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.)
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.
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.
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.)
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.
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.
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.
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.
I'm looking at an AC wall socket, with a 5 places multi plug with plugged in 2 phone chargers, my laptop's power brick, a fan. All of them are transforming AC into DC (well, maybe not all of the fan.) I guess that the problem here is that all those DC appliances have different voltage inputs so a hypothetical DC wall socket maybe would be 12 V or multiples of that and then we'd still need transformers to a different voltage. On the other side all my AC appliances are standardized at 230 V.
I think that the most common DC plug is a USB one. As you said, we pull the plug. I never saw an arc, not even in the dark, but they could be very tiny or the involved Vs and As are too small.
USB A single space modules for the Euro standard wall plugs are in stock at every shop. USB C are less common. The point is that all of them are 5 V and about 1 or 2 A, some about 3A. Furthermore they are still AC to DC converters inside the wall instead that outside of it. There is not a single central converter in the house.
DC will one day be ubiquitous, but I think that day is >50 years away.
Today, DC is common in small 'islands'. At a small scale, you have DC in your phone charger and USB power supplies. At a medium scale, DC is used in high speed electric car charging. At a large scale, DC is used for undersea electricity transmission.
DC has benefits of better making use of available conductors and insulators - for a given mass of copper and plastic, more energy can be transferred from A to B at a given efficiency. Modern DC/DC converters can convert voltages more efficiently and using less metal (ie. cost) than AC transformers.
However... AC is still the standard. And changing power standards is awfully slow, because power cables in the ground can easily have a lifespan of 50+ years, and there is a chicken and egg problem involved with deploying a new standard.
> However... AC is still the standard. And changing power standards is awfully slow, because power cables in the ground can easily have a lifespan of 50+ years, and there is a chicken and egg problem involved with deploying a new standard.
The lines can theoretically stay the same, at least in the distribution network - you'd "only" need to exchange the equipment like transformers and switches.
DC has some pretty challenging aspects in implementation, and that not just on the large grid scale:
- changing voltages requires active semiconductors instead of a (relatively) dumb transformer. This has been lessened by technological advances, but it's still more expensive.
- switching DC loads on and off is harder because the voltage never crosses the zero threshold - this is also the reason why relays and circuit breakers are always rated for way lower currents in DC than in AC, and usually have lower cycle ratings as well as there will be an arc that continuously burns.
- the same is also true for ground faults, say a tree branch: the arc isn't "automatically" extinguished once the voltage drops (which happens every 1/50 second)
- it's more difficult to have an actual grid, most current implementations are point-to-point only
- DC introduces the potential for very weird "stray currents" and resulting electrochemical corrosion
- unlike with multi-phase AC, the magnetic forces in a cable that are generated by current flow don't cancel each other out, so that needs to be taken into consideration to verify if cables are suited for DC transmission
Can you (or someone else) write more about the advantages of DC/DC voltage conversion? If I understand correctly this was the main advantage of AC, and the reason why was it chosen.
In the olden days, AC allowed easy voltage conversion with a transformer. The transformer converts the electricity into magnetism in a steel core, and then back into electricity at a different voltage.
This process is quite efficient but requires a lot of steel, since in a 60 Hz AC system, 1/120th of a second of the energy being converted has to be stored as a magnetic field in steel - and steel isn't a particularly good 'store' of magnetic fields...
Modern DC/DC systems actually have similarities! But instead of operating at 60Hz, they tend to operate at more like 1,000,000 Hz. That means far less copper and steel is needed. Unfortunately, 1,000,000 Hz power has a habit of leaking out of cables and becoming radio waves, so we can't send it long distances like that - so we convert it to DC before and afterwards. The conversion to DC is done with electronic switches switched at 1 Mhz or more - usually MOSFETS are used, and one promising but expensive type is a GaN MOSFET. It turns out that the DC->AC, transformer, and AC->DC setup can also be combined and simplified a bit, and we call the result a buck/boost converter.
