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nuckolls.bob(at)aeroelect
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 Posted: Thu Feb 25, 2010 10:46 am Post subject: Clearing some away some fog and fuzzy logic . . . |
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I've been invited in a private discussion where a builder is wrestling
with his panel fabricator. Seems his hired wire-slinger has been
discovered some stuff fuses vs. breakers and breakers only for crow-bar protected
alternators. For many of you, this is pretty old stuff but this
is a large group. For folks who have not followed this discussion
for the last 15 years or so, I'll offer the following. The
AeroElectric-List graybeards can delete and move on . . .
Q: The Aeroelectric is such a proponent of Autocar Fuses . . .
Bob: The PRIMARY attraction for fuses is equivalent protection
for MUCH less cost and conservation of panel space.
Q(continued) . . . instead of breakers because if it trips he says you should not attempt to put it back on while in the air, hence fuses makes sense...but in his diagrams he still adds a 5 amp breaker to alternator field instead of fuses.? But would one reset this breaker 'in flight' after it trips, cause re-setting breakers in flight could cause more harm, maybe you can explain a bit more.
A(reader): Engaging the field circuit while the alternator is spinning at rated speed (i.e. - in flight) is hard on the alternator and even harder on the regulator and (my opinion only) should not be done on a regular or casual basis.
Bob: Absolutely not true. This myth has been bubbling
in old mechanic's tales cauldron for decades and has
NO foundation in physics. There IS a phenomenon exhibited
by some alternator regulators where energizing the alternator
at light load and hi rpm produces an voltage overshoot
that may even trip WHAT EVER ov protection is in place.
This is a design feature of regulator dynamics that
can be disconcerting but in no way does it represent
a hazard to the alternator, regulator or the rest of
the system.
A(continued): IF you have a problem, however - having the option to do so is nice. Suppose you had a OV condition that trips the alternator offline and you are IMC or VFR-on-top and your battery is draining pretty bad before you can get down - I'll take a possible OV condition in that scenario over dead screens, and reengage the alternator field. If the battery is low already it will absorb a good deal of the excess current, dropping the voltage and protecting the avionics to a degree. As the charge state of the battery comes up the OV will trip out the alt field again when the voltage rises beyond the set point of the crow bar, but you've gained some battery charge in the process. Lather, rinse, repeat, and fly the airplane. You may fry the regulator (which was suspect already by default of your situation) and you won't do the battery any favors with the high current charge/discharge cycles - but you'll keep your avionics up and give yourself a chance to get down safely.
Bob: This reader demonstrates no appreciation for
failure tolerant design or understanding of what
constitutes a hazardous voltage condition for
the rest of the system.
The root rationale for resetting CB on the
alternator field includes . . .
(1) there are occasionally situations where an ov
protection system of ANY style by ANY manufacturer
can be induced to nuisance trip. The ability
to do a cautious/attentive reset is useful.
(2) legacy operating philosophy CB trips that
shut down especially useful equipment, resetting a
breaker is is allowed one time. In a failed-regulator
scenario, resetting the breaker once simply produces
a second trip. In no case does it put extraordinary
stresses on other components that are still working.
A (reader): Bob talks in absolutes. When there is over lap or preference, it gets a little confusing. Remember some of Bob's ideas are more in the preference area than technically necessary.
The main reason for the ALT CB is when using a "crow-bar" or B & C regulator, which has a crow bar in it. The CB is an integral part of the "crow-bar system", an over voltage protection device as you know. The crow bar works by dead shorting the CB and tripping it. The Plane power alternator that I recommend, also uses or needs a CB. You could still use a fuse from any CB, however, a CB makes more sense, if you plan on occasionally tripping circuit "normally". (read on)
Here is my interpretation of Bob's CB/Fuse philosophy.
Since the OV "crow bar" might trip "accidentally", the ability to reset it, is desirable; the CB is obviously more desirable than a fuse for something you expect to normally trip on occasion. Bob denies the crow-bar is finicky or subject to nuisance trips. OK? If its true, it never tripped unless, than you could use a fuse, right? Well the crow-bar is not that stable. There are other ways to protect from OV, and this is one that Bob likes, and it needs a CB to work.
