Thursday, August 20, 2009

Circuit Protection & Fire

While we're on the subject of electrical adventure, I'll start answering the question I was asked about resetting circuit breakers. Circuit protection is an important issue in a confined space where fuel and oxygen are readily available and where electrical power is needed for lighting, navigation, communication and control. The circuitry includes two generators, a battery, and multiple buses, so there is a lot to go wrong. As a result of this accident some people are discarding the usual rule of "reset once."

I'm not an electrical engineer and I'll try not to pretend to be one for this discussion, but here are some of the things I think I know about circuit breakers. Real electrical engineers can cringe along, and fill up the comments with useful corrections. Long-ago physics classes left me with the ability to talk about electricity in terms of voltage which is how hard the power source can push electrons through the circuit, current which is how strongly the electrons are flowing, and resistance which is how hard the the electrons have to be pushed to get through whatever is in the circuit. At this point in any discussion of electricity it is also required to mention that the convention for direction of current flow is in fact opposite to the direction of travel of the electrons. This is irrelevant, but if I don't mention it, someone else will.

The device that any given circuit exists to power is a source of resistance. The battery shoves the electrons though it and the thing responds by lighting up, moving a needle, transmitting radio waves, or whatever it does. The light bulb over my VSI requires less current than the starter motor or the hydraulic pump, and the airplane is designed with that in mind. If the light bulb suddenly starts drawing more current than it should, the electrical system gets suspicious and cuts it off, kind of like the power company does if they suspect you're running a grow-op, or the credit card company does every time I buy a thousand dollars worth of fuel in three different provinces on the same day.

The circuit breaker is the thing that cuts it off. To a pilot, a circuit breaker is a cylindrical thing a bit thinner and a bit longer than a pencil eraser. Rows of them are arranged with the circular ends towards the pilot in a breaker panel in the wall, ceiling, and/or dashboard. They all have labels underneath them, although some of the labels are illegible, ambiguous or wrong. Some are flush with the panel until they pop and some stick out a little ways so you can pull them yourself if you want to. When the breaker is in it completes the circuit. When it pops it sticks up and breaks the circuit. Hence the name. I think they work via a bimetallic strip, but there are probably multiple kinds.

There are two basic ways for electrical components to break. Either they don't let electricity go through them anymore, or they let the electricity go the wrong place. When a light bulb burns out, the filament breaks and electricity can't flow all the way through the circuit, so for the purpose of that bulb, the CB might as well be pulled. If a pilot pulls out that bulb and shoves a bulb in from the supply of spares, but gets the wrong kind, or twists it wrong, or there's a some metallic debris caught on it, it could allow the electricity to flow not in the base, through the filament and back through the tip of the base, but instead skip all that hard part about going through the filament and just race around through the metallic debris and out again. The electricity gets very excited about this shortcut. Increased current increases the temperature. Increased temperature increases resistance. I'm not sure which is the chicken and which is the egg in this resistance and current chicken coop, because I'd think at some point increased resistance would decrease the current but what do I know. When the current gets going past the limit prescribed by the designer of the circuit, the CB pops and all the instruments on that circuit are plunged into darkness. Cursing and flashlights are in order.

In that example, I would fly the airplane, turn on the cockpit light or the map light or something on a different circuit that would allow me to see the instruments, turn off the control for the suddenly darkened lights, remove the recently replaced bulb, check the breakers and reset the panel lighting one, and turn the lights back on. If the bit of metallic debris was still stuck in the socket and it did it again, I wouldn't know what had happened, so I'd continue my flight on emergency lighting and then later let some apprentice on the ground fish out the debris, look at me like I was retarded, and re-certify the system. If I did happen to look inside the socket and find the bit of steel wool that had caused the problem, I probably would even reset the circuit breaker a second time to see if I could get some lights. I'd let maintenance and other company pilots know it had happened, though.

