Blame the device not the CB, and the future
Kevin Horton said:
There was an interesting incident with one of our King Air A100s a few years ago. The windshield heat CB popped in flight. The crew wisely left it popped, and reported the snag to maintenance. The maintenance guy, without doing any other troubleshooting, decided to push the CB in and turn the windshield heat back on. The power connection at the corner of the pilot's windscreen started arcing, and it started an electrical fire.
If you have a CB or fuse that regularly trips, that indicates a design problem.
Kevin I agree if a CB trips the pilot has the option to NOT reset.
However I question if your story relevant. Something does not sound right with your logic.
A quick check of Beech Aircraft window heat shows where the problem is, the window heat. A FUSE would not have saved the day, and IF the CB did not trip a fuse would not trip.
Lets say it was protected by a FUSE. It blew if flight. Than later on the ground the maintenance guy popped in a new fuse, the same thing would have happened. Also the current draw of cool window and hot window may be quite differnt. There is more to it than what you're letting on.
As far resetting a CB in flight, in this case if they would have reset it and sparks appeared I am sure they would have turned the window heat back off. However I am pretty sure the CB would not have re-set in flight.
A CB is a thermal device and
accumulates heat and will trip over time with just a slight over load. In your example, if the pilots did attempt a CB reset, it would have re-tripped, since the CB was already hot. Later on the ground it was totally cool. Still the problem is NOT the CB it's the window heat.
The problem is the device, the window heat, which can arch and spark under the rated current limit of the protection.
To imply somehow a CB is NOT as good as a fuse because of this example is just unconvincing. It is interesting. It brings up the nuance of how CB's work and some devices, but I think the CB did just what it was suppose to do very well. I kind of blame maintenance for just pushing the CB in. From all the NTSB reports I ref below, this is not a rare event.
The window heat problem was not a CB issue. A fuse would have done the same thing and may have not blown in the first place (see list of NTSB reports below).
Some facts about CB's
CB's incorporate a trip free design, meaning they can't be reset with a fault. They mechanically will not engage or latch. You can't even override them by holding them in.
As I mentioned CB's are thermal devices. A fuse might take a slight overload indefinitely. A CB will eventually TRIP even if not way overloaded, as long as it gets hot enough. That's why
IF you do choose to reset a CB, you must wait a minute or two for cooling. Otherwise they'll not reset unless cooled, even if the fault is removed. That is a cool feature (pun intended
).
The maintenance guy I hope turned the window heat off or pulled the CB out he just reset as soon as he got the arch. I know of a few maintenance guys burning planes to the ground. I am fairly sure the cool window and CB aggravated the situation when the mechanic reset it on the ground.
I am totally convinced a fuse would NOT have done anything better than a CB here, except remove the ability or option of the pilots to reset. IF they would have attempted a reset, I am also fairly sure it would have not reset or popped quickly. If it did reset, the problem was obvious and they would have de-powered the circuit. It does bring up an important point, be careful resetting the CB. You don't have to reset a CB. Have you ever landed with out window heat and iced over windscreen? I did once. It was not the CB it was the hot plate window strip was open, dead. Good thing the runway was wide and the side windows where clear.
Any high amp device, window heat, coffee makers and ovens, can make smoke and sparks below the circuits CB/fuse current limit. The first thing you do in a big plane (or any plane) when you smell electrical smoke (notice I did not say fire) is turn the utility buss off, which has all non-essential and big current items: coffee maker, ovens and basically everything not needed. If you are flying a King Air you turn then window heat off.
If flying a RV and smell electrical smoke what do you do? You turn the
??????? off? Master off? Just FOOD FOR THOUGHT, what would you do?
Bottom line don't blame the protection, blame the device.
Why do CB's pop when there is no hard short?, they get hot. In the OLD days it was common to GANG several devices onto one CB, like we do in home wiring. At home we know you can blow a CB if you put too many items on at one time. Most home builders, including myself, install +20 CB's for discrete CB protection of almost every circuit. BTW, why did almost all homes and commercial/industrial wiring go from fuses to CB's?
TAKE A QUICK GLANCE AT THESE KING AIR WINDOW HEAT SHORT REPORTS
These NTSB reports sound similar to your story. Your maintenance folks must have known of this fairly common King Air window arching issue? Don't you think he would have looked for arching before pushing the CB?
arcing on the copilots windshield
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001208X07001&key=1
smoke arch's emanating from windshield
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001211X11404&key=1
smoke bottom left corner of F/O's windscreen
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001208X05914&key=1
Window arch smoke
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20060504X00513&key=1
Window cracked and started to burn
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20040413X00453&key=1
PS:
Better protection would be ARCH-FAILT interrupter protection like in your bathroom. There are ARC-FAULT protection CB's for aircraft. The Arc?Fault Circuit Interrupters (AFCI) detects potentially hazardous arcing conditions and prevents catastrophic damage caused by electrical fires.
http://www.sensata.com/products/controls/arcfault.htm
An AFCI CB would likely be a good choice for the Beech King air window heat. Cost? I don't know but no doubt more expensive. Do we need it in a RV? No, but I find it interesting.
You all might remember (1998) Swissair Flt 111 (Nova Scotia, overhead wiring fire behind cockpit) and (2000) TWA 800 (Long Island NY, short if fuel tank, explosion). A fault interrupter
may have saved those looses. CB's and fuse's both have limitations. The future is better circuit protection with logic built into them, like AFCI, which will be CB's not fuses. However for a single engine GA plane CB's or fuses are fine.
The key is GOOD wiring practice to avoid any short in the first place. I think we can agree on that.