Hacker News Reader

Comment by aetherspawn | Hacker News Reader

> "A new battery module isolation loss alarm has been added."

This is the worst fault possible in an EV 101 scenario right here (touch metal -> instant death), so I'm surprised that with all Tesla's experience, they didn't consider an isolation leak as a serious fault. And I'm surprised that they allow the battery monitoring system to be disabled during servicing, especially the IMD (insulation monitoring device).

View on HN

I am constantly surprised how many systems have no isolation when it would be easy and cheap to add.

For example, typical solar inverter systems don't have isolation between the AC and DC sides. That means any chafed wire can be deadly. They do at least have leakage detection, but set at a 300mA level, which probably won't save your life.

Considering solar inverters already uses high frequency switching inside the per-string MPPT boost converter, it would have cost mere cents to put two windings on that inductor rather than one and get isolation with no downside.

The cynic in me says manufacturers don't add safety features not required by law, even if zero cost, because in the future laws might be updated to require that safety feature, preventing reuse of hardware already sold, which is good for business.

> For example, typical solar inverter systems don't have isolation between the AC and DC sides. That means any chafed wire can be deadly. They do at least have leakage detection, but set at a 300mA level, which probably won't save your life.

This is very yikes. I just installed such a system.

Do you know any way to test if that's a problem? And is it possible to fix without opening the inverter?

It's normally set that high by design because the solar panels have substantial capacitance to ground (they are large flat conductors sat just a few cm off the ground after all). The capacitance to ground wouldn't be an issue if not for the fact the inverter is non-isolated. That means current can flow to/from the grid to charge and discharge that capacitor, which is indistinguishable from AC leakage.

Some inverters will allow you to lower the leakage threshold (usually with settings in a service menu), but then you're limited to just attaching 3 or so panels or getting false isolation trips.

In my case the panels are, in fact, mounted on wooden brackets a good 1.5 meters off the ground. On the other hand, there's 20 m^2 of them.

Huh? 300 mA should barely give you a tingle unless you're talking about some very high voltages. What am I missing here?

A basic knowledge of electricity. 100-200mA will kill you.

https://academic.oup.com/ptj/article-abstract/46/9/968/46379...

I have heard people say "that can't be right, because a car battery can source 100+ amps and touching those terminals doesn't kill me!" The reason is because you have a high enough impedance that 100mA doesn't flow from a 12V source through your body.

"It's not the voltage that kills you, it's the amps!" Wouldn't power (W) (or joules) be a better way to phrase it? Because as you said, you can't have 100mA flowing through your heart unless you ALSO have stiff enough source to actually supply that current - which implies a high compliance voltage, can't escape ohms law.

No, it's not the power, either, as low resistant paths can be extremely dangerous (such as wet skin). Really, it's a 2 dimensional space that makes for a less catchy aphorism. "It's not the voltage that kills you, it's the combination of voltage and conductivity".

Honestly I kinda hate that saying. It's like "it's not the fall that kills you, it's the sudden decelleration on impact." For most situations people encounter, it's high voltage sources with more than enough power which are dangerous.

"The combination of voltage and conductivity" is ... current, no?

GP mentioned impedance, not resistance. You won't feel anything touching the 12/24V terminals in your car because they're DC. Your skin has a high resistance, but when faced with AC, has a low impedance. This makes AC much more dangerous, as you conduct 120V AC much better than 120V DC.

Even 50mA will likely kill you, as outlined on the "death graph" as we electricians call it. This is independent of voltage.

https://upload.wikimedia.org/wikipedia/commons/7/7f/IEC_TS_6...

Blue is harmless and imperceptible, green is harmless, yellow is harmful, red is fibrillation.

Edit: This illustrates why 30mA (or less) RCD protection is common (and required) in various jurisdictions, depending on the kind of circuits you're serving.

Right, and in particular, "Type B" RCD protection is required for solar inverters in various jurisdictions. These provide protection against DC fault currents.

> This makes AC much more dangerous, as you conduct 120V AC much better than 120V DC.

But at higher frequencies, you should experience the skin effect, right?

I can't answer that question, as I'm only an electrician, not an electrical or RF engineer.

That said, we're referring to power delivery here, and the highest frequency I'm aware of in this space is the 400 Hz power distribution systems used on warships.

"It's the volts that jolt, but the mills (mA) that kills" - The phrase a schoolteacher used when I was a lad.