I can remember two incidents in my career that might be termed electrical fires, although only in the sense of "where there's a burning smell and melted plastic there's fire." Neither progressed to fire extinguisher-requiring flames, nor did either trigger circuit protection. The first was a faulty fuel flow gauge that overheated enough to melt the indicator needle to a crispy brown colour and heat up the case enough to burn a knuckle on the glass and melt itself into the dashboard in less than 15 minutes. The reason the circuit protection did not act was that someone had put a two-amp fuse in the slot protecting the half-amp circuit. The other was a live but loose wire from an instrument light swinging around and striking sparks off everything it contacted inside the dashboard. The display of sparks was impressive. It visibly lit up the cockpit in broad daylight, and the smell was such that 30 minutes after shutting everything off, the guy who opened the back door of the airplane needed no further paperwork or explanation to understand that I had a legitimate reason for landing with no radios. No CB tripped, possibly because I didn't give it a chance: I followed my airplane's electrical fire emergency checklist, so turned off the master and pulled every CB in the panel as soon as I saw the sparks coming out of the panel. Fire, smoke and burning smells must be heeded in the cockpit.

That starts to segue to the accident they wanted me to discuss, but I'm hungry and I've written lots, so I'll continue this later making this week an all-electrical blogging bonanza.


Guy said...

Couldn't find any glaring errors in your electrical description. Breakers for low current are typically thermal -- once they heat up past a certain point, they release a spring which separates the two contacts. If circuit has high transient current for some reason, it may trip but then be able to be reset after it cools down.

Wires do increase in resistance as they warm up, but not enough to significantly impede current flow before something bad happens (like something nearby melting or catching fire). This is the basic idea behind a fuse -- once past a certain point, the current melts the thin point and blows.

Joseph said...

My focus is mostly on computer engineering, but I had a decent number of electrical engineering classes, so here's my shot.

You let loose the magic smoke! Every electrical/computer engineer has had the magic smoke escape on them. And no matter how hard you try, once that smoke is out, you can never get it back in.

There are two different types of protection for circuits, circuit breakers, and fuses. Chances are, if it has a circuit breaker, its designed to protect against the occasional transient spike in power consumption. If it has a fuse, its to protect against a more long-term failure. That isn't always true, but it is a good general rule.

When its a loose (as in flopping around) wire causing sparking, unless it gets stuck, its unlikely to trip a breaker, because it is generally moving too quickly to keep current in one place. You definitely did the right thing by tripping all the breakers manually, because it can still cause damage even in those little bursts.

As guy said, the increase in resistance of a wire as it warms is still generally negligible, at least unless you only have a wire connected between two points with a high voltage potential. In that case, it is a very good fire starter/metal melter.


Anoynmous said...

If the light bulb suddenly starts drawing more current than it should, the electrical system gets suspicious and cuts it off,...

Electrical power distribution systems are rarely that savvy about exactly what current the loads "should" be drawing. All they typically care about is that the current stays below what the wire is rated to handle.

Circuit breakers are designed to protect the wiring from overheating. They can't be expected to do any more than that. If a light bulb malfunctions in such a way that it uses twice the current it ought to, it has already broken, and the circuit breaker had no chance to prevent it. If that doubled current is still within the capability of the wires to handle safely, an appropriately-sized circuit breaker won't cut off the power.

Aviatrix said...

If that doubled current is still within the capability of the wires to handle safely, an appropriately-sized circuit breaker won't cut off the power.

So if the bulbs on that circuit are expected to draw one amp and the wires can handle five, but there's a 2 amp breaker on the circuit, you're saying a 2 A current won't pop the breaker?

Of course I know that the system doesn't "know" what the bulb is supposed to draw, but I thought its "knowledge" came from the size of fuse or CB specified for that spot. Exceed that and it blows. But you say no?

Icebound said...

We have to be aware that electrical fires are not normally caused by a "failure" which causes an increase in current (such as a short circuit to ground). When that happens, the CB or Fuse will usually blow and the danger is over.

Fires start with there is some sort of unexpected increase in resistance along the path. Heat is generated as a function of the square of resistance. Remember I-R-squared.

So if you have a "loose" wire barely touching the panel, chances are that the point of contact is not very strong, thus a high resistance, thus that point gets hot.

