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Date: Sun, 18 Feb 2007 23:15:53 -0500
From: Chad Robinson <crj@lucubration.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: CBs and fuses
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Wendell Voto wrote:
> Concerning the PTC devices, I was interested in them and a friend also 
> building a Cozy wants to use the PTC devices but I am reluctant to say 
> to use them.  If correct, many of the ones Mouser lists that allow 
> high current (1 - 18 amp) seem to have an awfully high holding 
> current; that is, the current to maintain the device temp to hold it 
> in the tripped state.  Be careful of the device you choose, it's 
> holding current could be 50% of the trip value (Littel 30R series for 
> instance).  This would not be good for protecting the wiring.
That's not a bad point, but I would argue that wiring isn't the issue 
here. If you properly size your breaker to protect your wiring, then (by 
definition) 50% of the rated current shouldn't hurt your wiring at all. 
I think the bigger issue is that the device could be drawing 50% current 
- it's through the device that the current goes, after all, unless 
there's a short. Suppose you protect your nose gear with one. If it 
trips, but 50% current is still allowed to flow, it could still be 
pulling that wheel up against blockage and could still do some damage. 
Also, if you don't realize this is going on, it could be bad for some 
loads such as motors. I prefer manual reset devices in these cases 
because you KNOW what's happening. It's either tripped, or it isn't, and 
both states are well defined as long as the contacts don't weld (and 
that's another story).

May I make a suggestion? Fuse vs. CB discussions border on religion to 
some people, and this list isn't really intended for electrical 
discussions. I'd recommend moving the discussion to Aeroelectric or (if 
you don't like Bob's fuse bias) some other forum other than this one. 
It's been an interesting discussion, but frankly, both options have 
merits (so the discussion could continue for a LONG time) and neither 
one is really specific to a rotary.

Just my 2c.

Regards,
Chad