Comment by causesothere | Hacker News Reader

Comment by causesothere | Hacker News Reader

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Yes, there are serious safety concerns. Both islanding, which is where a circuit is hot when it shouldn't be, and from a circuit overload.

Your circuit breaker protects your circuits by putting all the load (eg your microwave) on one side, and your supply (mains power) on the other. If too much load is present, the circuit breaker trips.

However, plugin solar puts invisible supply on the load side, which your circuit breaker cannot see. Let's make an absurd example so you can see what the problem is, then scale it back.

Imagine someone buys 5 panels that can each supply 5amps. They plug them into outlets on a 40 amp circuit. They then proceed to run their toaster, microwave, vacuum, and hair dryer. They can draw 65 amps through the wires before the circuit breaker trips. (40 from mains, 25 from the panels)

Let's say they had a good electrician who installed 6AWG wiring on that circuit, which is rated for 50 amps. The circuit breaker used to trip at 40, so no problem. But now up to 65 can go across the wires and suddenly you are exceeding rating for wires.

Now you might say, "most of these are 3.5 watts, and they clearly say don't use more than one". But I say that's a "good intentions solution". We should make it mechanically impossible to do the wrong thing. Maybe these panels could try to communicate over the electrical wires, and if they detect a signal from another panel they should self limit. Maybe we need to rethink circuit panel design. But as it stands people will absolutely buy one of these, decide they love it, and then buy 4 more.

Plug in solar inverters turn off when they detect a loss of grid power. Anti-islanding is a requirement for them to be certified.

Ok, for those models, strike concern number one.

anti-islanding is required for all models of plug-in solar

This moves the problem though.

In the original example, RCD/circuit protection is regulated on the _property_ side, e.g UK has EICR for electrical install. Once its done for a property, its good until next inspection (in theory)

This moves the protection to the _appliance_ , which while it needs to meet certification, is much harder to police a consumer electronic device, than a physical unmovable premise.

I think these are great, but there will inevitably be devices that don't implement the spec correctly that still get sold to willful or unwillful consumers that result in danger to everyone.

> Imagine someone buys 5 panels that can each supply 5amps. They plug them into outlets on a 40 amp circuit. They then proceed to run their toaster, microwave, vacuum, and hair dryer. They can draw 65 amps through the wires before the circuit breaker trips. (40 from mains, 25 from the panels)

Circuit breakers trip bidirectionally, so the circuit breaker itself would protect if too much current is drawn through to the mains panel.

How, precisely, would you get 65A through the wiring on this 40A circuit, if I drew 40A of consumption and had 25A of supply?

The 25A cancels out the 40A, so the upstream feed would only see 15A total. You do get 65A of total current drawn through the breaker panel, which is a concern, but the supply wiring would not see 65A. Subpanels (which, this would be a subpanel if it only had 40A capacity) are typically going to be just fine.

Are you in the UK, perchance?

In the UK you typically make a ring circuit. In the US you make branching circuits.

In the US you get: Mains -> Breaker -> Outlet 1 -> Outlet 2 -> Outlet 3.

In the UK you get: Mains -> Breaker -> all outlets -> Breaker

In the US, assume I had 65A of load on outlet 3 -- I've plugged in too many things into a surge protector. Assume a bunch of solar panels are on outlet 1.

The the diagram looks like: Mains -40A-> Breaker -40A-> Outlet 1 -65A-> Outlet 2 -65A-> Outlet 3

Now, the breaker won't trip because only 40A are crossing the breaker. But 65A are on the wire between outlet 1 and 2 and between 2 and 3.

No, I’m in the US. What you’re describing does not exist in that capacity in the US. No multi outlet branch circuit exists that is backed by a 40A breaker. At most you’re going to see 20 amps for a regular outlet.

Either way, your scenario of a multi outlet branch circuit on a 20A outlet would also not be correct. Let’s just make it simple: 20 amps of solar gen and 20 amps of power gen, connected as follows:

Panel -> Breaker -> Solar outlet -> hairdryer outlet

The current would flow from the solar outlet to the hairdryer outlet fully, and the measured current through the breaker would be 0. All wires would be at 20 amps and therefore safe. (Lets ignore that 20 amp wire is only rated for that temporarily… continuous draw would be 16a…) No violation of any current limits and no danger.

Now, you could go crazy and connect 30 amps of solar and 30 amps of hairdryers and that would be dangerous, but we’re rapidly getting into the realm of incredulity. As soon as you removed the load/hairdryers you would pop the breaker.

Ok, but if you put 15 amps of solar on your hypothetical circuit, which is 3-5 panels, you would not trip the 20A breaker.

Then someone puts two space heaters at the end of their branch circuit, they could easily exceed 20A without tripping the breaker and without straining incredulity.

And you are right, 40A was a silly choice, I figured bigger numbers made it more obvious the problem. But it's not hard to construct a likely example on a 20A circuit either.

https://news.ycombinator.com/item?id=50070844 : UKGOV has gone for the warning notice solution.

I think this is probably fine, since UK circuits are typically 20A. Apart from fault current (which would have to be line to line, since line to ground would trip the RCD), it's quite hard to end up with more than 20A of stuff plugged into regular sockets unless you're overinvested in portable heaters. All the big stuff - cookers, heat pumps, showers - is on separate breakers from the sockets.

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