Can a short down to a small load wire blow a main battery T class fuse?

[filippomasoni]

Fuses and breakers are meant to protect wires and we place them close to the battery side as possible to each different size wire.

So for example a 200A class T fuse to the battery, than at the busbar each big load gets a fuse, than say a 30A fuse to a fuse box and then each small load (2-5A) gets a blade fuse. Once we get to a load, some actually have an integrated fuse to protect the internal wire, but when we get to small 5v or 3.3v circuits we usually don't find any fuses.

If a short happens down the line at a load, will the blade fuse of that specific load blow first or will the class T fuse, which is usually fast acting and therefore quicker?

As far as I understand the small wire of that load won't be capable of carrying the current necessary for the class T fuse to blow, essentially acting as a fuse itself which means the fuse of that wire has to blow first, but I'm not sure if that's correct.

Furthermore, using DC hydraulic magnetic circuit breaker (like airpax sensata) the magnetic feature should quickly close the circuit in case of a short before the temperature raises, but still will this be quicker than a class T fuse at the battery?

 

[rodrick]

Your down stream fuses should go first unless you are running right at the max amps of the class t fuse when a short were to happen then it could possibly blow

 

[filippomasoni]

So following the previous example, if I have a total load of 180A and a short happens on a small load downstream, a 5V circuit for example, that additional current can make the 200A class T fuse blow?

 

[1234enough]

The loads on anything, are the loads.
If a fuse is rated to blow at a given amps/time then it should blow at that amps/time( Bear in mind fuses generally do not blow immediately at thier given rating), doesn't matter the individual loads applied downstream, its the total load on that individual item.
In your example for Instance, if the 5v circuit running continually at 20a+ was protected with a 40a fuse which was supplied from a 200a fuse which was already at 180a load, then yes 200a fuse would potentially blow / be above its designed spec.

The sum of loads downstream from main fuse can often theoretical be greater, but liklihood of it happening should be allowed for in the design.
Just look at your house electrical DB - main breaker may be 100a, but sum of sub breakers are likely to be 100a +, but the liklihood of you pulling 6a+ from upstairs lights + 6a+ from downstairs lights + 6a from External security lights + 32a+ from cooker + 20a+ from the three socket circuits + 6a+ from fire alarm ect ect are theoretically low, so design is acceptable and fit for purpose.

If a breaker/fuse blows this indicates overload / malfunction or incorrect design and investigation should be undertaken to determine why.

In summary - design system protection that safely provides suitable protection.

 

[filippomasoni]

Regarding the example the 5V circuit, would probably have a 5A breaker (which is the smallest airpax available) or maybe a 2A blade fuse, as the wires for those small circuits will be small for the small load and the fuse sized accordingly. Not a 40A fuse like you suggested.

I will have many small low voltage (5V or 3.3V) circuits in my camper installation and they will be spread around the camper with small wires because their power draw is tiny, we are talking about 70mA at 3.3V. I'm planning to bring the 24V from the battery with small wires and properly sized fuses and then step down to 5V or 3.3V with a tiny dc-dc buck converter at the circuit to avoid voltage drop. The fuse/breaker will be at the 24V busbar/battery side protecting the wire run.
Having many of this tiny circuits around the camper, connected to various sensors or controlling pwm fans etc has a higher potential risk of short than around the big loads/chargers close to the battery in the enclosed electrical cabinet.

What I'm trying to understand is if a short downstream at a tiny circuit board can be a potential disaster as a short close to the battery.

If that's the case, will having an isolated dc-dc converter, for example 24V to 12V to run all of the small circuits (which is then again lowered to 5 or 3.3V) be a safety layer between the big loads and the tiny ones?

 

[1234enough]

Chances are the loads you indicate would have no effect on a large main fuse.

If the tiny circuits short and have associated tiny protection, it would be unlikely to cause issues with a much larger main fuse.

If you have many items at 3.3v or 5v, have you considered a central point of conversion to required voltage( one dc-dc converter for each voltage ) - then distribution of this around to required locations.

 

[mikefitz]

trying to understand is if a short downstream at a tiny circuit board can be a potential disaster

Assuming there is a lower value fuse in the path or the fusing value of the PCB track is much lower, (that it will be), then the 200A class T will allow 500A or more for up to 5 seconds. The down stream low value fuse will fail within this time.

 

[PVGeezer]

What you are concerned about is very legitimate and what you are looking for is what in professional power electrical engineering is called a fuse (or more generally a protection) coordination study. You are on the right track reviewing the fuse trip curves.

I suggest you create a full schematic of your electrical system (not a single line) with the location of each OCPD identified. Then methodically go through it placing imaginary faults on each conductor.

Consider that faults may be line to ground or line to ground, that the faults might be low resistance (resulting in high currents that are able to clear OCPDs with high current ratings) or high resistance (creating low currents that require OCPDs with lower current ratings).

Consider also the case of more than one ground fault occuring simultaneously on conductors at different voltages. This is especially important for systems without a grounded neutral and is why the electrical codes require OCPDs in all ungrounded conductors.

Also very important for our systems that have multiple current sources (parallel solar and battery strings, generator grid, etc) is to consider all the sources that can flow into each fault from different directions.

For each fault scenario above look at the trip curves of your OCPDs and determine which one will trip first and if all your conductors will be fully protected in each case.

This all sounds rather tedious and yes it can be but it's really the only way to be certain your system is safe. I promise that once you go through this you will feel confident that you understand and have addressed all the potential faults your system might encounter.

 

[filippomasoni]

Chances are the loads you indicate would have no effect on a large main fuse.

If the tiny circuits short and have associated tiny protection, it would be unlikely to cause issues with a much larger main fuse.

