Is this 12-VDC house system layout idea correct?

Mex-Busnut · December 03, 2011, 12:23:38 AM

Dear Friends,

I am basically plunging into my 12-Volt D. C. house install, as my 55-amp converter/charger arrived via the mail this week.

I would sure hate to mess something up! I want to ask if I can simply parallel these systems (all pluses together, all negatives together):

1. Output from battery isolator, which tees off of engine generator's regulator.
2. Output from 55-amp convertor/charger.
3. 12-volt house battery bank
4. input to 3,000-watt inverter.

...Or does each voltage-charging source (converter/charger and isolator) need to be switched separately?

Please click on the PDF file's link to get the idea.

Thanks in advance!

bevans6 · December 03, 2011, 04:33:29 AM

I like to use at least a couple of fuses or breakers in there to protect the wiring. I also like to give myself options so I use switches to allow me to separate things. My big load is the inverter, I have it on a switch that allows me to connect it to just the battery bank, or to both the battery bank and the alternator/start batteries. This switch also allows me to tie my house batteries to my start batteries. My converter/charger is permanently connected to the house batteries, but can also charge the start batteries if the tie switch is closed. My small 12 volt house loads - lights, mostly, and the fridge computer - are permanently tied to the house batteries through a fuse panel that has individual circuits for each branch of 12v.

Brian

bobofthenorth · December 03, 2011, 06:18:04 AM

Add a negative side fuse right at the house bank and a knife disconnect switch after that and you're good to go as far as I'm concerned. Like Brian said a crossover between the house and start banks would be good too so you can start off the house bank if something bad happens.

thomasinnv · December 03, 2011, 08:18:35 AM

On a negative grounded vehicle, fuses should go on the positive side. With the fuse on the negative side, If you end up with a negative to chassis short somewhere then you have no protection. You should have a negative lug connected to the chassis anyway.

buswarrior · December 03, 2011, 09:22:58 AM

A negative side fuse goes tight to the batteries, no opportunity for contact upstream of the fuse, barring a structural collision issue, at which point, all bets are off anyway.

Perhaps one of our more eloquent writers could put out the pros and cons of a negative side fuse and it's intended added layer of protection?

Then one may decide whether this option is appealing or not for one's own situation.

happy coaching!
buswarrior

Mex-Busnut · December 03, 2011, 09:32:48 AM

Dear Friends,

Thanks to all for youir comments.

I have already aquired three 450-amp switches. I plan to install one for house batteries, one for engine batteries and the third for connecting both together when needed.

I am also fusing the B+ lines.

bevans6 · December 03, 2011, 10:16:36 AM

Sean covered negative side fusing extensively about two weeks ago. The idea is that it's a catastrophe fuse, I think. You could search for that discussion.

Brian

Sean · December 03, 2011, 10:28:15 AM

Quote from: bevans6 on December 03, 2011, 10:16:36 AM
Sean covered negative side fusing extensively about two weeks ago. The idea is that it's a catastrophe fuse, I think. You could search for that discussion.

I think that would be this post:
http://www.busconversions.com/bbs/index.php?topic=22290.msg244760#msg244760
and the discussion that follows it.

-Sean
http://OurOdyssey.BlogSpot.com

thomasinnv · December 03, 2011, 10:34:59 AM

I think if someone wants to add a negative side fuse for catastrophe protection right at the batteries, as BW or Bob suggests, that is their prerogative. That's why we have buses, so we can do it our way. I think what you are saying though is that it would be added protection, but not the only source of protection. I certainly would never install a negative side fuse without a positive side fuse, and just depend on that for protection. A negative side catastrophe fuse as Sean suggests is probably not a bad idea.

bobofthenorth · December 03, 2011, 10:57:47 AM

If you have the negative side catastrophe fuse at the batteries and the individual loads fused on the positive side that's all you need.

My original post was unclear about the location of the disco switch - it could be read that the disco should be on the negative side but clearly it needs to be on the positive side.

TedsBUSted · December 03, 2011, 08:00:24 PM

I don't like the idea of a "catastrophic-fuse" partially disconnecting the grounded side of a battery bank.

A controlled complete disconnect is okay, but not a fuse that can open while still leaving potential stray paths connected. An over-current which opens a catastrophic-fuse will seek out and flow through
other still-connected stray paths. The stray paths may not have the ampacity to handle a current which "popped" a 200+ amp fuse, and so may go into meltdown. Or if some stray path has the ampacity to handle the over-current, such as a non-isolated starter motor circuit, then that heavy current path would make the catastrophic-fuse's (non?) opening a meaningless event.

Ted

Sean · December 03, 2011, 08:48:21 PM

Quote from: TedsBUSted on December 03, 2011, 08:00:24 PM
I don't like the idea of a "catastrophic-fuse" partially disconnecting the grounded side of a battery bank.

When a catastrophe fuse opens, it is not a partial disconnect -- it is a complete disconnect. The fuse is immediately adjacent to the battery, and once it opens, the battery is completely disconnected from the circuit.

Quote
A controlled complete disconnect is okay, but not a fuse that can open while still leaving potential stray paths connected. An over-current which opens a catastrophic-fuse will seek out and flow through
other still-connected stray paths.

