, jason


    Message 2


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    Time: 10:03:11 AM PST US
    From: Richard Tasker <retasker@optonline.net>
    Subject: Re: First Engine Start Problem
    If the current ratings are similar, the fuse will always blow first, so the breaker serves no purpose. If the fuse current rating is some significant multiple of the breaker rating that might work - which might be okay to protect against shorts in the wire up to the breaker - but what is the point. Dick Tasker Jason Beaver wrote: > --> AeroElectric-List message posted by: Jason Beaver<jason@jasonbeaver.com> > > > On Dec 30, 2011, at 7:14 PM, Robert L. Nuckolls, III wrote: > >> If you wanted to put a 20A, in-line fuse holder in its >> place, that would be fine too as long as you don't have >> BREAKERS downstream. > Bob, can you explain what is bad about having circuit breakers downstream of fuses? I wired my Lightspeed Plasma II ignition through a fuse on the battery bus to the breaker on the panel because I didn't want such a long, always-hot wire from the battery directly to the panel. Should I rewire this? > > thanks, > > jason > >


    Message 3


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    Time: 06:55:46 PM PST US
    From: "Robert L. Nuckolls, III" <nuckolls.bob@aeroelectric.com>
    Subject: Re: First Engine Start Problem
    At 02:16 AM 1/6/2012, you wrote: On Dec 30, 2011, at 7:14 PM, Robert L. Nuckolls, III wrote: > If you wanted to put a 20A, in-line fuse holder in its > place, that would be fine too as long as you don't have > BREAKERS downstream. Bob, can you explain what is bad about having circuit breakers downstream of fuses? I wired my Lightspeed Plasma II ignition through a fuse on the battery bus to the breaker on the panel because I didn't want such a long, always-hot wire from the battery directly to the panel. Should I rewire this? Probably. Put the one and only protective device (5A breaker or fuse) at the battery bus with no further devices downstream. If THAT protection opens, it is because the ignition system is hurt bad . . . and you'll need to rely on the second, totally independent system to bring your flight to an orderly conclusion. To have a 'breaker' downstream of a fuse turns the breaker into a switch. Breakers are so much slower than a fuse that a 5A fuse upstream of the 5A breaker will always open first. In fact, a 10A fuse upstream of a 5A breaker will still open before the 5A breaker. The "fuse" symbol often found upstream of breakers in aircraft power distribution diagrams is a CURRENT LIMITER, a robust, special kind of fuse with fusing dynamics that are very unlike their little plastic, plug-in cousins. For example: http://aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/ANL_Specs.pdf Note that a '35 Amp' ANL will carry 80 amps about indefinitely which is a 200% overload. The ANL and close cousins are suited for protecting fat-wire feeders to busses downstream that might mount circuit breakers. Here we see: http://aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/ATC_Specs.pdf that a 200% overload on a 5A ATC plastic fuse is expected to open it in something on the order of 300 milliseconds. In the last data sheet we see: http://aeroelectric.com/Mfgr_Data/Breakers/Eaton/Eaton_4200.pdf that a Series 4200 miniature breaker under 200% overload can take from 2 to 20 seconds to open and still be "in spec". As a matter of simple design goals, a single feeder needs only one protective device located as near to the power source (bus) as practical. Any device upstream of a breaker needs to be robust enough to stay closed during operation of the downstream device. Finally, all robust fuses and indeed any protective device over 5A is discouraged for use on always hot feeders. This design rule is cited in the FARS and generally refers to circuit breakers. We know that a 5A breaker will open a 10A fuse . . . so it follows that one COULD consider 10A ATC fuses on battery bus feeders without presenting potential fault energy worst than a 5A breaker. Bob . . .




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