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Fuse/Circuit breaker Sizing and placement.

FilterGuy said:
Yes. Jeff has a *lot* of videos out there and is a great resource for learning about low voltage electrical wiring. He is all about doing it right, he has a lot of experience and he knows what he is talking about.
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Agreed, a lot of the content in his videos is pretty basic but I appreciate how comprehensive they are and packed with info, and its the type of stuff that never hurts to learn and relearn a few times until its drilled into you, his content provides a good solid base of general knowledge, and I like his system based approach.
 
FilterGuy said:
Yes. Jeff has a *lot* of videos out there and is a great resource for learning about low voltage electrical wiring. He is all about doing it right, he has a lot of experience and he knows what he is talking about.
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Check out this page on making cables and give your opinion.... I haven't seen the different types of lugs covered much, or I just missed it.
The tip about rotating the lug 90 degrees and crimping again to get rid of the sharp corners is not something I have seen before.

https://marinehowto.com/making-your-own-battery-cables/
 
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Blue Sea Systems has some good resources on fusing and protection devices:

Each article is no more than a 3 minute read.
  • Also check out Blue Sea's circuit wizard (A very good wire size calculator, that explains the results, gives recommendations and links to wiring, and even has the ability to recommend protection devices)
 
Are there any rules of thumb on using circuit breakers over fuses- it it a $ for convenience thing? (My consideration is for charging sources to a battery)
 
FilterGuy said:
I found this nice fuse type comparison document.
http://assets.bluesea.com/files/res...o_Blue_Sea_Systems_Fuses_and_Fuse_Holders.pdf
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I also recently discovered this reference chart (and posted to the beginners section). Its a good reference, very information-dense, but reasonably clear. Here is a high resolution (click on the picture and click the magnifiying glass for full res) JPEG of the chart for anyone who doesn't want to download a PDF:

QyON0UX.jpg

And a link to Blue Sea's circuit wizard which will recommend wire gauge and circuit protection based on the parameters you specify. I have found there knowledgebase to be a great learning and resource.
 
Not sure if this belongs here or not, but I wanted to let people know about Victron's system schematics, because I have found them very helpful in understanding how all the components of a system fit together (or at least how Victron intends them to), and a lot can be inferred about circuit protection from the schematics.

Here are two example Schematics:
Multiplus | MPPT | Orion-TR | Lithium | Battery Protect (more complex)
Multiplus | MPPT | AGM (less complex)

The system schematics can be found in the same place as the datasheets and the manuals on Victron's website. Choose the product you are interested in scroll down to the 'downloads' section and if there is a schematic available it will be listed there.

Screenshot_2020-03-19 Model - 1 6KVA-12V-MultiPlus-230-Volt-system-example-4-PIN-VE-Bus-BMS-Li...png
 
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The Class T fuses are expensive, but I trust them the most for a main battery fuse. They have extremely high voltage and current interruption capability, and zero chance of melted debris dropping somewhere when they blow (unlike some ANL fuses). Everything is sealed inside a ceramic casing.
 
orangeminnie said:
The Class T fuses are expensive, but I trust them the most for a main battery fuse. They have extremely high voltage and current interruption capability, and zero chance of melted debris dropping somewhere when they blow (unlike some ANL fuses). Everything is sealed inside a ceramic casing.
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I believe they are also 'fast acting' which is one reason inverter manufacturers often recommend them between the battery and inverter.

Here is an excerpt from the Samlex Evo manual:
3.5.4 Fuse Protection In The Battery Circuit
A battery is an unlimited source of current. Under short circuit conditions, a battery can supply thousands of Amperes of current. If there is a short circuit along the length of the cables that connects the battery to the inverter, thousands of Amperes of current can flow from the battery to the point of shorting and that section of the cable will become red-hot, the insulation will melt and the cable will ultimately break. This interruption of very high current will generate a hazardous, high temperature, high-energy arc with accompanying high-pressure wave that may cause fire, damage nearby objects and cause injury. To prevent occurrence of hazardous conditions under short circuit conditions, the fuse used in the battery circuit should limit the current (should be "Current Limiting Type"), blow in a very short time (should be Fast Blow Type) and at the same time, quench the arc in a safe manner. For this purpose, UL Class T fuse or equivalent should be used (As per UL Standard 248-15). This special purpose current limiting, very fast acting fuse will blow in less than 8 ms under short circuit conditions. Appropriate capacity of the above Class T fuse or equivalent should be installed within 7” of the battery Plus (+) Terminal (Please see Table 3.1 for fuse sizing).

Marine Rated Battery Fuses, MRBF-xxx Series made by Cooper Bussmann may also be used. These fuses comply with ISO 8820-6 for road vehicles.
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More info in this video
 
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So I've been pondering inverter circuit protection lately, and thinking about how the conventional wisdom of circuit protection doesn't necessarily apply to an inverter.

While it is as important as ever to size your fuse to protect the wire, it may also be practical and advisable to size your fuse to protect the device. In the case of an inverter, the conventional rule of thumb that 'a fuse should be roughly 1.25x the max amperage of a device' needs to be... wait for it... inverted (sorry I couldn't help myself :rolleyes:). Point being, maybe the fuse on an inverter circuit should be <100% of the inverter rating, so your breaker will trip before the inverter is harmed.

With typical loads, you want to ensure your fuse won't blow everytime your device is pulling max amps, so a little breathing room makes sense, so you size the fuse a bit bigger than the load. But an inverter isn't a traditional load, its just passing along power to the AC loads.The implications of this are twofold:
  1. There is no reason an inverter ever needs to pull its rated output
  2. It can and will exceed its rated output--to its detriment--if an AC load exceeds your inverters rating, the inverter will try to deliver, even if it damages itself in the process.
For this reason, I'm thinking an inverter should be treated more like a wire than a load. I.E. make sure the fuse blows before the capacity of the inverter is exceeded so that an improperly sized load on the AC side doesn't damage your inverter or pose a safety risk.

