It's like a jigsaw puzzle that could kill me (setting up my bus bar) - my thoughts make sense?

[rhino]

I guess I'll know once the second fuse blows because it demands too much power through the only remaining cable.

That won't be guaranteed to happen. If something damages the one cable it's possible the fault clears before the 2nd fuse can blow (rodent chewing it). In any event you'd then be replacing two expensive fuses every time a "single" fault happens when you could just have one. You're also slightly increasing the total resistance of the battery->inverter cabling by having two fuses instead of one. Maybe someone can enlighten me but I don't see what advantage you get with that arrangement.

 

[Shimmy]

I have never understood the logic of protecting an inverter? What could go wrong if the DC inputs to an inverter are shorted? A fuse at the pack would protect the wire. If the short was internal to the inverter the inverter is probably fried anyway and further damage to the cables is prevented by the breaker or the fuse..

The inverter issue is that the IGBTs have an extremely low reverse current capability so an AC input (or output in some conditions) transient will cause them to blow. This can start a fire. For large pump VFDs you sometimes need to add a suplemental snubber circuit to ensure the system is sufficiently robust.

 

[629658]

The consultant we hired said:

100% correct. I’m expecting someone on here to suggest we hire someone to sit in our battery rooms 24/7 to watch them!!!!

I’m all for safety but how many redundancies is enough? 2,6,or 8.

 

[Ampster]

The inverter issue is that the IGBTs have an extremely low reverse current capability so an AC input (or output in some conditions) transient will cause them to blow.

Most inverters have circuit breakers on the AC side anyway. I thought the above discussion was about the DC fuse and that was what my question was about. How does a fuse at the inverter protect the inverter if the DC inputs are shorted? Do you agree that the purpose of fuses and circuit breakers is to protect the wires?

 

[629658]

So I have to ask the obvious question is it worth spending $500+ to protect $150 worth of wire worst case?

 

[Ampster]

So I have to ask the obvious question is it worth spending $500+ to protect $150 worth of wire worst case?

The wire is not the the ultimate risk. When the wire gets hot from overcurrent it could cause a fire. A fire could burn down a home. I did not spend $500 to purchase a Class T fuse. My 225 Amp Blue Seas Class T fuse cost $40. I bought it to ultimately protect my home.

I’m all for safety but how many redundancies is enough? 2,6,or 8.

Just one in the right location would work. I put mine at the largest current source which is my 42kWh pack which would be capable of releasing over a thousand Amps on a direct short.

 

[Shimmy]

Do you agree that the purpose of fuses and circuit breakers is to protect the wires?

Primarily, but not exclusively. The NEC focus (for consistency) is on the wires. But, what you are protecting the DC wires from is not overload but a short sourced by the batteries (highest source of fault current).

Most of the DC circuit breakers I have seen in inverters are not actually rated for fault load-break; they are just isolation switches functionally.

 

[629658]

The wire is not the the ultimate risk. When the wire gets hot from overcurrent it could cause a fire. A fire could burn down a home. I did not spend $500 to purchase a Class T fuse. My 225 Amp Blue Seas Class T fuse cost $40.

Just one in the right location would work. I put mine at the largest current source which is my 42kWh pack which would be capable of releasing over a thousand Amps on a direct short.

I know I’m just poking the bear. But some on here are putting in many class T fuses. One correctly sized is sufficient. But by the time you buy the fuse, holder, case and lugs you’re looking at least $100 maybe $120. If I took them seriously I’d be looking at over $1k (8 batteries and two from the inverter). They are advocating 1 for each pack into the buss. I did fuse my packs at the buss using Mega’s (I know they don’t have the arc stopping capability of class T but if I have an arc it’s not going to be at the buss anyway. Hopefully, the Mega’s would blow way before we get to arc time and then we have the DC breakers on the batts themselves and of course the BMS itself and then the DC breakers on the inverter and lastly the class T feeding the buss. Add to that the central smoke detector over the bank and the temp alarms.

This stuff will drive you crazy if you let it. I’m a safety first guy and I think it’s as safe as possible. But it kills me looking at pictures of the crap some hang on their walls with batteries and wires running everywhere. Now that would scare the hell out of me.

 

[AlaskanNoob]

The wire is not the the ultimate risk. When the wire gets hot from overcurrent it could cause a fire. A fire could burn down a home. I did not spend $500 to purchase a Class T fuse. My 225 Amp Blue Seas Class T fuse cost $40.