V = I * R

The voltage across the heart will be rather bounded if you fix the current at 100mA, though. Hearts will not differ significantly from each other.

Not really, whether it takes 50V with wet skin or 500V with dry skin, it's the current that kills you, not the watts

Here is medhi sadaghdar (electro boom) putting 11v at 150A through his tongue https://www.youtube.com/watch?v=XDf2nhfxVzg

Embarrassingly, I have managed to shock myself several times during my life on 240V mains. Once quite recently while working on a light fitting at a friend's flat and not realising that someone had flipped the circuit back on at the circuit breaker!

That shock was certainly quite unpleasant, but despite touching my sweaty skin against live wires, it seems to have been far from fatal.

Surely the dangerous situations are live-to-earth shocks, which could potentially run across your whole body (rather than just a hand or finger, as in my case). But that's why we have RCD/GFCI protection, right? Those will trip with tiny fault currents, certainly much less than 300 mA.

Secondly, aren't "Type B" RCDs required on solar inverters? Those will detect and trip on DC fault currents as low as 30 mA, not 300!

you need "Lock Out Tag Out" lol

https://en.wikipedia.org/wiki/Lockout–tagout

> Embarrassingly, I have managed to shock myself several times during my life on 240V mains. Once quite recently while working on a light fitting at a friend's flat and not realising that someone had flipped the circuit back on at the circuit breaker!

Next time, I suggest using a breaker lockout [0] in conjunction with the local disconnecting means (light switch, which you can also lock out! [1]) to do it safely :)

[0] https://www.walmart.com/ip/MASTER-LOCK-493B-Circuit-Breaker-...

[1] https://www.homedepot.com/p/Ideal-Wall-Switch-Lockout-44-789...

> "light switch, which you can also lock out!"

No you can't! Ceiling light fittings (in the UK, at least) are often always live regardless of the switch setting. This is because they are typically wired in a "loop" arrangement with separate loop and switch cables.

Example: https://www.practicaldiy.com/electrics/lighting-wiring/light...

Well, the rest of the world outside of the UK, Ireland, and Hong Kong uses a safer method of electrical circuiting, one that wasn’t invented to deal with post-war material shortages.

> A domestic power supply voltage (110 or 230 V), 50 or 60 Hz alternating current (AC) through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as 30 milliamperes (mA).With direct current (DC), 300 to 500 mA is required.

https://en.m.wikipedia.org/wiki/Electrical_injury

So you'd have to have a wire pressed against your chest with your back on the ground... Unlikely but yes possible hazard when working on solar panels

Think about the path the electricity would have to follow between, say, an arm and a leg, or even both arms.

These, and the GFCI outlets/breakers are calibrated against injury for adults. The thresholds are lower for children.

Mike Holt is an excellent instructor on the NEC and has quite a few videos that speak about this topic.

https://m.youtube.com/user/MikeHoltNEC

Aren't you are mistaking galvanic isolation with insulation?

Galvanic isolation is useful because it means that you have to touch 2 specific points on the affected unit simultaneously to die, rather than 1 point. Galvanically isolated systems can detect leaks on both the positive and negative sides by using a dual channel IMD. Any insulation issue is bad, but this method of detection can usually detect it before it's able to instantly kill someone per-se. Electric vehicles are an example of a galvanically isolated system (from earth) with dual channel IMD, hence you can have massive leaks such as 8-20mA before it becomes a safety issue.

Yes..., but what difference it makes to this fire?

Are you working in EE?

Galvanic isolation probably wouldn't have prevented this fire, it sounds like the cells just shorted together. They would have had to short over several cells, since a non-trivial voltage is required to break down coolant, so having maintenance contactors to break the pack up into 20V or so segments may have prevented this fire.

Using a mineral oil type coolant that is not electrically conductive (such as used in Tesla vehicles) would have also helped to prevent this fire.

The batteries should be galvanically isolated from the casing so that if liquid causes a current flow from any point in the pack, then the maintenance contactors can be opened and the opposite side current path for the leak is interrupted. We can only assume that they are already (galvanically isolated) since anything else would be bizarre.

I spent a bit working on various power supplies and sadly isolation is non-trivial nor cheap. Adding isolation (transformer vs inductor) requires changing control signals, control mechanism, size. It costs far more then mere cents, and loses some efficiency.

Reply on news.ycombinator.com