48v off grid system: battery cable advice needed please

[Sunking]

Thank you very much Sunking. Exactly what I was looking for. The SW4048 is max rated for 7000w for five seconds. Shouldn't I be using 7000 watts in the equation instead of 4000 watts to prevent blowing fuses if all the big loads hit at once?

No sir. Inetdog explained why. Your Inverter would burn up or shut itself down before the fuse operates.

I do understand the cost issue of the MRB fuses. They are expensive as you have discovered. Shop around, Blue Sea is not the only source for them. Any RV/Boat shop, Cabell's, Bass Pro, and even some Walmarts and Automotive places have them. Even cruise Ebay and Amazon for them. If you are uncomfortable with 150 amps, bump it up to 175 amps.

If you are going to error, do so on the side of caution. Too small is better than too large. I am confident 150 amps is the sweet spot. As I said earlier assuming your 4/0 cable is copper, 90 Degree Insulation, you can go to 225 amps, but I think those will be higher cost than 150. Go find out, I am not going to do all the work for you unless you send me a check for 15% of your total installation cost is my standard fee. .

[Mike90250]

......If I was crazy enough to cross the positive and negative cables would that blow a 225a fuse?

Only if you are lucky, Depending on how long it takes the fuse to blow, the batteries might start to boil, cables will heat up and start melting the insulation off them,,,,,,

[Sunking]

If I was crazy enough to cross the positive and negative cables would that blow a 225a fuse?

Yes but not sure you would live to see it. It would produce quite a magnificent Arc Flash like this guy who did not get to see it. Looks spectacular from an observer POV. Like a fireworks show. If you try it make sure to have your wife or kids take video from a distance so they can put on You Tube for everyone to see. Watch the video.

[hammick]

Obviously I would never cross the positive and negative terminals. But just so I know what I am dealing with would accidentally making contact with the positive and negative terminals at the batteries or in the terminals of a 48v 325ah system really be that sensational?

I'm thinking I want to fab some protectors for my + and - battery terminals to protect from dropping a wrench. Anyone done this? I'm thinking 6" x 6" pieces of non-conductive material and have my wife sew in velcro so I can wrap them around the terminals and cables.

[Living Large]

Obviously I would never cross the positive and negative terminals. But just so I know what I am dealing with would accidentally making contact with the positive and negative terminals at the batteries or in the terminals of a 48v 325ah system really be that sensational?

Do you mean touch both with your body? DC becomes dangerous to people at around 50V. You can research that for yourself, and make your own call. Shorting the terminals with metal is a whole different situation, as has been covered. Don't do it.

[Logan005]

kinda like a 9volt battery on your tongue, I do not recommend wetting each index finger and touching 48+ volts to make sure it's good.

[Sunking]

Obviously I would never cross the positive and negative terminals. But just so I know what I am dealing with would accidentally making contact with the positive and negative terminals at the batteries or in the terminals of a 48v 325ah system really be that sensational?

No but not far from it. Depends on how close to the batteries you are in cable distance and cable size.

Batteries, especially Lead Acid Batteries can supply tremendous amounts of fault current. It is a function of their Internal Resistance plus any added cable resistance along the way. Utility faults are measured the same way. You have the impedance of the upstream transformer outside the control room you seen plus the cable resistance. What you are seein gis copper and steel vaporize and the poor SOB wrapped in a cloud of white hot plasma.

I do not recall what AH you are running, but I do know 48 volts and with a 4000 watt inverter I would think at least 800 AH or more. But I can give you an idea of how much power a dead boat fault will produce by using some Rolls and Trojan data on Internal resistance and CCA ratings. With inter-cell cable resistance and connections will produce 3021 amps @ 22 volts = 66 Kw of heat. That may be difficult for you to understand so lets use an analogy. 66000 watts is equal to roughly 225,000 BTU's. So imagine taking a 40 pound block of ice (5 gallons of water) at 32 degrees, and boiling it in 0.1 seconds. You would not survive it and nothing to bury. If the plasma did not do enough damage 64 gallons of boiling hot battery acid would wash the rest of you away.

[inetdog]

Note that at those currents even a hefty chunk of metal like a wrench or a ring makes a good but spectacular fuse. They would probably protect the battery, but not the person making the short.

[Sunking]

Note that at those currents even a hefty chunk of metal like a wrench or a ring makes a good but spectacular fuse. They would probably protect the battery, but not the person making the short.

I know what happens to a wrist watch, wrench, and wedding ring can do on 48 volt 1000 AH plus size batteries.