Just one in the right location would work. I put mine at the largest current source which is my 42kWh pack which would be capable of releasing over a thousand Amps on a direct short.

Yeah, the fire potential is my concern.
On the inverter side, it can pull 520A surge, and 250A continuous.

Anybody think I have a greater fire risk if I size my t-fuse based on the surge rather than the continuous number? Unless there is a safety issue I'll stick with the fuses I got.

 

[time2roll]

If the previous poster is correct that I should have sized my t-fuses for the max continuous inverter draw (250A) rather than what I did, which was sizing for max surge (520A) then I'm going to have to get smaller t-fuses anyway for the cables to the inverter. Maybe I'll put fuses on both ends of the cables.

200% goes to about 50 seconds. Easily 10 seconds.

https://www.bluesea.com/products/5112/Fuse_A3T___Class_T_110_Amp

 

[HRTKD]

Hopefully not a dumb questions, but is the time spent over the rated amps cumulative? I don't know what goes on inside one of these Class T fuses.

 

[rhino]

Hopefully not a dumb questions, but is the time spent over the rated amps cumulative? I don't know what goes on inside one of these Class T fuses.

They aren't like MOVs which degrade after each surge they protect against. There is no aging factor. There's just a piece of metal inside connecting the two ends like you normally see in a fuse but with "sand" surrounding it to help extinguish any arcing when the metal breaks/melts.

 

[Bluedog225]

I thought fuses did age/deteriorate.

 

[Shimmy]

Hopefully not a dumb questions, but is the time spent over the rated amps cumulative? I don't know what goes on inside one of these Class T fuses.

Not so much on Class T fuses, but some of the fuses have a series of parallel ribbons where you can see one starting to melt. This typically happens on low-level faults rather than overload; these segments are designed for high fault current instantaneous protection.

 

[Ampster]

Anybody think I have a greater fire risk if I size my t-fuse based on the surge rather than the continuous number?

Yes it is a greater fire risk if you size the fuse for the surge and your wire size is smaller. If you ignore the conscept of the fuse is to protect the wire then you are on your own and have learned nothing from your consultant or this thread. Sorry to be so blunt.
EDIT: The OP subsequently clarified that the fuse was correctly sized to protect the wire in the above question.

 

[AlaskanNoob]

Yes it is a greater fire risk if you size the fuse for the surge and your wire size is smaller. If you ignore the conscept of the fuse is to protect the wire then you are on your own and have learned nothing from your consultant or this thread. Sorry to be so blunt.

My consultant is the person who sized the fuses. When I passed along the thoughts here that the fuse should have been sized for the continuous power and not the surge, he responded as I stated above, re-affirming that it doesn't matter and that the fuse size is based on what the cables can handle. So we got fuses that can handle the surge (and apparently smaller fuses also can for 50 seconds or so), and we made sure the cables could handle that.

There is a lot of confusion about overcurrent protection on forums etc
Overcurrent protection (fuse or breaker) is to protect wires only, not equipment.
Thus, overcurrent protection is sized based on wire size. In this case, 300A is perfectly suitable for 4/0 cables.
You could certainly downsize the fuses, but there's no advantage that I can see in doing that and you risk nuisance tripping of the fuse.

The consultant and you disagree, perhaps, so if that's the case, can you better explain why the consultant we hired is wrong on this issue? You mention "and your wire size is smaller." Smaller than what? I hadn't mentioned wire size I don't think. The inverter can demand 250A continuous, 520A surge. Two sets of 4/0 cable each with a fuse holder and a t-fuse. So that means each set of 4/0 cables can expect to see 125A continuous, 260A surge. The 4/0 cable is rated for 411A at 104F.

The two t-fuses I have right now are 300A fuses that were chosen because of the potential 260A surge current each set of cables could potentially see and the rating of the 4/0 cable. The question is, should those 300A fuses be reduced instead and sized for 125A current. You apparently say yes. The consultant we hired says no.

So please explain why there is a greater fire risk if we don't ditch the two 300A fuses for smaller fuses?

Could it be because the 300A fuses can actually take 600A or so for 50 seconds during a short, and that would be above the 411A rating of the cable, and that would cause a fire? I suppose that could be a possibility, although I would expect if there were a short it would be more than 600A and would blow the fuse but I have no idea how shorts work.