If you run 500 feet of extension cord, the resistance of a long cord is greater than a short cord... if you try to force the same current through both, the long cord with get hotter.

The plug-in is not as good a conection as the solid wire itself is. Hence at a given current, the area around the plug will probably get hotter than the rest of the wire.

Note that the breaker does not help you.... you are still carrying the same rated current, which may be well within the breaker specification so the breaker does not blow..

But if the resistance along the path gets too high, heat will result.

Corrosion at connections, terminals, plug-ins etc. is the usual suspect for such increase in resistance and heat.

Broken wires also.


Anonymous said...

It seems to me that you calculate the heat generated using I^2*R - current squared times the resistance.

Wikipedia seems to agree:

Dadette said...

"...inside the socket and find the bit of steel wool that had caused the proble, I probably would even reset the circuit breaker a second time..."

Does anyone else see a problem here? (Totally unintended)

Aviatrix said...

Yes, heat generated per time as I^2R makes sense, as that's equivalent to power. I guess you'd have to throw a t in there and some factor for heat dissipation to really know how hot things got. I edited the V=IR and P=IV equations out of the initial blog entry because they really weren't adding anything to what I had to say: people who knew them knew them and people who didn't would get scared off. I didn't have anything to prove with them.

Problem? Yeah, that's the problem. I can't see in the dark. If I thought I'd identified and solved the problem and really wanted that electrical service, yeah, I might try a second reset. It's different from mindlessly resetting it just to see if it will work this time.

If you mean that I would electrocute myself solving the problem, or that steel wool and lightbulbs don't act that way, then you may well be right. It was a made up example.

A squared said...

Icebound said: "If you run 500 feet of extension cord, the resistance of a long cord is greater than a short cord... if you try to force the same current through both, the long cord with get hotter."

This is a common misconception, but it's just that; a misconception. A given length of wire of a given gauge and a given material will have a specific resistance. (a foot of 16 gauge copper wire, for example will have a resistance of .00473 ohms)

for a given current, the same quantity of heat will generated in that foot of wire, regardless of how may of these feet there are linked together. The fact that the *overall* resistance of the entire cord is greater is irrelevant, the amount the cord will heat up is a function only of how much heat is generated per unit of length.

IOW, a 500 ft extension cord will generate 5 times as much heat for the same current as a 100 ft extension cord of the same gauge, but that 5 times as much heat is spread out over 5 times as much cord, so the actual temperature rise will be the same.

To take that a step further, with a purely resistive load connected, the longer extension cord will be less, because the greater resistance of total circuit will result in less current in the circuit, and less current in the circuit means that *less* heat will be produced per foot of extension cord.

Where you may get a hotter cord is if you are powering an electric motor; some types of electric motors will draw more current when connected to a lower voltage, and a longer cord will mean lower voltage, therefore a higher current draw. If you are drawing *more* current, than the heating per foot of wire will be greater

Anonymous said...

"Does anyone else see a problem here? (Totally unintended)"

I don't!

And since I don't know anything about circuit breaker procedures in cockpits I can only answer in a literal sense of the question (hint hint...)



Anonymous said...

Your explanation is pretty good.

A breaker usually has a reaction time; if a current spikes for a short period of time, the breaker may not blow. This is especially true if the current is at or around the breaker's rated current. To blow a larger slow-blow aircraft breaker quickly (<1s), you need to be running 300-600% of its rated current. Putting current limit circuitry into avionics is a science.

Go look at this link for a larger breaker:

Timo Kiravuo said...

So if the bulbs on that circuit are expected to draw one amp and the wires can handle five, but there's a 2 amp breaker on the circuit, you're saying a 2 A current won't pop the breaker?

Exactly. If you have unlimited 12 Volt power supply and plug in 12 Watt bulb between the terminals, you draw 1 Ampere of current from that unlimited (we assume zero internal resistance), voltage regulated power supply.

If the wiring can handle 5 A and circuit breaker 2 A, they do not enter the picture at all (their resistance is not worth calculating).