Bob: NEVER have I suggested that the OV crowbar devices
manufactured by me, B&C, Plane-Power or OV disconnect
devices by Perhelion, Electro-Delta, or BF Goodrich
ARE or ARE NOT "finicky".
Anyone who understands legacy OV protection philosophies
will offer due diligence in designing and qualifying
their device for intended purpose in accordance with
DO-160/Mil-STD-704 design goals.
ALL diligent designs are subject to nuisance tripping
from transients for reasons that have nothing to do
with design of the OV protection device.
Even the most diligent designs are subject to errors
in understanding ALL the potential nuisance trip
sources in the targeted airframes. Since the LR-1
first flew on Voyager's closed circuit tests of the
coast of CA up to the present LR-3 configuration
there have been THREE changes to design to address
new discoveries AFTER the product was fielded.
The first such condition was noted before Voyager did
the around the world flight where LR-2 regulators
were installed. The last two changes were prompted
by discoveries that were not brought to light until
thousands of regulators had been giving satisfactory
service for years. My current production OV modules
have benefited from lessons-learned in the last
two modifications. Further, those lessons learned
would have been the same whether the ov module opened
a circuit breaker or operated a relay.
A(contintued):In general his "logic" or philosophy on "fused" items, is you can live with out them or should if it blows, with proper design of your system. In most production planes (read all) only CB's are used or resettable thermal current limiting devices. Bob is right, fuse are cheap, simple, light and work to protect the wires as good as any CB. The down side is you have to carry extra fuses and it's difficult to fix or re-set in flight.
However the latter issue, resetting in flight, is moot in Bob's opinion. He proposes you don't want to replace a fuse, ever, until you land. Obviously if a CB pops, you might consider not resetting it, like if you smell something. At least you let a CB cool and only allow one reset. So bottom line, if a fuse blows it's OK to leave it blown till you land since you don't need the device and it might harm something to reset that circuit, in the scenario in his mind.
Bob: this writer chooses to ignore my suggestion
that fuses are PREFERABLE for equivalent protection
ONLY when the builder understands and strives for failure
tolerant design. This design philosophy produces a
system were fuse replacement is NOT NECESSARY because
the airplane has no device 'critical for continued flight'.
i.e. every devise with a potential for critical
operation has a PLAN-B. Hence, whether the fuse is
nuisance tripped or the device simply dies, no immediate
hazard to flight is created.
If one chooses to design and operation his/her OBAM
aircraft in the spirit and intent of a C-172, then
by all means, use breakers throughout. You may indeed
wish to do a one-time reset on several of the airplane's
electro-whizzies. When I fly, it's ALWAYS a rented
TC aircraft, it's ALWAYS fitted with breakers and
I ALWAYS have this failure-tolerance-package in the
flight bag . . .
http://aeroelectric.com/Pictures/Misc/Failure_Tolerance.pdf
A(continued) IMHO, for heavy current items like gear retraction motors, CB's are generally preferred. A electric motor can have a transient overload and be fine. So CB reset is hand.
Bob: Fuses are NOT suitable for hi-inrush loads or
the protection of fat-wire feeders and they've never
been recommended for that application. In these instances
fuse-like CURRENT LIMITERS and or fusible links are
suggested in my writing (TC aircraft use current
limiters too). The reader demonstrates no appreciation for
legacy design goals and recipes for success that go
back nearly 100 years.
A(continued): The Alternator is a little different? You could use a fuse. Bob makes grand statements of philosophy or "Bob rules", but than it "appears" he contradicts himself or changes the philosophy to fit his current opinion. That is not a criticism, we all move the goal post sometimes. However if you think you can live with out your alternator, say fly on battery power, than the fuse on the ALT is not critical. You could sub in a fuse for simplicity and less cost/weight. If you have a stock internal voltage regulated alterantor the CB on the "IGN" lead is even less useful. The current is like 0.10 amps or less.
If you are using an internally regulated alterantor than the fuse can be say 0.5 or 1 amp since the "IGN" lead is only a 'signal' to wake up or sleep the alternator, not a "field wire" or power to the voltage regulator. All the power goes through the b-lead. If you are using a Plane Power, than use a 5 amp (or what ever size) CB they suggest, since that is also a "crow bar" type CB popper as well. As you might know a crow-bar dead shorts the CB to trip it, removing power to the regulator, which removers power to the the alternator "Field", which de-energizes the alternator, if its miss behaving.
