Help re: ground fault protectors and circuit breakers

vince

I have a mobile solar system on a trailer. I'm wondering what needs to be protected with circuit breakers, with ground connections and is a ground fault connector needed? I have three parallel panels protected with in series 20A breakers in a combiner box. Do the panels require grounding or can they float. What about the other components of the system? Do I still need a circuit breaker for the load circuit if I down convert from 48V to 24V or 12V and the converters have a build in protection against overcharging. Do I hook the breakers up only to the positive side or also one in the negative side if the system floats. Some general guidelines about how to protect the systems would be helpful as it seems that so much can go wrong and cause fires.

BB.

Re: Help re: ground fault protectors and circuit breakers

A very complex set of questions...

Are you supplying DC power to devices inside/mounted to the trailer (say radio gear) and/or supplying +/- cables to external gear?

Are you supplying AC power (120/240 VAC 60 Hz)?

In general, the first protection is based around your Battery Bank... Any reasonably sized lead acid storage battery bank can output 100's to 1,000's of amperes into a dead short.

The breakers/fuses are there to protect the wiring from overheating and failing/catching fire. So, generally if you have 1 or 2 strings of batteries, they would go to a common battery +/- set of bus bars/connection points. If you have three or more battery string in parallel, then you should think about a fuse/breaker per battery string.

Normally, we would attach a ground wire from the battery negative terminal to the metal chassis of the trailer. This allows us to put a fuse/breaker on the + leads only... The negative leads do not need over current protective devices because they are always very close to zero volts with respect to the chassis. And any short between + and - or trailer chassis, causes the breaker to trip.

However, there is the option of "floating" your power system. You do not bond negative ground to the chassis of the trailer. However, when this is done, usually you need "double pole" breakers to protect against shorts in both + and - leads. It is a good method for safety (you can touch + or - to ground and there is no current flow--if everything is floating correctly)--But it is not commonly done in the US (need 2x the number of breakers/fuses, need to check for proper floating of power system, etc.).

In general, the DC battery bank is the source of all high current in the DC side of the system, so every wire + wire leaving the positive battery bus needs its own breaker/fuse rated for wire size (i.e., a 14 AWG wire would need around a 15 amp breaker and would be expected to not carry more than 80% of rated current in normal operation or 15a*0.8=12 amps continuous current).

The solar panel combiner box is a special case because if you put 3 or more panels (or strings of panels) in parallel, and short one panel, the others can exceed Isc rating of the one shorted panel.

Note that 12 volt systems are considered relatively safe to touch. A 24 volt system can "bite you" if you have damp/salty skin. And anything over ~60 volts is considered to hazardous to touch directly without risking electrocution (general guidelines). Note that a 48 volt lead acid battery can charge at 60 VDC.

Rather than go on for, too long, on the general theory behind DC and AC system safety--Perhaps you can tell us a bit more about your exact application.

It will probably be much easier to discuss your needs/solutions directly. And you will get a good understanding on the overall design for safety from the discussion.

-Bill

vince

Re: Help re: ground fault protectors and circuit breakers

Thanks Bill, I was hoping you might respond. Your response helped me already resolve some confusion about floating a system and grounding it. My application is a Tiny house on wheels (but it can be off loaded and anchored to the ground when it is stationary). The main components are in my signature, and my daily usage is mostly a solar fridge, circulation water pumps, fans (computer), phone charger and led lights that can all run on 12V or some on 24V. Average use is about 1.5 KwH/day. Wiring from panels to CC is about 8 ft and 12awg wiring. All wiring is inside except for a few lights that are on the porch. I was planning three load circuits that stay below 15A each and will each have a step down converter from 48-12V or 24V. As the Tiny House is build from wood, I have no ground connections to a metal frame (unless I make one). As the house sits on a trailer or may be stationary and anchored to the ground, I'm wondering how to work with ground and also a GFC. AC appliances are optional and primarily "good weather" loads.

BB.

Re: Help re: ground fault protectors and circuit breakers

OK--Basically your system is "floating"... But because there is no metal around and it is on wheels, there is really not much reason bother with double pole breakers as there is nothing for your wiring to "short circuit" with. Of course, you should still follow NEC (national electric code) or other good practices wiring system (boating, etc.) to size wiring for loads and breakers/fuses for wiring.

I like to use the NEC because the wire is cheap and available from local hardware stores, and the NEC tables are relatively conservative. To compare:

Wire Current Ampacities NEC Table 310-16
ABYC Wiring Standards

If you are mixing 12 and 24 volt systems... It may work out easier (120 VAC wiring is much easier than 12 or 24 VDC--You can send AC power much father and on smaller diameter wiring vs low voltage DC).

If you just use an efficient 120 VAC inverter instead. Many of the 120 VAC loads these days are just about as efficient as the DC versions. And DC to DC converters have their losses too.

You also asked about ground fault protection (in your thread title). On the AC side, that is pretty easy to do with standard GFI outlets (best on True Sine Wave inverters, Modified Square Wave inverters may cause issues with some GFIs).

On the DC side, there is a GFI requirement for the Solar Array--But the common method uses a fuse (or breaker) between Battery Bank negative and Earth/Safety ground. Personally, I do not like this circuity (putting a breaker between safety and earth ground)--But it also does not make sense for a building that has no ground rod/metal electrical panels and conduit.

There are new some new DC GFI breakers out there that can trip with a few amps of ground fault--But it is expensive and does not do the "safety" of 5 milliAmps (~0.005 amps) limit that is easy to measure with AC circuits.

Ground Fault Protection Device » Morningstar Corporation

But these are not generally used for DC power systems (at least so far).

-Bill

vince

Re: Help re: ground fault protectors and circuit breakers

BB. wrote: »

OK--Basically your system is "floating"... But because there is no metal around and it is on wheels, there is really not much reason bother with double pole breakers as there is nothing for your wiring to "short circuit" with.

-Bill

I was going to use a fuse in both the + and - line near the battery and CC terminals with a GE airconditioner disconnect. The fuses are the cartridge type that are rated for DC. Are you suggesting that I only need to do that for the + sides?

BB.

Re: Help re: ground fault protectors and circuit breakers

I would tend to just fusing the positive leads.

If you have any "metal" (plumbing, fuse/breaker panels, solar panel frames, etc.)--You should ground all of those to the battery negative bus. that would "justify" the single fuse/breaker per positive lead/branch circuit.

One less thing to "go wrong" or debug if something does fail.

-Bill

vince

Re: Help re: ground fault protectors and circuit breakers

I appreciate your help. Thanks