Overall, buck/boost converters can normally convert DC voltages for less money and at higher efficiencies than their AC transformer counterparts - mostly due to the higher operating frequency allowing use of far less steel and copper, and allowing other engineering tradeoffs be made in the direction of efficiency.
However, neither DC/DC nor transformers have any theoretical cap on efficiency - and with an unlimited budget, you could make either with an almost arbitrarily high efficiency.
Changing properties of electricity (ie current to voltage or vice versa) requires a trip through the magnetic field and that means varying the current through a conductor. With AC that's easy as it's already changing and so you can trivially (passively) convert between voltages at the same frequency with the efficiency cost of heating the conductors of the coils used in the transformer.
With DC it's harder because you don't have the time changing nature necessary for the magnetic field so you have to turn the DC on and off which requires a switch. Nowadays we have very fast switches (transistors) that allow us to tune a circuit to the power required and temporary energy storage (capacitors and inductors) available. Ignoring (or shielding) the RF interference that's created with fast switching we have systems that can efficiently convert between one DC voltage and another.
I'm not so sure we'll have DC to the home for supply, a zero-crossing is helpful to keep circuit breakers small and reduce damage in brief, accidental contact (eg broken insulation on a lamp etc).
One big advantage is that you don't have a wave you have to sync with (Which is why DC is used for connection between grids). One of the harder parts of AC grid management is that every generator on the grid has a timing component to make sure it's producing power in the same waveform as the grid. It's not enough to produce AC at 60hz, if that 60hz is misaligned then you generator turns into a load on the grid.
It also means that if 2 grids aren't in sync, they can't connect (even if they are both 60hz) without some expensive equipment to sync the waveform. A 60hz grid cannot connect with a 50hz grid without a DC phase.
There will never be a switch to DC in the home. The advantage is small, and it would involve changing everything. Would have to replace every outlet and switch, every appliance, every light socket and light bulbs. The only way that would happen is if building in new place, like on Mars.
There aren't any standards for DC in the home, no plugs, and no voltage. The voltage would be pretty high, 480V is likely, that would have to step down for USB and lights. It is telling that boats and RVs, which use 12V and 48V, just have inverters and AC plugs.
The only place where DC in home makes sense is between battery backup and solar panels. That way can have one, large, efficient inverter instead of inverters on the batteries and each panel. There isn't much difference between AC inverter and DC power converter. Although, I think will still need DC converters between solar panel, which varies, battery, and inverter/DC wiring.
Maybe. It sort of depends on how renewables shake out.
I could see DC gaining in popularity because you don't have to invest the hardware to convert renewables to AC for transmission. But at the same time, IDK, The AC grid works just fine so I have a hard time envisioning replacing the whole grid with a DC grid. Just doesn't seem like there'd be enough gains.
I do think intergrid connections will be more common. Micro/macro grids might also be more popular. Perhaps we start seeing subdivisions with their own grid/battery backups to improve reliability and allow the overall grid to disconnect them temporarily under load?
I'll take those 'islands' to include examples you mentioned.
Outside such specific cases, advantages of AC usually outweigh advantages of DC. Or: DC dis advantages outweigh those of AC. Especially safety related.
In short: AC will stay for utillity scale & in-home power distribution. Regardless of history.
But within eg. a solar farm, or a vehicle, yes DC may be more practical. And thus... used there.
That's right: we should postpone all other attempts to improve our energy infrastructure till our network of highways is destroyed or at least reduced to 2 lanes everywhere.
It was really mind blowing to me the first time I did an inventory in my house of appliances and gadgets that had to convert AC back to DC after being collected as DC and converted to AC. Not just the electricity being used but also just the wasteful resources needed for wall warts and built in transformers.
But this is about 380V DC which is not going into your living room anytime soon.