 

[Ampster]

So please explain why there is a greater fire risk if we don't ditch the two 300A fuses for smaller fuses?

If your wire cannot handle 300 A then the fuse is not sized to protect the wire,and there is a risk. The risk is that some event could occur that could exceed the capacity of the wire and the fuse would not protect the wire. That is the concept that your consultant and I agree on, that the fuse is to protect the wire.. I also agree that the fuse does not have to be sized for the surge since the surge will not exceed the time illustrated above and a brief surge will not overheat your wire enough to start a fire.

and we made sure the cables could handle that.

Clearly I did not understand that from your earlier post, in which case my apologies.

 

[629658]

My consultant is the person who sized the fuses. When I passed along the thoughts here that the fuse should have been sized for the continuous power and not the surge, he responded as I stated above, re-affirming that it doesn't matter and that the fuse size is based on what the cables can handle. So we got fuses that can handle the surge (and apparently smaller fuses also can for 50 seconds or so), and we made sure the cables could handle that.



The consultant and you disagree, perhaps, so if that's the case, can you better explain why the consultant we hired is wrong on this issue? You mention "and your wire size is smaller." Smaller than what? I hadn't mentioned wire size I don't think. The inverter can demand 250A continuous, 520A surge. Two sets of 4/0 cable each with a fuse holder and a t-fuse. So that means each set of 4/0 cables can expect to see 125A continuous, 260A surge. The 4/0 cable is rated for 411A at 104F.

The two t-fuses I have right now are 300A fuses that were chosen because of the potential 260A surge current each set of cables could potentially see and the rating of the 4/0 cable. The question is, should those 300A fuses be reduced instead and sized for 125A current. You apparently say yes. The consultant we hired says no.

So please explain why there is a greater fire risk if we don't ditch the two 300A fuses for smaller fuses?

Could it be because the 300A fuses can actually take 600A or so for 50 seconds during a short, and that would be above the 411A rating of the cable, and that would cause a fire? I suppose that could be a possibility, although I would expect if there were a short it would be more than 600A and would blow the fuse but I have no idea how shorts work.

You have a good consultant and stay with his recommendations. You are on the right track.

 

[AlaskanNoob]

Your setup is very similar to mine. Same Blue Sea bus bars and double cables to the inverter. Below is a picture of my compartment. The batteries and inverter are at the top of the picture. I've added two temperature sensors and exhaust fans since the picture was taken.

It's good to see that I'm not the only one that had a problem fitting fuse holders into place. It bothered me to see the connecting bus bars at an angle.

The bus bars have room underneath them to run cables. However, I don't think 2/0 will fit unless you elevate the bus bar. I did elevate my bus bars so that the fuse holders and the shunt were at the same height as the bus bars.

Your fuses on the bus bar for the solar charge controllers is wrong. The over current protection should be on the solar charge controller end, not the bus bar end.

My Class T fuse is immediately downstream of the battery. I use twin MEGA fuses on the bus bar going to the inverter.

I'll echo the suggestion to make your own cables. I buy high quality cable and lugs. I've had zero failures with my crimps and I do test them. No wiring plan survives contact with physical placement. Making my own cables allowed me to play with the layout.

View attachment 147076

Finally took a gander at your system...is this an RV setup? Good to know about the cable fit on the bottom row of those bus bars. I'm going to get a 4/0 cable tomorrow so that I can start sizing things a bit to see if I need to elevate my bus bars.

Question on your mega fuse holders. Why the horizontal chunk of copper at the top of the two fuse holders? Could you have put the inverter cable lugs on the fuse holders directly?


And what is this blue box and red cone looking thing you have going to your positive bus bar?



Lastly, did you take the plastic covers off your bus bars for this photo, or do you not use them?

 

[HRTKD]

Question on your mega fuse holders. Why the horizontal chunk of copper at the top of the two fuse holders? Could you have put the inverter cable lugs on the fuse holders directly?


And what is this blue box and red cone looking thing you have going to your positive bus bar?

I went a little crazy with my copper bus bar. As I recall, my thinking was that if one fuse blew, I still wanted two cables going to the inverter. I should have just connected the cables to the studs on the fuse holders.

The blue box is the Victron BatteryProtect 65.