Now if you change the light bulb to 24 W bulb, you draw 2A and the circuit breaker is still happy. With a 60 W (12 V) bulb you draw 5 A and even if the wiring can handle it, the CB will pop, because the designer had decided to limit the current on this circuit to 2A, for example because he thought that 60 W of power in the console would make stuff overheat.

If you replace the bulb with a paper clip, you create a system, where the resistance is very low and the current is 12 V divided by the overall resistance of the system, which is very little and you get a load of Amperes running through the wire. As P = U x I, you are changing a lot of electricity to heat, but fortunately the fuse or circuit breaker is designed to detect a current over 2 A and to break the circuit.

So the CB does not normally enter into calculations, until it pops. The actual load (bulb, motor) controls the current over the circuit. CB controls only overloads.

Hope this helps.


Unknown said...

Interesting and erudite discussion which seems to have covered all the bases, 2 things arising.

1- power-cords which are coiled up or on a reel should be fully unwound otherwise they act as an inductor and "slow down the electricity".
2- don't worry about daisy-chains- in practical terms, all connectors are beefier than the tiny single strand of wire we call a fuse.

I have seen a bit of aircraft wiring and was shocked at how small the margins of safety were...even the nasty "made in China" washing-machines have more robust and better-insulated wiring...and Kapton is nowhere like as chafe/crush/abuse-resistant as PVC.

In an ideal world, everything would be "beefy" up to the breaker bus-bar..from there-on each circuit has it's own breaker as a "safety-valve".

Aviatrix, you never fail to impress with your in-depth grasp of the Physicsand practicalities.

Very curious about why you'd carry a lump of wire-wool.....I think you should tell your "friends" ;-)

Ed said...

1- power-cords which are coiled up or on a reel should be fully unwound otherwise they act as an inductor and "slow down the electricity".

I'd have thought the inductance at 50 or 60 Hz would be negligible. The reason for unwinding power cords is to dissipate the heat produced more effectively.

Aviatrix said...

kiravuo Hope this helps.

No, it doesn't. Kiravuo has just described my model of how circuit breakers work: they are irrelevant until the current exceeds the number stamped on the end of the CB, and then the interrupt the circuit.

Anonymous of Aug 20 03:09 seems to be stating that as long as the wires can handle the current, the CB will not blow. But rereading it, I suddenly realize that s/he is pointing out that the CB does not pop to protect the light bulb, and that by the time the current exceeds the CB rating, the lightbulb is already destroyed, and the only thing left to protect is the wiring. The implication that the CB would not blow when the rated current was exceeded was an accidental side effect of the explanation.

I hadn't meant to imply that the CB was going to jump in and save the equipment before damage was done. I was just being cute.

Aviatrix said...

And I do not carry steel wool for stuffing in empty light sockets!

Anoynmous said...

So if the bulbs on that circuit are expected to draw one amp and the wires can handle five, but there's a 2 amp breaker on the circuit, you're saying a 2 A current won't pop the breaker?

What the wires can handle is irrelevant to what the breaker will do. The key is that the breaker has been sized "appropriately". Device failures can't be relied upon to do anything specific, so the breakers in systems I work with are almost always chosen to reflect the capacity of the wiring. (It's often actually the other way around, with the wire size chosen to safely handle the current permitted by the breaker.)

2 amps through a 2-amp circuit breaker is effectively an indeterminate case. More than 2 amps for "a while" is supposed to trip it. Less than 2 amps for "a long time" is not supposed to.

I completely recognize the description you gave and I know exactly what "collared" means, but I'm not familiar with the internal details of how those breakers work. Are they thermal "slow acting" or are they magnetic "fast acting"?

A thermal circuit breaker has to heat up enough to bend a piece of metal and pull a latch out of the way. That can happen because of a slight overload for a few minutes, or an extreme overload for a fraction of a second. A constant load of just below the rated current will "preheat" it some and render it more sensitive, so it will trip more quickly if the current increases to just above the rated current.

A magnetic circuit breaker is closer to an ammeter that flips a switch when the measured current reaches a predetermined value. It'll trip quickly whether the overload is slight or severe.

But in either case, a current at the limit might or might not trip the breaker, and if it does, it might take minutes or hours to do it.

