Should Negative on Battery Bank Be tied to Ground?

Solarguy1 · December 2020

Hello,

As the title states, should your battery bank be tied to your ground? I have read and seen several threads where it shows this, however, I'm wondering if this is always the case.

My inverter/charge controller is a Phocos PSW-H-3kW-120/24v and my batteries are Battle Born lithium ion BB5024.

Thanks for any advice!

BB. · December 2020

Ok, first stop, the manual:

https://www.solar-electric.com/lib/wind-sun/Phocos_PSW-H_Series_manual.pdf (page 3)

WARNING: Because this inverter (AC output) is not isolated from the PV input, only solar panels are acceptable for use which do not require positive or negative grounding as grounding the positive or negative PV cables is not allowed.To avoid any malfunction, do not connect any PV modules with possible current leakage to the inverter. For example, positive- or negative-grounded PV modules will cause current leakage to the inverter. Grounding of the PV module frame is permitted and frequently required by local law.The battery is galvanically isolated from the inverter and PV input, therefore the battery positive or negative terminal may be grounded if required.

Very clear (not all manuals are written with clear grounding instructions).

Next... Why ground (or not).

A floating battery (ungrounded) cannot shock you/cause a short circuit when one terminal is connected (short circuited) to ground. Isolated/floating output power systems are designed for this and (with proper testing/certification/ongoing maintenance) are "safe".

HOWEVER, if a short circuit between + or - and ground does occur, then the system is no longer floating, but is now (inadvertently) a ground referenced system. And that is when the issue(s) begin. The short answer is that unintentional current paths in shorted floating power systems. For an example, imagine you have a DC light fixture that has a short to "metal" with the negative wire (metal as in a vehicle body, electrical conduit, plumbing, green wire safety ground to fixture, etc.). "First connection" is no issue--The second connection is the problem--Say you get a short between a heavy battery cable to the AC inverter + to "grounded metal". If you have a 200 Amp fuse in the + lead, and no fuse in the return lead (-), then you can get >200 amps in that little wire going to the light fixture... That is why floating systems require two fuses (or breakers) in both the + and - leads of the wiring.

To only use 1 fuse/breaker in DC wiring... Normally what is done is to ground reference the battery bank (to a ground rod, cold water pipe, and the same ground rod/water pipe as your AC system green wire safety ground too).

When you ground the battery bank (negative battery bus ground bonding to ground rod/cold water pipe/etc.) it makes sure that the negative terminal can never get above zero volts. So shorting the negative wiring cannot cause a "short circuit" or over current situation and you only need fuses/breaker in the + leads (DC input to inverter, any 24 volt loads you may have, 24 volt chargers).

For a home or vehicle, connecting the battery bank negative terminal to your home's (or vehicle's) "ground system" is the way to go normally.

Another reason to ground your battery bank to a ground rod/cold water pipe is, I believe, it also helps direct lightning energy to the ground, instead of somewhere else in your home.

If you have lightning in your area--You should also look at surge suppressors...

https://www.solar-electric.com/search/?q=surge+suppressor

One where your solar array wiring enters your home... And a second suppressor at your 120/240 AC main panel (both suppressors need to be connected to a safety ground, sheet metal electrical box, to ground rod with 6 AWG minimum wiring path). Note that there are different suppressors for different voltages (low voltage DC, AC voltage, etc.).

-Bill

Solarguy1 · December 2020

Thanks Bill! Lots of great info.... I saw that section in the manual, but was not sure on the "if required" part of it.

One more question regarding SPD's. So, since my installation is off-grid, I'll have an AC panel coming off from the inverter. No grid tie AC panel. (I do have a SPD for the combiner box already.) Would I still need a SPD on my AC panel that is downstream of my inverter? If so, how does that get hooked up to a 120 volt panel? I see their directions for 220...looks like the black and red both go to the positive?

BB. · December 2020

My humble suggestion... A properly grounded battery bank helps limit lightning damage.

For AC power off grid--Our founder (now retired), in his experience, the #1 damage from lightning for off grid power systems was to the output stage of AC Inverters... So using SPDs on your AC output/main panel is a good idea.

