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Comment by wesleyd | Hacker News Reader

One of the fascinating challenges with HVDC is that a circuit breaker - an off switch! - is a surprisingly complex and expensive piece of engineering. If the poles aren’t separated quickly enough, an arc can form and current continues to flow. This is usually undesirable.

(Technically AC has the same problem, but the problem is much more acute for DC at the same voltage.)

You know the joke/story about how nasa spent millions of dollars developing a pen that can write in space, and the soviets used a pencil? Well, the circuit breaker version of this is that the west built crazy complex huge high voltage circuit breakers, and the Soviets just built, uh, “single use” circuit breakers. With explosives.

(I don’t know if this is true, I have never found a source, but it feels true, it should be true, I so desperately want it to be true.)

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Explosive circuit breakers are standard in electric car safety systems (in case the regular one fails). I can't imagine anyone using them in normal operation though.

The space pen story is made up as well: https://www.snopes.com/fact-check/the-write-stuff/

It's not quite wrong either. The truth is more complex. See https://www.scientificamerican.com/article/nasa-spent-millio... and https://www.reuters.com/article/factcheck-nasa-pens-idUSL1N2...

I think any of us would balk at a government agency spending 1300 bucks on a single pencil (first link). And the graphite conductivity problem is a non sequitur, given that the Russians used wax (or grease) pencils, not graphite. This is really the point of the anecdote: wasteful government spending.

Though apparently the Russians also find the pens useful and bought some as well.

> I think any of us would balk at a government agency spending 1300 bucks on a single pencil (first link).

Only people who don't understand how engineering and manufacturing projects work. $1300 is completely reasonable for a very low volume manufactured product, which was specified to meet specific criteria.

The reason it sounds unreasonable is because it is being called a "pencil/pen", instead of what it actually was, which was speciality equipment designed to be compatible with the conditions inside of the spacecraft.

At face value in meme format, the implied "free" solution would be buying a pen or pencil off the shelf, when in reality, that alone wouldn't have solved the problem. It is possible that someone could have evaluated off-the-shelf options to see if one would have met the criteria, but that would still have significant labor cost.

> which was speciality equipment designed to be compatible with the conditions inside of the spacecraft.

Right, that's the point.

It's also how you get the other very expensive things that the Pentagon gets dinged for.

What everyone forgets is how valuable each second of astronaut time actually is. 1300$ for a slightly better pen is probably a great deal for the agency.

The shuttle program (excluding ISS) worked out to roughly 1 million dollars each hour an astronaut spent in space including when they were sleeping. Space stations are significantly better but anything that marginally improves astronaut effectiveness is worth quite a bit.

But those space pens are available to civilians now. In plastic disposable versions for a few bucks(uniball power tank).

They are the only pens I know that stand up to modern use where you might only touch them once a week or less. Now they exist, and probably will be available indefinitely, and cheaply, and the problem of pens that just work anywhere is basically solved.

Isn't part of the point of NASA to develop tech that benefits everyone? Granted it's not a major life changing thing, but it's not just tossing money in the toilet either.

As I understand it, the point of the explosive is to have finer control over when the fuse blows. An ordinary fuse is more or less a resistor, and it blows when part of it gets hot enough. There is a fair amount of error, and this means they work best when there is a considerable margin between the current they must carry without blowing and the conditions under which they must blow. Conventional thermal-magnetic or hydraulic-magnetic circuit breakers are similar.

AIUI some high performance cars may draw so much current under maximum acceleration that the fuse needs to be dangerously large to avoid blowing when flooring it.

The solution is an electronically triggered fuse. A reliable and precise electronic circuit detects excessive current and blows a small pyrotechnic charge that opens the fuse. The analogous technology for circuit breakers is fairly mature in the commercial/industrial world — you can buy an electronically tripped circuit breaker, and there is likely one in an office building near you.

(Electronic trip devices for circuit breakers have ludicrous list prices, and there is no way a car company would pay anything resembling those prices for a car component that lets them eke out a bit more performance. I bet Tesla’s cost for its pyro fuses is quite low.)

> Electronic trip devices for circuit breakers have ludicrous list prices

The last one I bought was a GFCI trip unit for a 1000A 480v Siemens breaker and it was around $6000, and that doesn’t include the cost of the circuit breaker.

