2 pole vs single pole breakers.

Craig

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So when I purchased my SCC the instructions recomended 2 pole breakers with the positive in one pole and the negative in the other. I see most people here using single pole breakers. What are the pros and cons off either type.
 
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HighTechLab

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Usually it varies based on the way that the solar system is configured.

If your solar battery bank is grounded, then you do not need to have a double pole breaker on the solar array (or the battery output). Just single pole breakers and fuses are fine.

Why? When you negative ground your battery bank, there are no, or almost no, solar charge controllers that have isolation from solar array to the battery bank. So, negative grounding, solar panels are not floating, but ground referenced.

When you ground reference a power system (DC power, North American Ground bonded white/neutral conductor), the ground referenced conductor cannot get above "zero volts" with respect to frame/safety/green wire ground. So, there is no reason to put a fuse/breaker on the ground referenced conductors.

The reason, for example, we use double pole breakers on 240 VAC split phase North American circuits, is that both L1 and L2 (Black and Red) conductors are 120 VAC with respect to ground (and 240 VAC from L1 to L2). The pole transformer is a center tap configuration (L1-Neutral center tap-L3) connection. And the Neutral is bonded (typically in the main panel of the home), so only L1 and/or L2 need circuit breakers.

Some systems use this bipolar configuration where the center point is grounded and you end up with - and + voltage with reference to ground exactly like a 120/240v split phase system. These REQUIRE a 2 pole breaker.

For "floating power system". In theory, they are very safe. If you short one of the output conductors (whether 12 VDC or 120 VAC), since there is no connection to the frame/earth ground elsewhere, there is no electrical return path for you to get shocked (unless you touch other output wires).

The reason you are supposed to (and few people do) have to put a breaker on both +/- (if DC) output for a floating supply is if you get two short circuits to ground. Say your first ground is a 20 AWG gave wire in an LED ceiling lamp in an RV where the negative wire gets cut and shorted to frame ground on the RV.

Now, you get a second short circuit from Battery positive bus (and you have a 100 Amp breaker/fuse only on the positive bus and no breaker/fuse on the "return or negative wiring anywhere). The short circuit path goes from Battery + (100 amp breaker) to the return/negative 20 AWG wire in the LED fixture, and follows the 20 AWG wire back to the negative battery bus and back to the battery.

Your 20 AWG wire is now your "fuse" in the circuit (and a very real threat of fire). If you had a floating system with double pole fuses or breakers, then you would have one on each wire that leaves the battery + and - buses. The overcurrent protection device would be rated to protect the "down stream" wiring from over rated current (say a 2 amp fuse on 20 AWG +/- wire for your LED fixture).

That is why two fuses/breakers on "floating power systems" and only one breaker/fuse is required for ground referenced systems.

For large ships, they have "isolated" AC power systems (floating) so that the hull does not become a conductor during a short circuit (safety). A simple method (probably not used anymore, very old) is to connect a small 120 VAC lamp from L1 to ground and a second lamp from L2 to ground. If the AC circuit is OK, each lamp glows at 1/2 voltage. If L1 or L2 become shorted to ground, the shorted leg "lamp" goes out, the the second leg lamp goes bright.

So, for small portable floating power supplies (small solar panels and battery), generally, people, at best, have a single fuse/breaker on the + output. There is no way for a single short circuit to create a path where the unground return wire can get more current than it is designed for (i.e., 15 amp fuse, 14 AWG wire + out to load and - return back to battery).

For larger floating power systems, where you may have large conductors (50+ amps going to a solar charge controller, 150 amps to an AC inverter, 2 amps to an LED lamp), you should have properly rated double pole breakers/fuses for each pair of cables that leave the battery bus.

There are other issues why and how grounding is done in AC and DC power systems (lightning, static discharge, reducing the possibility of electrocution, cathodic protection systems for buried pipe/boats/telecom/natural gas piping/etc., fluorescent lighting, spark ignition for propane/natural gas burners, etc.)...

And there are reasons to (possibly) put fuse/breaker per solar panel on your RV (in positive lead only required for DC ground bonded systems)--Typically with 3 or more panels in series (usually 100+ Watt per panel size) that help prevent solar wiring overheating if one panel gets shorted and the other 2+ panels feed excessive current to the shorted panel). But this may not even be an issue for a small solar power system.
 
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Wait, is this the beginner section? I almost fell over from the gust of wind as that passed over my head :ROFLMAO::ROFLMAO:. Great info, though! I have my google assignments for the evening.(y)(y)
 

Dzl

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Wait, is this the beginner section? I almost fell over from the gust of wind as that passed over my head :ROFLMAO::ROFLMAO:. Great info, though! I have my google assignments for the evening.(y)(y)

I had the same reaction! Valuable, info-dense response, but so much info, so far over my head it's making my head spin. If I can find and read a few introductory resources on the subject of floating vs negative grounded systems, or isolated vs non-isolated systems (are these two terms for the same concept), I imagine I can circle back to this thread and learn a lot!
 

kernel

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Ungrounded pv systems require overcurrent protection on the positive and negative pv source conductors and like the texbook above lays out!

If you check your manual and the safety labeling you will see reference to ground fault and ungrounded pv input.
 

Dzl

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Ungrounded pv systems require overcurrent protection on the positive and negative pv source conductors

And does 'ungrounded pv system' mean that the panels themselves are not grounded, or that the entire system is floating or 'isolated' from earth ground / chassis ground?
 

evad

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Another reason that the neutral conductor is not fused on a normal 120 volt circuit is if that is the fuse or breaker that opens the circuit is dead in that it will not function but the 120 volt line is still hot a is a danger to a person checking a device which does not function.
 

kernel

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And does 'ungrounded pv system' mean that the panels themselves are not grounded, or that the entire system is floating or 'isolated' from earth ground / chassis ground?
Negative and positive pv inputs are isolated. There is likely no transformer. Switching power supply.
 
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