No, it can't. Again, once the fuse blows, there is no way for current to return to the battery.

As I said in the other thread, "positive" and "negative" are arbitrary concepts and neither the batteries nor the currents care one whit which side of the battery gets disconnected. The only thing that sets them apart and makes which side gets fused an issue is the fact that we normally choose one terminal to connect to the frame and thus forcing the frame to be at that potential.

Opening a "frame-side" catastrophe fuse disconnects this frame connection and absolutely terminates all current flow and severs the frame-potential relationship.

The reason for using a frame-side fuse (when possible) is to protect everything from the positive battery terminal(s) to the main fuse or breaker panel. A positive-side fuse can not protect the batteries themselves and all the wiring between the batteries and the fuse, whereas a negative-side fuse does.

As I also wrote in the other thread, there are some configurations that simply do not permit a negative-side fuse, such as a battery bank with multiple taps for different voltages, or a dual-battery-bank system where multiply-served loads (e.g. an inverter) are not individually fused. But in a single-bank, single-voltage system, a negative-side fuse provides greater protection than a positive-side fuse. Note that the main disconnect as well as individual load fuses should still be on the positive side, as I wrote in the other thread.

-Sean
http://OurOdyssey.BlogSpot.com

Brassman · December 03, 2011, 09:43:10 PM

This link is for a fuse that connects directly to the battery terminal. That should make everyone happy (maybe). Max. fuse size is 300 amps. Might not be rated for 28 V.

http://bluesea.com/viewresource/640

TedsBUSted · December 03, 2011, 10:24:00 PM

Quote from: Brassman on December 03, 2011, 09:43:10 PM
This link is for a fuse that connects directly to the battery terminal. That should make everyone happy (maybe). Max. fuse size is 300 amps. Might not be rated for 28 V.

http://bluesea.com/viewresource/640

Not yet.

Did you notice Brassman, that the fuse is installed in the ungrounded side and only protects part of the loads?

Quote from: Sean on December 03, 2011, 08:48:21 PM
. . .Opening a "frame-side" catastrophe fuse disconnects this frame connection and absolutely terminates all current flow and severs the frame-potential relationship. . . .

Sean, While I technically agree with that limited statement, I must respectfully doubt its possible real-world application.

Yes, in the ideal simplified model of one battery having all loads combined to connect at one fuse-protected cable, and without a charging system, yes it's true. But in the real world application of multiple battery banks, their charging systems, their isolators and combiners, and cranking motor circuits to boot, it's suddenly not so simple.

I can think of a myriad of "what if" possibilities but I'll concentrate on one example of a fairly common high-current failure scenario:

At a bump in the road the driver notices that the lights dimmed and the voltmeter took a momentary dive. But all seems well now so he keeps rolling. Unknown to the driver is that the dip was caused by loose battery hold-down hardware that shifted to touch the battery's ungrounded terminal. Typically, a wayward bracket may short for a split second, but then having a "nip" burned out of the bracket or terminal quickly opens the short. Although a very high-current short has burned away the wandering bracket, and caused the entire system to "dip" for a second, all will seem normal again until the mystery may repeat many miles later. Hopefully the wandering bracket problem is soon found and corrected.

Now let's consider what could happen with the same scenario with a catastrophic-fuse located on the battery's chassis ground side. The current which was high enough to burn metal away from the wayward bracket would most likely be enough to open the catastrophic fuse. With the fuse open the entire electrical system is disconnected from the batteries. Yes? Yes, disconnected, but NO, not powered down! Not in the typical system. Almost everything is disconnected, however, the charging system and starter motor are still (depending on isolation) somehow connected to the system with heavy-current capable paths.

So... when the loose bracket shorts and the catastrophic-fuse opens to disconnect the batteries, a new current path is created, directly between the charging system and the connected loads. But now, unfettered by a petty 12V or 24V battery bank, the alternator voltage would at least momentarily spike and head for the sky. Maybe close to 100 volts even, but certainly high enough voltage to send most components to the promised land. And depending on the wiring of the regulator and field circuit, the alternator also may futilely try to raise the disconnected battery bank to normal charging voltage. Of course the voltage of disconnected batteries isn't "sensed" and won't rise, but the full-fielded alternator will die trying, meanwhile taking most connected components with it.

Basically what I'm saying is that as described the hypothetical total disconnect by ground-side fuse is correct. However, in real world application there are lots of devils in the details and there are no simple "one size fits all" fuse or disconnect solutions. Draw the circuit with a ground-side fuse, and I'll try to figure out how to short it!

Ted

Brassman · December 04, 2011, 11:04:05 AM

Lazy me did some research: the BlueSeas terminal fuse is good to 58 Volts. At 32 volts it has a 5000 amp circuit interupt.

http://bluesea.com/category/81/22/productline/overview/379

At the bottom of that previous link I posted it states the ABYC (boat) code requires a fuse within 7" of the battery. I assume the starter cable is excluded.

In that previous link they have a picture of a bus bar feeding loads through those terminal fuses. I have installed my house batteries, and was wondering how I was going to feed and fuse the major loads. Seeing that picture solved that problem.