I inquired once of UL what it would take to have the Hydra UL approved. Their reply was that the "initial engineering consultation," which likely means I send them one and they destroy it and send back a list of what's wrong with it, would cost US$10,000. So a full UL approval, by my estimation, would be Tesla money.

So it's not happening.

But friendly birdies have supplied me with a few insights into the process for an EVSE.

One fall-out from that is that the current design won't do.

The GCM test is fine, but surprisingly enough, the relay test is the problem. UL is concerned about situations where one pole of the relay/contactor gets welded shut and the other stays open. The current design doesn't catch that.

So the design is going to morph back to something similar to the very first design I had a while ago, which yielded a combined GCM and relay test.

Two diodes from each hot line have a common cathode. That cathode runs to the current GCM test circuit unchanged. Repeat that for each car. In the firmware, if the car is shut off, then the relay test line must be low or it's a stuck relay failure. If the car is turned on, then the relay test line must be high or it's a GCM failure.

What concerns me slightly is that compound failures would not be detected. That is, if you have a stuck relay *and* no ground connection, then that's not a failure until you attempt to start charging. Meanwhile, you have an energized J1772 plug and the user is not alerted. To be fair, though, with the current design, the behavior is the same if only one pole is welded.

One other problem has surfaced as well - one person who's built their own Hydra has reported GFCI problems. It's occurred to me that the GFI is *before* the GCM, which results in the GCM current adding sensitivity to the GFI. Now, it's only 1 mA of current - not even enough to set off a household GFI outlet - but it's something I didn't think needed to be taken into account. The difficulty is that the GFI coil is physically so small that trying to cram both cars' hot lines through wouldn't be tenable. The only alternative to that is doubling the GFI circuit so that each car is independent. I can only hope that won't be necessary. It should be noted, however, that he's only having this problem with a BMW i3. His other car - a LEAF - is fine. So it's just a border condition, not something pathological.

So there will need to be another revision of the HV board with the new combined relay/GCM test. The de-sensing of the GFI merely requires swapping out a resistor on the board, so that's no big deal.