X-av8r said...

Hi, 'Trix,
1. A CB in an aircraft is there to protect the wiring. If an item in the cicuit is drawing more amperage than it's supposed to, the CB heats up, along with the wiring. When the CB pops it can be reset after it cools, but the wire envolved may not be able to cool due to it being buried inside a bundle along with a hundred other vital wires. Continually resetting of the CB may cause the wire in the bundle to become hot enough to begin melting insulation on other wires and an electrical fire may result. One of the worst things that can happen to an aircraft!
Ergo, the one reset rule.

2. Hooking a whole bunch of extension cords together will reduce the voltage and amperage to the unit on the end that you need powered. It may not work so well.

Aviatrix said...

Anoynmous: while I have flown aircraft with slo-blo fuses designed to protect starter-generators that handle huge loads briefly, I do not believe the CBs in my current ride are slo-blo. I don't know whether they are magnetic or bimetallic.

I wasn't actually asking about the boundary condition. I was just sloppy and said "2A" when I meant "in excess of 2A."

Unknown said...

The C310 accident you referenced was certainly an eye opener to many regarding the potential danger of CB resets. For me the wake-up call was Air Canada 797 Modern aircraft have so much systems redundancy it's difficult to justify any CB reset inflight.

Another pet peeve of mine are the (often) lengthy checklists most manufacturers supply for inflight fire/smoke, smoke removal etc. I would add as a preface to any of these "Perform only with the nose of the aircraft pointed at the ground (if applicable)."

Sarah said...

Angus, thanks for the link to the AC 797 report. While giving chilling emphasis on the results of in-flight fire, it doesn't appear this one was the result of CB reset policy. The breakers likely tripped because of the fire, not as the cause. On Page 55:

"Another possible source of ignition near the area where the fire was
discovered was the flush motor wiring harness. The tripping of the three circuit breakers
accompanied by the arcing sounds recorded by the CVR occurred at 1851:14. The three
circuit breakers tripped almost simultaneously indicating that the circuitry of all three
phases shorted at the same time. The only evidence of wiring damage was found .where
the flush motor wiring harness passed through the lightening hole in the partition between
the amenities’section and the toilet section of the lavatory. The damage noted in the
wiring harness at this location could only have been the result of fire and heat, and the
Safety Board concludes that the damage to the wiring which caused the three flush motor
circuit breakers to trip was caused by heat and fire."

Sarah said...

This has been such an instructive thread. I learned that while coiling up unused of even a high-power extension is likely not dangerous ... except in possibly concentrating the heat of an overloaded cord.

And, that the voltage drop of a substandard long cord even if not over-spec with current draw *can* be dangerous to motors.

It turns out AC motors presented with low voltage but high torque will draw *more* current to compensate, to the point they may toast their own windings.

The Bad Astronomer forum is nearly as interesting as the BA himself.

Unknown said...

You're certainly correct regarding the AC 797 accident Sarah. However it did get me (and many others at that time)thinking about the wisdom of resetting any breakers.

Dadette said...

"Does anyone else see a problem here? (Totally unintended)"

Was supposed to be a clever pun, since problem was misspelled in the original post as proble,.

A bit embarrassed that nobody likes my puns :(

Aviatrix said...

blavey, it looks like everyone else is so fluent in typo that they didn't notice. Fixed now. Thanks for pointing it out.

zb said...

Using a coiled extension cord may actually help due to the inductance of the coil: Even though the inductance is small because it's basically just an air coil with no magnetic core like the ones you find in transformers or many other inductors, it does work:

If you have a load with a high start-up (inrush) current like a motor, and the outlet is protected with a fast-acting fuse that keeps blowing every time you switch on the motor, the small inductance of the wound up extension cord may likely limit the inrush current spike and the fuse will remain set. The inductance won't do much harm once everything runs steady-state at 50/60 Hz.

This trick is taken out of the toolbox of redneck engineering, though, because, as has been mentioned, the roll might heat up significantly due to the cord's resistance in case the steady-state current is large.

zb said...