Sadly, it is less safe than AC. Automakers tried to increase DC voltage beyond 12V but it causes sparks that cause mechanical switches to fail. AC sparks are extinguished whenever the voltage wave crosses zero. Ubiquitous power DC needs high power/high voltage solid state switches to become cheaper and more reliable than mechanical ones. Perhaps SiC or GaN transistors will do eventually.
re: 12V automotive LV systems: it’s a little more nuanced than that. teslas cybertruck will be running a 48V LV system[1] and while relays fuses efuses and DCDC converters across the LV system have to be rated for higher voltages - you gain efficiency back with I^2R losses across the entire harness, and can drop your required wire gauge since the necessary current carrying capacity is reduced by 4.
So it’s a nuanced trade off, and if the industry shifts (which tesla is banking on since they’re the ‘leader’) then economies of scale can be reached with higher voltage fuses switches relays etc.
It is possible for high voltage DC to be made safe. Currently, techniques to do so are neither cheap nor off-the-shelf.
For example, imagine I want a 3000 volt DC wire to power a portable air conditioner. The air conditioner will be 10 kilowatts, so 3.3 amps. The wire can be thinner than headphone cables (two 0.3mm conductors, +-1500 volts, 150um PTFE coating) if desired.
Obviously, with such a thin insulation, the system needs to be human safe when chewed through by a baby. To ensure that, the current flow through the baby must be under 1 milliamp, or 10 milliJoules through the baby's heart. That can be ensured by tracking the current through each conductor, accurate to 1 milliamp, and shutting off the supply if there is ever more than 1 milliamp unaccounted for (either to earth, or to the other conductor). When the shutoff occurs, it must therefore happen within 1 microsecond (assuming the worst case fault, that is all three amps direct to the baby's heart). That in turn puts capacitance and therefore length limits on the cable - it wouldn't be possible for this cable to be safe longer than ~1000 feet.
TL;DR: It is very possible, with today's technology, to design very high voltage DC systems safe enough for use within a home. However, no hardware available off-the-shelf yet can do this, due to no demand.
They would have to do conversion regardless. It might be a little more efficient DC DC, but it's not like you could just have 5v lines everywhere, you might wind up using as much copper in the thicker lines as you do in the power adapters.
Especially now with USB C. I suspect in 20 years a lot of today's PD supplies will be perfectly good. It's less wasteful when they're that reusable.
The #1 purpose of such wall warts is to provide safety by means of galvanic isolation (in use, any part you can touch on wall wart or device it powers, has no direct electrical connection to HV side). That's where a transformer comes in.
When using a transformer, the voltage conversion comes 'free'. Modern electronics makes this smaller, lighter & using less metal (not more reliable, btw ;-)
On the generation side (for example rooftop solar), it isn't a big deal to have 1, powerful, high-efficiency converter.
Yes though AC is still safer and easier to transport. It's just the conversion that is wasteful. DC grids might be a good option but not without risks, especially at higher voltages.
There is a DC long distance line from the Dalles, Oregon to LA that has been around since the early 70's. Its been upgraded a few times, and now runs 3GW of power over it. its very distinctive from the other lines nearby, since it only has 2 wires:
The grounding loops are very impressive. 6 mile loop of buried cable at either end.
I might misremember, but aren't some of the submerged sea power cables (Norway, Germany, UK) DC? If so, why? And how does that rhyme with AC being more efficient?
Readit News
(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.)
The space pen story is made up as well: https://www.snopes.com/fact-check/the-write-stuff/
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.
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.)
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.
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...
They rely on the 0-point crossing to quench the arc.
https://www.youtube.com/watch?v=GMbN9nb3qyk
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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.
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A DC plug isn't hard, considering this already assumes the user will pull the arc by ripping the plug of an e.g. flaming appliance.
USB A single space modules for the Euro standard wall plugs are in stock at every shop. USB C are less common. The point is that all of them are 5 V and about 1 or 2 A, some about 3A. Furthermore they are still AC to DC converters inside the wall instead that outside of it. There is not a single central converter in the house.