It may be ideal to have two separate protection devices in the circuit, one at the beginning of the circuit to protect the wire and sized using the traditional method, the second (resettable breaker) at the inverter sized to trip before the inverter exceeds its max load. See below:

inverter-protection.png

If my explanation wasn't clear this video is what got me thinking about the idea, and might do a better job explaining than I can.

I'm curious to hear everyone's thoughts.
 
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Craig said:
I think its a fair concept. I still have not figured out why we are so worried about protecting the wire. Wire is cheap Inverters not so much.
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I would say to protect against a fire would be one reason to "protect" the wire. We're not so much protecting the wire, we're protecting everything from the bad that could happen if we seriously overloaded the wire.
 
Sgt Raven said:
I would say to protect against a fire would be one reason to "protect" the wire. We're not so much protecting the wire, we're protecting everything from the bad that could happen if we seriously overloaded the wire.
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Fair enough but if your wire goes something else is gonna go with it provided you have properly sized wire.
 
Craig said:
I think its a fair concept. I still have not figured out why we are so worried about protecting the wire. Wire is cheap Inverters not so much.
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If you watch the first 5 or 10 minutes of the video I posted above, It explains it decently. I believe the reasoning is twofold:
  1. Safety, 'protecting the wire' really means, protecting yourself and your equipment by protecting the wire. Placing the fuse at the beginning of the circuit, and sizing the fuse to blow before the wire overheats means reduced risk of fire.
  2. Feasability. Its within a builders ability to fuse a circuit to protect the wiring, but from what I understand its not always possible to size a fuse to effectively protect a device, at least not enough to count on.
I think the other thing to keep in mind about the saying is that it doesn't mean the sole purpose of a fuse is to protect the wire, just that primary purpose usually is.

If you look at the first example in the diagram, it (at least attempts) to accomplish both wire/short circuit protection, and device protection. (There should ideally be a fuse on the + battery terminal, but I elected to leave that out for simplicity's sake).

Dzl said:
...
For this reason, I'm thinking an inverter should be treated more like a wire than a load. I.E. make sure the fuse blows before the capacity of the inverter is exceeded so that an improperly sized load on the AC side doesn't damage your inverter or pose a safety risk.

It may be ideal to have two separate protection devices in the circuit, one at the beginning of the circuit to protect the wire and sized using the traditional method, the second (resettable breaker) at the inverter sized to trip before the inverter exceeds its max load. See below:

inverter-protection.png

If my explanation wasn't clear this video is what got me thinking about the idea, and might do a better job explaining than I can.

I'm curious to hear everyone's thoughts.
Click to expand...
 
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I guess my point is the wire should be ample to supply whatever device you have. the fuse/circuit breaker should also be prepared to protect device. That fuse would also "protect the wire" Because the wire rating is larger than the device rating. So the wire gets protected by default.
 
Craig said:
I guess my point is the wire should be ample to supply whatever device you have. the fuse/circuit breaker should also be prepared to protect device. That fuse would also "protect the wire" Because the wire rating is larger than the device rating. So the wire gets protected by default.
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Sort of yes. I agree with you as it relates to wire/fuse size (if you properly size your wire and your fuse the wire is protected by default) but fuse placement is also very important. I believe the phrase gained popularity to try to correct the widespread practice among newbies of placing the fuse @ the device rather than as close to the beginning of the circuit as possible. As well as a way to teach 'intent.' If you understand why you need to use a fuse, its easier to be confident in how to protect a circuit.
 
Craig said:
I guess my point is the wire should be ample to supply whatever device you have. the fuse/circuit breaker should also be prepared to protect device. That fuse would also "protect the wire" Because the wire rating is larger than the device rating. So the wire gets protected by default.
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Think about short circuits. If you drop a wrench across positive and negative, it will be drawing whatever the battery can supply. At this point the only thing in the circuit is the battery, the wire, the wrench and the fuse. The fuse is the only thing protecting the wire (and it's insulation) from turning into a very large cloud of very nasty smoke and possibly a ball of fire.
 
If you watch the whole video that @Dzl linked to, it has some great (and sobering) videos that demonstrate the point of 'protecting the wire'.

Note: That video is one in a series. In total they are several hours long but I watched them all. A lot of it is review of the basics but it was still worth watching and I recommend them to anyone working with batteries. (I set the Youtube playback speed to 125%)
 
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FilterGuy said:
Think about short circuits. If you drop a wrench across positive and negative, it will be drawing whatever the battery can supply. At this point the only thing in the circuit is the battery, the wire, the wrench and the fuse. The fuse is the only thing protecting the wire (and it's insulation) from turning into a very large cloud of very nasty smoke and possibly a ball of fire.
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The BMS is also an OCPD device and they are usually spec to open pretty quick.
I don't have the specs in front of me but from memory its single digit milliseconds.
Faster than a fuse or breaker I believe.
I wouldn't bet my house on it though :)
 
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FilterGuy said:
Think about short circuits. If you drop a wrench across positive and negative, it will be drawing whatever the battery can supply. At this point the only thing in the circuit is the battery, the wire, the wrench and the fuse. The fuse is the only thing protecting the wire (and it's insulation) from turning into a very large cloud of very nasty smoke and possibly a ball of fire.
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If I drop a wrench across the terminals unless the fuse is the terminal how is a fuse gonna help. Please Im not trying to be argumentative. I just think I want my fuses protecting my devices and by default the wires will be protected.
Trust me I know from much experience the utility of fuses. Every time I trip a breaker I say WOW glad I had that there.
 
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