On the inverter output or the main panel for SPDs (that is the question?)... If you have a relatively short run from the AC inverter to the main panel (and especially if you use metal conduit for that wiring), I would put the SPDs in the AC main panel. This will also help prevent a surge on one circuit (say outdoor flood lamp circuit) from bringing the lighting in and sending the surge to other AC branch circuits. (again, in my humble opinion).

Regarding SPDs, is this the manual you are looking at?

https://www.solar-electric.com/lib/wind-sun/SPD-installation.pdf

And your question--You are talking about AC panel but "positive" terminal... Do you mean L1 to Neutral/Ground on AC panel?

Not an expert, but as I understand the Midnite SPDs, they are actually two protection circuits. A "ground" lead, and two protection leads. You can connect Red to L1 and Black to L2 (in a 120/240 VAC split phase system) or both Red and Black (in parallel) to L1 in a 120 VAC only system.

They are not polarity sensitive... You can use the same protection device on DC or AC power systems (and there are NO limitations requiring that Red goes to + and Black goes to - DC connections).

-Bill

Solarguy1 · December 2020

Thanks Bill... Yes, that is the installation manual i was reading.

I watched a video on YouTube from Midnight Solar. It makes sense now...yes, each SPD has 2 channels as they call them. Both red and black would go to L1 in a 120 VAC system.

Thanks again for the info! I'll add a SPD to the AC panel as well...

notes · December 2020

Bill,
RE: Battery negative ground
I am currently starting my off grid install. Charge control is a Midnite classic 250. I am reading this in there instructions manual.
The Classic has internal ground fault protection (GFP) built-in. Since 2008, the NEC requires a DC- GFP on all PV systems in the USA. The built-in DC-GFP eliminates the need to purchase and install an external DC-GFP. If the internal grounding jumper is installed in a Classic, the battery negative and DC source negative must not be connected to the system grounding conductor anywhere in the system. Grounding of these circuits will defeat the GFP function.

Would you suggest removing jumper and disabling the DC-GFP function and grounding as explained above.
Sorry for jumping in on op comments.

BB. · December 2020

That is my personal suggestion... Placing a fuse between system ground and safety ground (fuse or breaker is typical method to detect a "ground fault")--Just really a bad idea.

Ground the negative bus to ground rod/green wire/safety ground. And disable the ground fault detect in the Charge Controller...

There are other methods to detect ground faults and other faults. Midnite on some of their controllers does have an Arc Fault detect/shutdown (detects the frequencies of arcs in the wiring). This is what the "original" NEC "ground fault fuse/breaker detection" system was supposed to prevent.

Arc fault "breakers" have always been a little bit iffy... They are known to false trip at times--For example "universal motors" (such as brushed motors in vacuum cleaners) can trip arc fault breakers (which are intended to stop arc fires from cords being run under carpets in bedrooms and such).

If you want to read the (sleep inducing) details, here are a couple threads sort of written in parallel:

https://forum.solar-electric.com/discussion/9345/system-grounding
http://midniteftp.com/forum/index.php?topic=142.0

The NEC (at that time?) was focused on trying to prevent DC solar array fires--Most of which were Arc Faults (hot to ground arcing, or a bad connection wire to wire arcing). The problem is that DC current is really good a sustaining arcs (vs AC). And it looks like the "powers that be" tried to construct a system using existing NRTL (UL, CSA, etc.) rated/Listed hardware (like fuses and circuit breakers).

Their solution would detect Hot wiring to Frame ground faults (shorts, arcs). But not wire to wire arcs (such as a failed crimp connection). And they did this by doing a huge NO in electrical safety... The put a 1 amp fuse (or equivalent) between the system power ground/neutral/negative terminal, and the frame/chassis/earth ground... That fuse pops, and now the ground referenced power system or floating. It causes all sorts of failure modes that are both shock and fire hazards.

The above links are from 2011--So my little battle did not really do much. Oh Well.

-Bill

notes · December 2020

Thanks Bill, great debate. I will follow your lead. Thanks again.

Should Negative on Battery Bank Be tied to Ground?

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