... I have to add that I really love the description about voltage and current and their relationship. You really can teach well! Never have I read such a good text-only explanation anywhere.

By the way: On the topic of which ends of the battery are called + and -...

zb said...

Sorry... Link works this time, hopefully?

A Squared said...

Steve said: "I have seen a bit of aircraft wiring and was shocked at how small the margins of safety were...even the nasty "made in China" washing-machines have more robust and better-insulated wiring...and Kapton is nowhere like as chafe/crush/abuse-resistant as PVC.

I don't know about Kapton, but having spent a lot of time working with aircraft wire and other types I can assure you that the insulation on aircraft grade wire much tougher then the insulation you will find on wire from your local electrical or electronics supply house.

At any rate, you have to bear in mind that your Chinese washing machine is not the result of a design process in which weight reduction is one of the highest priorities.

You may also be shocked to discover that the structure of an airplane is engineered on much thinner margins than an automobile, and the hydraulic systems are engineered on much thinner margins than are farm equipment, and virtually all other components will be designed wit thinner margins than most non aviation counterparts. The reason is weight.

Paul B said...

CBs have a couple of advantages over fuses... first of all they can be reset. Second is that they tend to act faster.

There are 2 general type of fuse "fast blow" and "slow blow". You might be amazed how SLOW a slow-blow fuse can be. Many years ago, I had about 20 amps running via a 13 amp socket, and it took about a minute to blow! (that was a fan heater, a kettle and a toaster, if I remember right!!!!)

Fast-blow fuses work a lot faster than that.

In terms of wire, you might be surprised how much current a wire can take.... it's all about the heating of the wire. If you take a pice if 5 amp wire, in a "protected environment", then 5 amps will probably raise its temperature by a few degrees. 10 amps will probably make it quite warm. However, if there is ANY cooling, then you can get loads more current through the wire.

(Think of a fan heater: normally the coils glow slightly, but if the airflow stops, they will start to glow a lot more brightly!)

We once tested a thin track on a printed circuit board... I believe we had 5 amps (the max of the PSU) running through it, and a finger on it could detect no noticible heat rise!

zb said...

"..., because I'd think at some point increased resistance would decrease the current..."

Finally, the week of work is over and I find some time to add some geek's stuff on this topic:

Most conductors like copper or aluminum will increase their resistance when their temperature rises, but the resistance increases only very slightly. The resistance will not increase enough to provide a protective feature. In most practical applications, copper will melt before it limits the current enough to put the heat-vs-resistance process into an equilibrium. Usually, you need not care about this effect. Only in extreme applications, this becomes significant: For example, high-voltage power lines that run cross-country need special attention on hot summer days. They are very long, long enough that their resistance causes significant losses, and they carry plenty of current. The associated large transformers face the same problem...

There are devices, though, that have not only a slight, but a very strong increase in resistance when the temperature increases. These devices are called positive-temperature-coefficient resistors (PTC resistors, or PTCs). They are made of special ceramics that are ground to tiny pieces and then sintered again. Where two of the tiny pieces touch inside of the device, magic happens when you get beyond a certain temperature: The ceramic structure changes and the resistance of the device increases dramatically.

Tiny PTCs are used as sensors for over-temperature shut-down features in electronic equipment. Large PTCs are used as protective devices: Unlike copper, they will have a very big resistance at high temperatures and can be put into a circuit to limit the current to safe levels during fault conditions. They act just like you guessed: The load current runs through the PTC, the PTC becomes hot and finally won't allow more current to flow. The PTC will settle in a state where current and temperature reach a balance. Once the PTC cools down, the process can start over again: They have an auto-reset feature, if you like to call it that way. Another very, very common application of PTCs where they are used by the millions is in good old cathode-ray tubes of TVs or computer monitors: Every time you turn on the monitor, the steel mask at the screen needs degaussing. The degaussing coil at the back of the CRT is powered through a PTC. While the PTC is still cold, much 50/60Hz current runs through the coil. The PTC becomes hot and reduces the current through the coil more and more. The alternating magnetic field produced by the coil becomes weaker and weaker until the individual magnetic parts of the mask finally will not know if the last field they have seen was South or North. Some remain facing S, some N; and on average, no one has the majority and the mask is degaussed. But I get carried off topic... So much that I bust the limit of characters for one comment... to be continued...