Bottom line. Bob's way is not the only way or best way, just his way which is totally fine. Some times its over kill. Some times the reasons he gives are opposed to other opinions (but there is always a justification). It's just not that important, and if you follow his philosophy you will be ok, albeit may be on the overkill side. The only thing I really think is wrong is putting a crow bar & over voltage relay on the b-lead of a internally regulated alternator. That is heavy and a Jury Rig. It also will damage the alternator in the event of a nuisance trip.
Bob: discussed, researched, and demonstrated not to be true.
See recent updates to chapter on alternators.
A(continued): If you are worried about your internally regulated alterantor (and you will be if you believe everything Bob says, which he does not have proof of) than get a Plane Power unit. Also good is B&C alterantor with an external regulator, but I would get a Transpo V1200 regulator not a B&C voltage regulator. The Transpo V1200 uses solid state OV protection and not a CB tripper crow bar. It also cost 1/3rd or 1/4th the cost of B&C voltage regulator price.
Bob: The really cool thing about the Internet is that
it's ALL out there for reading. The really bad thing
about the Internet is that it's ALL out there for the
reading.
Individuals who make due-diligence searches of the
archives for guidance must be wary of advice that carries
just enough truth to give the appearance of knowledgeable,
well considered, recipes for success.
But like the exchange detailed above, Some writers
place their otherwise good advice in question
when they carry tar bushes in one hand and buckets
of tomatoes in the other. The writing is also
suspect when the reasoning demonstrates no understanding
of the physics or simple ideas that go into recipes for
success that have rich histories of performing to design
goals.
Bob . . . [quote][b]
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user9253
Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan
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| Posted: Fri Feb 26, 2010 8:43 am Post subject: Re: Clearing some away some fog and fuzzy logic . . . |
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The main issue in this debate seems to be whether to use fuses or circuit breakers. Like many decisions in life, there is more than one way to accomplish a goal. One way may have certain advantages over the other way, and visa-versa. Many times the decision boils down to a matter of personal preference; and I believe that is the case here. I have replaced hundreds of fuses during my career as an industrial electrician, but only a few circuit breakers. Fuses can blow from a variety of reasons like old age, operating near maximum rating for long time, heat from a bad fuse-holder connection, and of course overload or short circuit. And circuit breakers will trip for most of those same reasons. The blade type fuses used in modern cars are much less likely to develop a high resistance connection with the fuse holder as compared to the old cylinder shaped fuses. Some people might argue that fuses will blow quicker than circuit breakers will trip, thus offering better protection. Although technically true, I do not think the time difference is significant. One way to avoid nuisance blowing of fuses is to use a larger size. For instance, an avionics manufacturer might recommend using a 1 amp fuse. If you use a 3 amp fuse instead, there will be a lot less chance of it blowing. The fuse should be sized to protect the wire, not the load. If the equipment manufacturer is depending on you to protect their device from internal failure, then choose another brand.
I am going to use fuses (where applicable) in my plane because they are lighter, cost less, take up less room, and are easily replaceable. My fuses will have indicator lights that will illuminate when the fuse blows. Van's Aircraft is using fuses in the RV-12 and it meets the ASTM standards. If you want to use circuit breakers, that is OK with me. To each his own.
Quote from technician, | Quote: | | "The only thing I really think is wrong is putting a crow bar & over voltage relay on the b-lead of an internally regulated alternator. That is heavy and a Jury Rig. It also will damage the alternator in the event of a nuisance trip." |
End Quote.
Quote from Bob, | Quote: | | "discussed, researched, and demonstrated not to be true." |
End Quote
I am taking Bob's side here. The technician is basing his statement on rumors. Bob has done experiments with expensive lab equipment. I trust his work. Even if the technician is correct (which I doubt), I would rather ruin an alternator than have an over-voltage condition ruin thousands of dollars worth of avionics.
Although the technician is highly critical of Bob's recommendations, I do not take his statements as being a personal attack. Still, he was expressing his opinions as fact and pooh-poohing Bob's, which is not a nice thing to do. If I say something that others disagree with, it is OK to tell me, as long as it is done diplomatically. LOL
Joe
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