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Comment by loeg | Hacker News Reader

> as it'll have to be able to break the fault if it happens

Are you talking about the case where both of the ground wires fail while charging? If either is non-functional when charging starts, it disallows charging. If one fails during charging, charging disconnects.

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Imagine a catastrophic fault: a dude in another car drives into your car, and a conductor in the car breaks off and energizes the car body. The current now goes through the car body into the ground, and also through the grounding wire back to the charger (EVSE). It's a phase-to-ground short.

The charger tries to interrupt the flow, but the short circuit fries the power electrics in the buck converter (MOSFET/IGBTs can fail _short_). The charger will have contactors that physically disconnect the line from the converter, but they normally interrupt the line with zero voltage when the controlling IGBTs are closed.

In this case, they'll have to interrupt high voltage and high current flow. Can they do it reliably within the fractions of a second?

Thanks!

Imagine a catastrophic fault: a dude in another car drives into your car at the petrol pump. The petrol pump starts dropping huge amounts of petrol on the ground. Part of the car wreck falls back on the ground and ignites a fire, causing the whole station to blow up, killing everyone within 100 meters. Clearly, we cannot allow petrol stations to exist.

Modern petrol pumps have break-away detection, it can recognize when the nozzle/hose is broken along the chain - I _think_ it works via the nozzle being lower gauge, which means when the flow rate suddenly increases there must be a high volume leak in the hose. And even then, it's only a few liters a minute. That's enough for a nasty fire, yes, but you can usually extinguish it with a handheld extinguisher.

Cars, at least modern cars, are also built with safety systems in mind. The fuel line from inlet to tank isn't open as it used to be, nowadays there's a spring actuated flap that is pushed aside by the nozzle (as well as vapor suction systems in the nozzle) so vapors cannot escape and form an explosive atmosphere.

On top of _that_, all parts from nozzle over hose to pump to tanks have safety features to prevent a catastrophic explosion: flame arrestors and inertization, and buried tanks.

So, we didn't ban petrol stations, but in the interest of safety and environmental protections (reduction/near elimination of VOC escape) we mandated changes in how cars and petrol stations are made so that even the dumbest possible user (aka, someone smoking a cigarette right next to his Ferrari getting refilled with 102 octane fuel) will more likely than not either not explode at all or the explosion will be relatively harmless in scope.

There is really no difference between the two cases, because any EV chargers will also have multiple levels of over-current, over-voltage and short-circuit protection, i.e. both at each charger and for the entire charging station.

So not even an exploding car would be able to prevent the disconnection of the chargers by some of the protection circuits.

The issue here is that interrupting a 7.2kV 50A current is not easy. If an arc ignites, it can keep the current flowing for quite a while. Long enough to kill anybody unlucky enough to be a part of the ground fault path.

Modern petrol stations are designed with this very scenario in mind. The above-ground structure can burn down, but the underground gas tank will still be fine.

And more to the point, right now with galvanic isolation this scenario fails safe. The current won't go through the ground. In fact, you can peel away the insulation from the charger cable and touch one conductor while a fast DC charging session is in progress. You will be fine, as long as you don't touch the second conductor at the same time.

When a fault occurs (say a battery fluid leak to the chassis) and is detected, a mechanical relay has to open to disconnect the power from the grid. That can take 200 ms or so, which is a long time to have 400v flowing through you.

One can have in series a very fast solid-state relay (which would disconnect in some tens of microseconds at most; e.g. with IGBTs) and a mechanical relay for permanent disconnection.

This would increase the cost, but by many times less than the insulated power converters used today.

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