Anonymous said...

PTCs have a counterpart: NTCs (negative-temperature-coefficient resistors). They are also made from ceramic materials. While there are a lot of free electrons in metals, there are only few in these ceramics. Hot temperature in metal conductors mean that the electrons will bounce into each other instead of carrying the current straight through the material. On the other hand, hot temperatures in NTC ceramics mean that more free electrons may leave the ceramic lattice and thus become free electrons that may carry current through the device. A scientist would say: The mobility of the charge carriers increases. Semiconductors like silicon act the same way and conduct better at hot temperatures for the same reason: They also have only very few free electrons when cold, and a bit more free electrons when hot.

NTCs also come in small and large varieties. The small ones, again, can be used as sensors. While PTCs cut off the current in a fairly sharp manner at their rated temperature and are best used as detectors for shut-down features, NTCs have a smooth characteristic and are fairly well suited when a somewhat-precise and very inexpensive thermometer is needed. They are often used together with cheap micro-controllers: The micro-controller measures what the NTC does with its non-linear characteristic and has a table to look up the non-linear values so it can provide a fairly linear temperature value. Big NTCs are used in insane quantities in very, very many types of electronic devices as inrush-current limiters. A great majority of power supplies for PCs, notebook computers, printers, fax machines and whatnot use them. They work just the opposite way compared to the PTCs in the degauss circuits: You turn on your PC. The NTC is still cold and has a high resistance, allowing only a small current to flow into your power supply. The current heats the NTC, its resistance becomes smaller and smaller, until finally, after some ten milli-seconds, it will only dissipate so much power as to stay hot enough to allow a continued current flow into your power supply. These inrush-limiting NTCs are dirt cheap compared to other means of better inrush limiters and this is why they are so popular in consumer electronics. They have two big disadvantages: They continuously waste power in order to remain hot and low-ohmic while turned on, typically between 0.3 and 1.5W. And they only work well when you turn your equipment on while it's cold. If you have a PC, a monitor and a printer connected to the same outlet, and you turn them on and off together with one switch at the outlet, your fuse is almost guaranteed to blow if you happen to turn off the switch for just some seconds.

Electrical. It's PFM. If Electrical stuff contains NTC inrush limiters, it tends to work. Under ideal conditions, that would be...

Have you noticed how many abbreviations with three letters there are in electrical contexts? Maybe more than in aviation, even?

Scott Johnson said...

Everybody has covered this so well that I can think of only one thing to add.

Sometimes a circuit breaker isn't enough. I have personally seen overhead high-voltage lines arc over continuously for half an hour after a tree blew into them. There were circuit breakers and fuses upstream, but in this case one very destructive fire consumed less power than peak domestic loads would have -- the power and resistive heating were just concentrated at a single point. The fuses and breakers have no way of knowing a good load from a bad one.

In the case of the NASCAR aircraft, so long as the circuit fault drew less than 5 amps, it was free to do whatever damage it might. 120 watts can heat things up significantly if concentrated into a small enough area.

mattheww50 said...

Three points.
1). Coiling an extension cord does NOT add to the inductance. There are two conductors in the cord carrying current in opposite directions. The resultant magnetic fields cancel so the net inductance is also zero. The inductance of the extension cord is independent of whether or not it is coiled, and unless it is exceptionally long (miles) at 60Hz, the reactance produced can be safely ignored in any analysis.
2). Circuit breakers provide a lot less protection than most people realize. Typical Circuit breaker specs require that the breaker open within 4 Hours (yes 4 hours) at 110% of rated load, and within a few seconds at 400% load. This is to allow the circuit breaker to remain closed long enough for things like electric motors to actually 'spin up' or light bulbs to actually light up. Starting current for most motors and light bulbs is a significant multiple of run current. So in general if the circuit breaker trips, something bad has probably happened on the equipment it is supposed to protect. While very fast acting circuit breakers are available, they are used only in special situations. If you want protection that trips in a hurry at even slight overload, use instrument fuses...
3). Resistance of conductors does rise with temperature, but it really has to get hot to make much of a difference. The temperature coefficient for Copper wire is .4%/degree Celsius. So a temperature rise of 100C translates to a 40% increase in resistance. It is only when you get to heating elements and light bulb filaments that the temperature change is big enough to cause big changes in resistance. The resistance of a 100 watt light bulb at 20C is about 12 ohms. It is about 120 ohms when fully lit.