Wrench was an installer working above batteries on cable rack. Dropped wrench and wrench contacted positive battery terminal post and steel frame battery rack. Fortunately the wrench bounced laterally and did not happen below installer. Arc flash about 8 feet to his side. Wrench vaporized, his hair on the side near flash was gone, and his pants got filled up with mud. All we found of the wrench was a few little pieces that looked like BB's and bird shot.

Wrist watch was on the buss above battery racks through a pair of 750 MCM cable. Contact was Battery Neg to frame. Blew his hand off, and he lost his arm up to elbow with sever burns on 30% of his upper body. He lived but never seen or heard from him again. Only thing that saved his life is he was working above his head.

The Ring was in a Distribution Frame 100 feet from batteries. Contact from back of 100 amp breaker with a wrench in his hand and frame. Burned the ring off his finger and was a half circle, Notched notch at contacts points on the wrench of about 1/4-inch deep. Burned the flesh down to the bone. To my surprise he was back on the job next day, and I worked on/off with him for 20 tears. He was one of my best contractors. We called him Moran even before the accident. That is what he called himself. His name is Ron Moore aka MoRon. He was lucky his nick name was not Stubby after the accident. He kept the wedding finger and pinky finger. He laughed about it and said: "Damn it would have been easy to flip the Bird". We found most of the missing gold ring as BB's on the floor when crew swept up.

[Sunking]

Hammick to protect your self. Remove all jewelry, wear cotton clothes, use insulated tools, and wear PPE like heavy leather gloves and face sheild when ready to use tools. Never work above where you are working. Always to the side or above you with an escape route. Arc flashes always goes out and UP. You do not want to be above it.

[solar pete]

This thread rocks, I was thinking maybe good enough to become a sticky. Nice work SK on bringing to bare how dangerous off-grid systems and electricity in general can be, this stuff is dangerous people and needs to be treated accordingly. The large hybrid system at my brothers has a heavy duty fire proof door and is locked at all times, no one is permitted entry except for qualified people.

[Sunking]

325ah Trojan L16RE-A

That tames things down a little, but still potentially fatal under the right conditions. The highest danger area with any battery installation is at the battery itself. The term post are the most dangerous area. Arc Flash is still a threat with your batteries. The worse scenario would you be terminating cable on one term post with an uninsulated wrench, it slips and makes contact with the other post. In that position you are in double jeopardy. One is the Arc Flash, second is a battery explosion. Not an explosion of fire, but of boiling hot battery acid as it is turning into steam. If your system is Grounded, meaning one polarity is bonded to ground and metallic frames, Contact is with Un-Grounded polarity on battery term post of fuse, term block to equipment frame. This is why I say Grounded systems can be very dangerous.

Second most dangerous are is first downstream point from batteries. You have 4/0 cable and those will be very low resistance not offering much current limiting. Arc Flash is still a threat, but not quite as severe at batteries. One saving grace downstream if you follow my advice by putting Fuse Blocks on the Battery Term Post. That will add some resistance, and will open and clear the fault very quickly. Likely enough so there is minor damage, mild burns, but muddy pants you can walk away from.

Just keep in mind the worse faults are what is called Bolted Faults. A Bolted Fault is where you have a large surface area of a heavy contactor making strong mechanical contact. Like a wrench in your hands while tightening a battery connector and slipping into a frame or opposite polarity. If that happens is YOUR FAULT because you failed to use Insulated Tools and heavy leather gloves. The other one is dropping something on top of the batteries. A good custom made plexi-glass shield using Insulating Cherries to support it works wonders.

Keep in mind unlike your home AC system is al sealed preventing you from accidental contact, your batteries are not and open to contact waiting to bite you. It is a Rattle Snake, it will not warn you like a barking dog you are about to Eff up. It will strike without warning or mercy. Sad and scary thing is it will be all your fault even if it is not you getting hurt like a kid. You are responsible and liable. .

[Mike90250]

I've used a Blue Seas battery switch in the middle of my bank, (near the doorway) so I can power the system all the way down, and then I can flip the switch OFF. I have not had to switch it while running (that would cause wear in the the contacts) but I could in an emergency, Once the system is off, I use the switch to disconnect the middle of the string, and it becomes much safer to work on, A wrench in the wrong spot cannot access the full bank, but is limited to the batteries it can touch.
I've since heard reports of the battery switches getting high resistance, but I hope not switching under load, would prolong it's useful life.

[Sunking]

I've since heard reports of the battery switches getting high resistance, but I hope not switching under load, would prolong it's useful life.

Eventually yes. Real easy to monitor if you think about it, no special equipment needed, although infrared thermometers are handy, just using the back of your hand touching the switch while loaded will tip you off. By loaded I am not talking about you after two 6-packs. Just make sure to put he other hand in your pocket.

[Mike90250]

I am a firm believer of a IR thermometer in every battery shed tool box.