Heavy duty busbar advice (+600A)

[FilterGuy]

I don't see the distributor adding anything to the above scenario?

What I really like about the distributor is that it allows you to combine Fuse Holders with the busbar.



Note that on the Lynx, two crimps and a connection are eliminated for each fuse.

This is more than just making it easier to build and/or nicer looking. This is a functional improvement. Each connection and each crimp adds resistance and it adds up. This cumulative resistance can create significant voltage drops as the current goes up. Just two days ago I was working on a system that had a very clean build with excellent crimps and heavy-duty bus bars but the system was suffering a significant voltage drop between the battery and the inverter. We are now considering re-wiring things to reduce the number of crimps/connections in order to reduce the voltage drop.

I don't really want fuses in my system if I can avoid it, would rather have breakers trip in case of a fault.

I used to be a never-fuser but over time I have changed my mind.

Quality DC breakers are going to be large and expensive. Furthermore for anything over about 150A you are going to find it hard to find one that does not require a breaker box to mount them. (Warning: There are a lot of cheap 200+A breakers out there that are more likely to start a fire than prevent one)

Meanwhile, quality, name-brand fuses are much more compact, easier to mount, and less expensive.

About now the never-fusers are saying "But what if the fuse blows..... then I have to get a replacement and I might not have one handy....the convenience is worth the extra cost!!" That used to be my point of view as well. However, blowing a 250A fuse is a catastrophic event that should never happen in normal operations. Something went really big wrong for that to happen. (Or the design is *really* bad). If a 250A Fuse blows, there is a good chance that replacing the fuse will be the least of your worries.

 

[John Frum]

One other consideration is a breaker allows someone to thoughtlessly open a breaker under load.
Depending on the breaker that can be much worse for the breaker than tripping on over-current.

 

[da9el84]

What I really like about the distributor is that it allows you to combine Fuse Holders with the busbar.

View attachment 106828

Note that on the Lynx, two crimps and a connection are eliminated for each fuse.

This is more than just making it easier to build and/or nicer looking. This is a functional improvement. Each connection and each crimp adds resistance and it adds up. This cumulative resistance can create significant voltage drops as the current goes up. Just two days ago I was working on a system that had a very clean build with excellent crimps and heavy-duty bus bars but the system was suffering a significant voltage drop between the battery and the inverter. We are now considering re-wiring things to reduce the number of crimps/connections in order to reduce the voltage drop.


I used to be a never-fuser but over time I have changed my mind.

Quality DC breakers are going to be large and expensive. Furthermore for anything over about 150A you are going to find it hard to find one that does not require a breaker box to mount them. (Warning: There are a lot of cheap 200+A breakers out there that are more likely to start a fire than prevent one)

Meanwhile, quality, name-brand fuses are much more compact, easier to mount, and less expensive.

About now the never-fusers are saying "But what if the fuse blows..... then I have to get a replacement and I might not have one handy....the convenience is worth the extra cost!!" That used to be my point of view as well. However, blowing a 250A fuse is a catastrophic event that should never happen in normal operations. Something went really big wrong for that to happen. (Or the design is *really* bad). If a 250A Fuse blows, there is a good chance that replacing the fuse will be the least of your worries.

Well, it doesn't eliminate the resistance, it just changes it to a fuse(which also has connection points) instead of crimps plus cable.

I'll see what the installer comes back with on the breakers front and take it from there. I can always upgrade the Lynx Power In to take fuses if need be.

 

[da9el84]

One other consideration is a breaker allows someone to thoughtlessly open a breaker under load.
Depending on the breaker that can be much worse for the breaker than tripping on over-current.

Isn't this exactly what a breaker does in normal operations in a house? It trips due to something going wrong, then you rectify that error and then you turn it "on" again once the reason for it tripping is fixed, instantly having it under load?

 

[John Frum]

Isn't this exactly what a breaker does in normal operations in a house?

AC crosses zero voltage ~50 times a second which makes it much easier to break AC.
DC is a whole other ballgame with very interesting physics.

It trips due to something going wrong, then you rectify that error and then you turn it "on" again once the reason for it tripping is fixed, instantly having it under load?

Not sure of your context or your meaning here.

I'm talking about administratively opening a breaker which can have a different mechanism than a breaker automatically opening on over-current.

 

[John Frum]

Explaining the physics is way above my pay grade.

 

[FilterGuy]

Well, it doesn't eliminate the resistance, it just changes it to a fuse(which also has connection points) instead of crimps plus cable.

The fuse resistance is there in both models. The difference is that the number of crimps and connections between the fuse and the bus bar is greatly reduced.

Isn't this exactly what a breaker does in normal operations in a house?