zb said...

mattheww50, I have to admit I've never tested a coiled extension cord on an LCR meter. Thinking about what I was told on the subject of coiled extension cords and inrush current spikes, I have to say that you appear to be right and my previous explanation was not really correct: A coiled extension cord acts much like a common mode choke, sometimes also called current compensated choke. These are used, for example, at the inputs of switch-mode power supplies to reduce electro-magnetic (radio) interference (EMI) going from inside of the power supply to the outside world. They keep common-mode noise form getting outside, i.e. noise that would travel through both the L and the N connection in the same direction and at the same time. They do not keep differential mode current from flowing, i.e. current that goes in through the L wire and out through the N wire.

Two things, though:

1. No choke is perfect. Under ideal conditions, it would be that the magnetic fields of the two wires cancel. However, a real choke has a stray inductance in addition to its main inductance. I have tested EMI chokes for their stray inductance. While they have a common mode inductance of, maybe, 5...10mH on each winding, they typically have a stray inductance of just 10 or 12 ┬ÁH: roughly 1000 times less. Even though the stray inductance is so small, it does help a bit when it comes to inrush spikes and unwanted tripping of circuit brakers. Then again, because the N and L wire in an extension cord are wound in a very parallel fashion because they share the same cable, you have a bifilar type of winding which is very good when it comes to low stray inductances. I really have to admit that I can't give any numbers here.

2. I absolutely agree that no effects will be noticeable at the low mains frequency of 50 or 60 Hz. An inrush spike, however, is very short in the time domain and thus extends to very high frequencies in the frequency domain. That's why a radio sometimes goes 'pop' when you turn something on. Very high frequencies will be rejected from flowing through even very small inductances. Looking at it the other way round, if only the low-frequency parts of an inrush current spike pass through an inductance, the spike will be reduced to some degree. It may be the case that the small stray inductance of a coiled extension cord will do some good if it comes to reduced inrush spikes, compared to a straight extension cord of the same length. But I have never tested, simulated or measured extension cords.

As a guess, I would still say that the coiled-extension-cord trick is worth trying if you are in the situation where a circuit breaker keeps disconnecting your drilling machine or air compressor every time you try to turn it on. It will depend heavily on the exact setup: The characteristic and rating of the particular circuit breaker or fuse, the shape of the inrush spike caused by the machine, the phase angle of where the 50 or 60 Hz sine wave is when you toggle the switch on, and maybe even the way the extension cord is wound: A 'clean' winding will have a different stray inductance as a 'heap' winding. As I said before: Do not use this setup if you run the machine for more than a few minutes. The cord might get too hot due to plain old resistive power dissipation.


zb said...


mattheww50, you are right when you say that circuit breakers and fuses are truly nasty devices. Their data sheets often only give vague hints at what they will really do. The worst thing I have seen was a fuse rated for AC that was tested with a DC current of maybe 150% of the fuse's rated current: The thin wire inside of the fuse tries to melt, once it opens, an arc bridges the gap, the arc does not stop because DC does not go through zero twice a period, and at the end, you have a melted chunk of ceramic, sand and metal. It's a hell fire. Consider how much temperature it takes to melt ceramic materials and sand!

The worst thing a heavy overload conditions will do to a fuse is an explosion, but at least the circuit disconnects. 120%...250% of the rated current is where things get very interesting... Even more at DC.

Yay electrical! Even more fun when it comes to parasitic effects...

When is the next electrical nerd week on Cockpit Conversation? I hope for ideal conditions, under which this would be very soon.