See some examples of those modules at https://www.amazon.it/bticino-living-usb/s?k=bticino+living+...
Today, DC is common in small 'islands'. At a small scale, you have DC in your phone charger and USB power supplies. At a medium scale, DC is used in high speed electric car charging. At a large scale, DC is used for undersea electricity transmission.
DC has benefits of better making use of available conductors and insulators - for a given mass of copper and plastic, more energy can be transferred from A to B at a given efficiency. Modern DC/DC converters can convert voltages more efficiently and using less metal (ie. cost) than AC transformers.
However... AC is still the standard. And changing power standards is awfully slow, because power cables in the ground can easily have a lifespan of 50+ years, and there is a chicken and egg problem involved with deploying a new standard.
The lines can theoretically stay the same, at least in the distribution network - you'd "only" need to exchange the equipment like transformers and switches.
DC has some pretty challenging aspects in implementation, and that not just on the large grid scale:
- changing voltages requires active semiconductors instead of a (relatively) dumb transformer. This has been lessened by technological advances, but it's still more expensive.
- switching DC loads on and off is harder because the voltage never crosses the zero threshold - this is also the reason why relays and circuit breakers are always rated for way lower currents in DC than in AC, and usually have lower cycle ratings as well as there will be an arc that continuously burns.
- the same is also true for ground faults, say a tree branch: the arc isn't "automatically" extinguished once the voltage drops (which happens every 1/50 second)
- it's more difficult to have an actual grid, most current implementations are point-to-point only
- DC introduces the potential for very weird "stray currents" and resulting electrochemical corrosion
- unlike with multi-phase AC, the magnetic forces in a cable that are generated by current flow don't cancel each other out, so that needs to be taken into consideration to verify if cables are suited for DC transmission
This process is quite efficient but requires a lot of steel, since in a 60 Hz AC system, 1/120th of a second of the energy being converted has to be stored as a magnetic field in steel - and steel isn't a particularly good 'store' of magnetic fields...
Modern DC/DC systems actually have similarities! But instead of operating at 60Hz, they tend to operate at more like 1,000,000 Hz. That means far less copper and steel is needed. Unfortunately, 1,000,000 Hz power has a habit of leaking out of cables and becoming radio waves, so we can't send it long distances like that - so we convert it to DC before and afterwards. The conversion to DC is done with electronic switches switched at 1 Mhz or more - usually MOSFETS are used, and one promising but expensive type is a GaN MOSFET. It turns out that the DC->AC, transformer, and AC->DC setup can also be combined and simplified a bit, and we call the result a buck/boost converter.
Overall, buck/boost converters can normally convert DC voltages for less money and at higher efficiencies than their AC transformer counterparts - mostly due to the higher operating frequency allowing use of far less steel and copper, and allowing other engineering tradeoffs be made in the direction of efficiency.
However, neither DC/DC nor transformers have any theoretical cap on efficiency - and with an unlimited budget, you could make either with an almost arbitrarily high efficiency.
With DC it's harder because you don't have the time changing nature necessary for the magnetic field so you have to turn the DC on and off which requires a switch. Nowadays we have very fast switches (transistors) that allow us to tune a circuit to the power required and temporary energy storage (capacitors and inductors) available. Ignoring (or shielding) the RF interference that's created with fast switching we have systems that can efficiently convert between one DC voltage and another.
I'm not so sure we'll have DC to the home for supply, a zero-crossing is helpful to keep circuit breakers small and reduce damage in brief, accidental contact (eg broken insulation on a lamp etc).
- Switching AC is much more easy than DC.
- The grid we have today was not made for small small cellphone chargers. It was made for light and motors.
It also means that if 2 grids aren't in sync, they can't connect (even if they are both 60hz) without some expensive equipment to sync the waveform. A 60hz grid cannot connect with a 50hz grid without a DC phase.