That's pretty impressively expensive, especially given that the innards should not have any particular dependence on the rated current. (I suppose the GFCI sensing circuit needs to tolerate an increased amount of induced current as the available fault current goes up.)

All switches have this problem, even low-voltage or AC ones. Slow switches, or switches that bounce, create arcs. This damages the contacts and can be hazardous.

This is one reason switches are "clicky". The action of completing or breaking the circuit must happen quickly. Switches have springs in them, which ensure the switch goes between the two extremes as quickly as possible. The springs oppose the motion for first part of the travel. Partway through the travel, they suddenly start to assist the motion and force the switch the rest of the way.

Rather than separating the conductors more quickly, wouldn't it be preferable to replace the space they occupied with a material in which arcs cannot form? Or is this a cost/risk thing? e.g. I know SF_6 is sometimes used for this purpose, but that is problematic as it is a potent greenhouse gas.

A typical miniature or molded case low-voltage (under 1000V) circuit breaker will have arc chutes to extinguish the arc. [0]

Higher power low-voltage circuit breakers as well medium and high voltage breakers can use oil [3], (compressed) air [1], gas [4], and vacuum [2] to extinguish the arc flash.

SF-6 is used for high voltage applications while vacuum and air are common for medium voltage gear installed indoors, oil breakers are used in outdoor installations at utility substations and similar installations.

[0] https://wiraelectrical.com/what-is-an-arc-chute/

[1] https://www.se.com/us/en/faqs/FA360729/

[2] https://en.m.wikipedia.org/wiki/Vacuum_interrupter

[3] https://www.electricaltechnology.org/2021/08/ocb-oil-circuit...

[4] https://www.electricaltechnology.org/2021/08/sf6-sulphur-hex...

Most high voltage breakers already are SF6.

They rely on the 0-point crossing to quench the arc.

But is there not 0-point crossing in DC currents ?

The current is always positive except when it's turned off. It never reversed so it never passes through 0v to get to negative volts. Which is also the big advantage, since you're not spending part of the time at lower voltage.

Like one metal blade that is placed in between the wires (current flows) and slid up to an insulating part (current does not flow) ?

I am pretty sure if it was that simple it would have been done that way already. Breaking high currents is something we had to do for more than a hundred years.

Arc flashes are no joke. We are talking about 2800 to 19000 degrees Celsius here. I don't know about your insulator but it has to survive a multiple of the surface temperature of the sun. And ideally it withstands that more than once or twice.

Thats not that easy. As soon the two metals are seperated, there will be an arc. The arc is really, really hot and will burn everything and also as soont there is an arc, the arc stays can bee so much much longer in discance than just air. Since there is 0 in DC, large DC-switches are much more difficult to build. There are several options from SF6 (today no), to vacum or blowing magnets (blow the arc to cooling cambers).

Would it be feasible to use induction to briefly induce a reverse current on a short section of the line that is of the same magnitude as the forward current so that there won't be arcing when you break the circuit there?

Kinda; actively sucking current/voltage difference away from a circuit breaker can allow you to interrupt a long DC line/part of a larger DC mesh without interrupting sufficiently distant users. Think train line/streetcar grid.

Bzzzz :)

https://www.youtube.com/watch?v=GMbN9nb3qyk

Tesla has pyro-fuses in their newer inverters and I believe in some battery packs as well.

Worth noting for each pyro fuse in the cars what it protects against.

The fuse in the motor inverters protects against failure of the mosfets (switches) inside the motor inverter. If any one of those switches get stuck 'on', the motor ends up doing full-power braking. That happening while driving along the highway would be catastrophic - hence the pyro-fuse to prevent such things.

The fuse in the battery pack is to prevent a short circuit anywhere in the high voltage system of the car short circuiting the battery. Obviously a short would cause some pretty huge currents to flow, probably causing the battery to overheat in a few seconds, giving out large amounts of flammable gas, which would immediately ignite (TV-style fireball explosion). And again, the pyro fuse prevents that.

What's the difference between a single use circuit breaker and a fuse?

A fuse is triggered by excessive current flow on the line that it breaks. A single use circuit breaker is triggered manually or by some external signal.

Nobody is confused about how to make a single use circuit breaker. But things like train stations have a need for repeated DC circuit breaking at high voltages.

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