As @John Frum mentioned a DC breaker is a different beast than an AC breaker. It is a lot harder to build a reliable DC breaker than an AC breaker. On top of that, the market for AC breakers is orders of magnitude larger than for high-current DC breakers so the economy of scale makes the AC breaker cheaper. Furthermore, the scenario you are talking about is typically tripping a 15A-30A breaker. (Have you ever seen the 100A or 200A main breaker pop? Probably not. It is extremely rare)

Finally, in the scenarios you are talking about, the system has lots of receptacles that just about anything can be plugged into.... The chance of a short or just someone doing stupid is large. The DC systems we are talking about are designed as a complete system.... at design time it is known what will be on the system and the current protection devices should be selected to handle all known 'normal' scenarios.

 

[OzSolar]

What I really like about the distributor is that it allows you to combine Fuse Holders with the busbar.

Could I get some help on a few things?

I've used a couple of the Victron Distributors but as I recall they only have a place for four MEGA fuse connections. Are you ganging two together in order to accomplish the 6 fused connections?

And then what about a Class T fuse? MEGA's have an AIC of 1000 amps (right?) and it's often said here that by themselves aren't enough to safely stop a dead short in systems with LiFePO4. Why aren't any shown? I'm just asking questions because I'm here to learn. Thank you

 

[John Frum]

OP I suggest you look for arc flash/blast videos on youtube.
I won't post links here because gore is not ok on this forum.
Yes, I'm kind of trying to scare you.

 

[FilterGuy]

AC crosses zero voltage ~50 times a second which makes it much easier to break AC.
DC is a whole other ballgame with very interesting physics.

Expanding on that a little:
When a breaker opens, there will almost always be an arc. The size of the arc is dependent on a lot of things relating to the current load and the characteristics of the circuit the breaker is opening. (A pure resistive load will have a much smaller arc than an inductive load).

Once an arc forms, it ionizes the air, creating a low resistance path for the current to keep flowing. All breakers are designed to snap the contacts open as quickly as possible. This reduces the chance for the arc to form the ionized path. There are many other ways to snub the arc, but as @John Frum mentions AC breakers have the huge advantage of the current going to zero twice during each sine wave (it will cross zero 120 times a second for a 60hz system) This alone is enough to kill the arc in many situations.

With DC current, once an arc forms it tends to keep going unless something changes. One common way to snub an arc is to use magnets to draw the arc out through a path that is so long that it goes out. They can also use magnetics to 'blow' it out. Many of these magnetic techniques are dependent on current direction and therefore the breaker is polarized. ( Nonpolarized DC breakers tend to be less common and more expensive because they are denied the easier ways of snubbing the arc.) This is very important to understand because 1) a polarized breaker will not pop if the current is backward and 2) if the breaker is manually turned off with the reverse current, it won't snub the arc and can easily start a fire.

In the design diagrams above, the breakers are between the battery and the inverter. Consequently, the current could be flowing in either direction and the breakers must be non-polarized breakers.

 

[FilterGuy]

I've used a couple of the Victron Distributors but as I recall they only have a place for four MEGA fuse connections. Are you ganging two together in order to accomplish the 6 fused connections?

Yes

And then what about a Class T fuse? MEGA's have an AIC of 1000 amps (right?) and it's often said here that by themselves aren't enough to safely stop a dead short in systems with LiFePO4. Why aren't any shown? I'm just asking question because I'm to learn. Thank you

You have touched on something I do not have a good answer for and I wish I had a better understanding of. Victron is a quality company and I have no problem following their recommendations. People trust their systems on boats that go hundreds of miles out to sea. Even small problems can quickly turn deadly. Having said that I don't understand why the AIC of the MegaFuses is not a problem.

Note/warning: Run-of-the-mill mega fuses are only rated for 32V systems, but Victron sells Meag fuses rated for 48V systems.

Warning: The following is likely to be very controversial!!

I still recommend using a Class T Main fuse..... but I also wonder if a class T is really necessary.
The first reason I wonder this is that Victron does not use them and I have not heard of issues.
The second reason I wonder is that there are a lot of people building systems without Class-T fuses and yet we don't hear about systems where the fuse blows with a sustained arc.
The third reason I wonder is that many folks have a T-class main fuse and then other fuses downstream of the main fuse.... Sometimes even tiny automotive fuses. We don't hear about these tiny fuses with small AIC ratings having sustained arcs.... but if the problem was real, why wouldn't they?

None of my points above proves there is not a potential problem..... but it makes me wonder.

So.... what is going on? I can only speculate.
The whole argument for having a T-Class fuse is that since LiFePO4 is such low resistance the current can instantly go to many thousands of AMPS.
This is true, but the argument does not consider the resistance of the rest of the system. Crimps, connections, BMSs, wires, disconnects, etc, all add resistance to the system. Could it be that the feared 20,000A short circuit never really happens? I wish I had the money time and equipment to set up tests to actually measure the actual short circuit surge on a few systems.

 

[John Frum]

You have touched on something I do not have a good answer for and I wish I had a better understanding of.