There aren't any standards for DC in the home, no plugs, and no voltage. The voltage would be pretty high, 480V is likely, that would have to step down for USB and lights. It is telling that boats and RVs, which use 12V and 48V, just have inverters and AC plugs.
The only place where DC in home makes sense is between battery backup and solar panels. That way can have one, large, efficient inverter instead of inverters on the batteries and each panel. There isn't much difference between AC inverter and DC power converter. Although, I think will still need DC converters between solar panel, which varies, battery, and inverter/DC wiring.
That means they can run off USB C even though it's only speced to 180 W.
Converting an electric geyser to run from USB C should even be practical: Especially if it's a bachelor who typically only use it for showering.
https://en.m.wikipedia.org/wiki/USB_hardware#USB_Power_Deliv...
I could see DC gaining in popularity because you don't have to invest the hardware to convert renewables to AC for transmission. But at the same time, IDK, The AC grid works just fine so I have a hard time envisioning replacing the whole grid with a DC grid. Just doesn't seem like there'd be enough gains.
I do think intergrid connections will be more common. Micro/macro grids might also be more popular. Perhaps we start seeing subdivisions with their own grid/battery backups to improve reliability and allow the overall grid to disconnect them temporarily under load?
I'll take those 'islands' to include examples you mentioned.
Outside such specific cases, advantages of AC usually outweigh advantages of DC. Or: DC dis advantages outweigh those of AC. Especially safety related.
In short: AC will stay for utillity scale & in-home power distribution. Regardless of history.
But within eg. a solar farm, or a vehicle, yes DC may be more practical. And thus... used there.
Sadly, it is less safe than AC. Automakers tried to increase DC voltage beyond 12V but it causes sparks that cause mechanical switches to fail. AC sparks are extinguished whenever the voltage wave crosses zero. Ubiquitous power DC needs high power/high voltage solid state switches to become cheaper and more reliable than mechanical ones. Perhaps SiC or GaN transistors will do eventually.
So it’s a nuanced trade off, and if the industry shifts (which tesla is banking on since they’re the ‘leader’) then economies of scale can be reached with higher voltage fuses switches relays etc.
[1] - https://auto.hindustantimes.com/auto/electric-vehicles/tesla...
For example, imagine I want a 3000 volt DC wire to power a portable air conditioner. The air conditioner will be 10 kilowatts, so 3.3 amps. The wire can be thinner than headphone cables (two 0.3mm conductors, +-1500 volts, 150um PTFE coating) if desired.
Obviously, with such a thin insulation, the system needs to be human safe when chewed through by a baby. To ensure that, the current flow through the baby must be under 1 milliamp, or 10 milliJoules through the baby's heart. That can be ensured by tracking the current through each conductor, accurate to 1 milliamp, and shutting off the supply if there is ever more than 1 milliamp unaccounted for (either to earth, or to the other conductor). When the shutoff occurs, it must therefore happen within 1 microsecond (assuming the worst case fault, that is all three amps direct to the baby's heart). That in turn puts capacitance and therefore length limits on the cable - it wouldn't be possible for this cable to be safe longer than ~1000 feet.
TL;DR: It is very possible, with today's technology, to design very high voltage DC systems safe enough for use within a home. However, no hardware available off-the-shelf yet can do this, due to no demand.
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Especially now with USB C. I suspect in 20 years a lot of today's PD supplies will be perfectly good. It's less wasteful when they're that reusable.
When using a transformer, the voltage conversion comes 'free'. Modern electronics makes this smaller, lighter & using less metal (not more reliable, btw ;-)
On the generation side (for example rooftop solar), it isn't a big deal to have 1, powerful, high-efficiency converter.
The grounding loops are very impressive. 6 mile loop of buried cable at either end.
https://en.wikipedia.org/wiki/Pacific_DC_Intertie
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(I'm not affiliated, just know of the project due to my work in DC grids in offices)