I'm going to ask an obvious question. Assume you switch your system to be all DC fed. Are you SURE you need a GFCI? 110v AC will kill you, and if not, sure as heck hurts to get zapped by it. (I'm ignoring the chiller&heater challenge)
I thought the risk with home 110v AC circuits is Black wire is hot, and current is supposed to be returned via White/Neutral wire... Which is also connected to earth ground. My understanding of risk is Hot being in contact with water, and you or I reaching into the tank, and having our body act as the return path going back to earth ground - instead of going through the white/neutral wire. I don't think this happens with DC, right? No earth ground.
A 65W DC pump needs, ~3A at 24v, ~2A at 36v. (most DC pumps I'm aware of are fed by 24v or 36v)
So the topic here is really about Amperage being fed out by each DC circuit, in the event of a short in the tank water, right? Isn't this the primary risk to your health?
(Apply all safety caveats here - I'm asking questions, from the perspective of not being an expert.)
I'm by no means an expert, but this is my understanding.
On an AC system a GFCI does two things. One, watches for an imbalance of current between the hot and neutral, two, looks for unexpected current on the neutral.
The first is indeed obvious, 100mA going out on the hot leg should translate to 100mA returned on the neutral. Unless the current has found an alternative path to ground.
The second isn't so obvious, because it's only going to happen if wiring elsewhere is screwy. You can run into situations where two different breakers are sharing one neutral. There may not be any current flowing between the outlet you're working on and neutral, but if that neutral is shared with a second breaker, that neutral is effectively a continuation of the second breakers hot line. If there's an active load on that second breaker, and you touch that shared neutral, you become an alternative path to ground for that second breaker. That's why breakers that share a neutral are supposed to be tied together. Ideally, breakers shouldn't be sharing a neutral.
There are two applications of DC systems. Floating, which is your battery powered systems that are self contained, and there's your non floating systems which are your systems that use rectification to get DC from AC and transformers to step the voltage down. With a non floating system, that DC current is ultimately looking for a path back to ground, since it's being derived from a grounded AC system to begin with. Looking at the block diagram in my particular power supplies datasheet, the positive and negative rails are tied to ground through capacitors.
With a non floating DC system, which is what my system will be during normal operation, if I'm in the water with a faulty piece of equipment, and I bump into for instance a lights grounded metal enclosure, I'm going to complete the circuit.
As I think through this, I'm starting to doubt whether ground fault protection will be of any use when the system fails over to battery and becomes a floating system. I'll be monitoring the AC side of the power supplies, and when they lose power, relays will disconnect the power supplies from the circuit, and switch over to batteries. Effectively removing the ground reference from the system
I believe I would need a faulty positive in one tank, and a faulty negative in a separate tank, on two pieces of equipment on the same power supply to complete a floating circuit in a way individual conductor current monitoring would catch. Though I suspect since I can't reach any two tanks that aren't sharing plumbing at the same time it's unlikely to be an issue.
I'm not too concerned about fire hazards. My NDR power supplies have overload protection and I'm putting fuses rated for 1.5x each individual piece of equipments normal draw on their respective circuits. My supply to bus feeds are 12awg, and individual circuits are 16awg carrying max maybe 2A 20 feet.
Though I might rethink some of this when I'm able to move equipment not in the water over to DC.