The mega fuses are ok for branch circuits.
The branch circuit fuse's goal is be the first line of defense and hopefully isolated a fault to a single circuit.
If it fails to isolate the fault the feeder(battery) fuses are the last line of defense.
The feeders are fused as close as possible to the battery positive terminal and should have sufficient breaking capacity for their intended use.

 

[FilterGuy]

The mega fuses are ok for branch circuits.
The branch cicuit fuse's goal is be fire first line of defence and hopefully isolated a fault to a single circuit.
If it fails to isolate the fault the feeder(battery) fuses are the last line of defence.
The feeders are fused as close as possible to the battery positive terminal and should have sufficient breaking capacity for their intended use.

If the branch circuit has a short, why wouldn't the current instantly go to the feard thousands of amps? When the branch fuse blows and a sustained arc results, will the current still be high enough to blow the feeder fuse? If not, it is a fire issue. I am confident in saying the arc path is resistive, but I do not know how resistive it will be so I can't guess what the magnitude of the current through the arc would be. I have tried to work through the math on this but I just don't have enough info to work through it. The assumptions I would have to make would be pulled out of a dark spot. (and I am not talking about Alaska in the winter)

I brought this up in a post many months ago but the resultant discussion did not provide any satisfactory answers on why we are not seeing problems on systems that have low AIC-rated fuses.

Calculating AIC for fuse selection.

Not only this, but a fuse with an insufficient AIC rating will simply allow an arc to set up and current will continue flowing, plus the intense heat of the arc will cause obvious problems.

diysolarforum.com

(I see @John Frum was on that thread too ).

 

[John Frum]

If the branch circuit has a short, why wouldn't the current instantly go to the feard thousands of amps?

It may but then the battery fuses will blow if the branch fuse does not.

When the branch fuse blows and a sustained arc results, will the current still be high enough to blow the feeder fuse?

Yes the lowest rated fuse is 80 amps which means the lowest rated wire is 6 awg.
6 awg should survive long enough for the battery to trip like dominoes.
That is the idea anyways.

If not, it is a fire issue. I am confident in saying the arc path is resistive, but I do not know how resistive it will be so I can't guess what the magnitude of the current through the arc would be. I have tried to work through the math on this but I just don't have enough info to work through it. The assumptions I would have to make would be pulled out of a dark spot. (and I am not talking about Alaska in the winter)

This really would be a good video topic for thunderfoot.

I brought this up in a post many months ago but the resultant discussion did not provide any satisfactory answers on why we are not seeing problems on systems that have low AIC-rated fuses.

Calculating AIC for fuse selection.

Not only this, but a fuse with an insufficient AIC rating will simply allow an arc to set up and current will continue flowing, plus the intense heat of the arc will cause obvious problems.

diysolarforum.com

(I see @John Frum was on that thread too ).

I made a spreadsheet for playing what if with dead short current based on path resistance.
I think I linked it early on in this thread.
If not I will attach it here.

 

[John Frum]

Here is the spreadsheet I mentioned.

 

[John Frum]

In that thread I posted this link https://www.bluesea.com/support/articles/Circuit_Protection/98/DC_Circuit_Protection

It shows requires different breaking capacity for feeders and branches but the values are pretty low I suspect relevant to lead acid batteries.

 

[FilterGuy]

Here is the spreadsheet I mentioned.
View attachment 106851

Where did you get the resistance values for the various items? Do you include the resistance of the crimps anywhere? The reason I ask is that from experience systems have higher resistance than what is shown in the 'total ohms' column.

This speaks to why I am questioning things. The theoretical numbers and scenarios don't seem to match what is actually happening in the field.

 

[John Frum]

Where did you get the resistance values for the various items? Do you include the resistance of the crimps anywhere? The reason I ask is that from experience systems have higher resistance than what is shown in the 'total ohms' column.

This speaks to why I am questioning things. The theoretical numbers and scenarios don't seem to match what is actually happening in the field.

Those numbers are order of magnitude guesses and joinery resistance is ignored. I suspect joining resistance is quite variable.

 

[FilterGuy]

In that thread I posted this link https://www.bluesea.com/support/articles/Circuit_Protection/98/DC_Circuit_Protection

It shows requires different breaking capacity for feeders and branches but the values are pretty low I suspect relevant to lead acid batteries.

The Blue Sea article talks about lower AIC ratings downstream from the main protection device but does not address why they can have a lower AIC rating. (And yes, the article primarily had numbers for Lead Acid)

 

[Steve_S]

Ohhh DANG IT, Here's me tossing a Rock in the Pond snicker snicker...

It has to do with reading Battery Pack Voltages.
Every Fuse, Breaker or device between the Solar Controller & Actual physical battery pack "has a price" ! Voltage Drop ! It may not be big pending on "gadget" and it is cumulative and does add up. This also applies to what the Inverter Subsystem is reading. Remember to use a Good DVOM (preferably 2 decimal place accurate) to measure the differences. People, you'd be surprised at how much drop there can be. Lithium Based is NOT Brute force like Lead Acid and it counts.