If you have many items at 3.3v or 5v, have you considered a central point of conversion to required voltage( one dc-dc converter for each voltage ) - then distribution of this around to required locations.

That was my initial though. For example I've had a minifridge fail and it blew the internal glass fuse, but didn't trip the breaker inside the house.

I've though of having a central dc-dc for 12V, 5V and 3.3V but even if the runs are relatively short they can still be 3-4m in some cases, not very practical for 3.3V the voltage drop would be too much and require unnecessary big wires. It's simpler and cheaper to bring 24V everywhere and step down at the device, I'll 3d print small enclosures with the converter all soldered inside to the esp32. Also in some cases I would need multiple voltages, like 12V for a fan and 3.3V for the esp that controls pwm.
I could have a central isolated dc-dc to make 12V and then run that throughout the camper only if the fact that's isolated makes it safer, which I'm not sure.

What you are concerned about is very legitimate and what you are looking for is what in professional power electrical engineering is called a fuse (or more generally a protection) coordination study. You are on the right track reviewing the fuse trip curves.

I suggest you create a full schematic of your electrical system (not a single line) with the location of each OCPD identified. Then methodically go through it placing imaginary faults on each conductor.

Consider that faults may be line to ground or line to ground, that the faults might be low resistance (resulting in high currents that are able to clear OCPDs with high current ratings) or high resistance (creating low currents that require OCPDs with lie current ratings).

Consider also the case of more than one ground fault occuring simultaneously on conductors at different voltages. This is especially important for systems without a grounded neutral and is why the electrical codes require OCPDs in all ungrounded conductors.

Also very important for our systems that have multiple current sources (parallel solar and battery strings, generator grid, etc) is to consider all the sources that can flow into each fault from different directions.

For each fault scenario above look at the trip curves of your OCPDs and determine which one will trip first and if all your conductors will be fully protected in each case.

This all sounds rather tedious and yes it can be but it's really the only way to be certain your system is safe. I promise that once you go through this you will feel confident that you understand and have addressed all the potential faults your system might encounter.

I'll look into fuse coordination study. I'm currently making a full schematics of the system and I'll post that when done, probably in the Vehicle Mounted Systems category as I'm also concerned with properly grounding everything to chassis.

Whenever possible I want to have breakers, the 80VDC rated Airpax IEG, which I can easily find here in Italy from the many marine distributors or online shops for about 15-20 euro, but they only go down to 5A and, at least the ones I found, are the trip curve number 72, which is long delay.
Here's the specsheet https://www.sensata.com/sites/default/files/media/documents/2018-04-24/iag.pdf at page 16 the trip curves.

As these are Magnetic Circuit Protectors, my understanding is the magnetic will trip instantly when an inrush short is detected, is the trip curve then applicable only for the thermal part of the breaker?

 

[PVGeezer]

As these are Magnetic Circuit Protectors, my understanding is the magnetic will trip instantly when an inrush short is detected, is the trip curve then applicable only for the thermal part of the breaker?

Yes that is the advantage of thermal-magnetic breakers. This shows up in the trip curves which characteristically have a two step curve, very fast response to high current surges followed by a slow response to low currents. In this case, the trip curves should show both the thermal and the magnetic functions.

 

[PVGeezer]

I'll look into fuse coordination study. I'm currently making a full schematics of the system and I'll post that when done, probably in the Vehicle Mounted Systems category as I'm also concerned with properly grounding everything to chassis.

If you are grounding the negative side of your system to the chassis as is standard practice with vehicle mounted systems then there is no need to provide OCPDs in the negative (neutral) conductors.

For systems using the newer all in one inverters, which often don't allow for the DC conductors to be grounded, there need to be OCPDs in both the positive and negative legs of the battery conductors to assure safety in the event ground faults occur. If those aren't there and ground faults are present in both legs then the unprotected conductors can overheat a cause a fire.

 

[filippomasoni]

If you are grounding the negative side of your system to the chassis as is standard practice with vehicle mounted systems then there is no need to provide OCPDs in the negative (neutral) conductors.

For systems using the newer all in one inverters, which often don't allow for the DC conductors to be grounded, there need to be OCPDs in both the positive and negative legs of the battery conductors to assure safety in the event ground faults occur. If those aren't there and ground faults are present in both legs then the unprotected conductors can overheat a cause a fire.

Yes that was the idea. I will have separate victron mppt, AC charger and dc-dc and for the inverter I'm planning on getting a Meanwell NTS-2200 (good compromise of quality and size/weight, as Victron inverters are almost 3 times as heavy) and it will be used only for the induction cooktop (limited to 2000W), everything else will be on DC.
I haven't found if the meanwell inverter has neutral and ground tied together, but it's not an all in one inverter and it has a dedicated grounding lug.

 

[PVGeezer]

Yes that was the idea. I will have separate victron mppt, AC charger and dc-dc and for the inverter I'm planning on getting a Meanwell NTS-2200 (good compromise of quality and size/weight, as Victron inverters are almost 3 times as heavy) and it will be used only for the induction cooktop (limited to 2000W), everything else will be on DC.
I haven't found if the meanwell inverter has neutral and ground tied together, but it's not an all in one inverter and it has a dedicated grounding lug.

The Meanwell inverter will be ok for you even without a grounded neutral on the AC output. Because you are dedicating it to a single load you will be running two 15A rated conductors (assuming you are using the EU 220V version) directly to the cooktop. Only the single (internal) circuit breaker is required because there are no branch circuits with small conductors that need to be protected.

Edit: of course you should ground both the inverter case and the cooktop case to the chassis.

 

[filippomasoni]

Yes I'll be using the EU 220V version. That's great to know, I didn't know a breaker wasn't required if only a circuit was connected, but it makes sense.
But in reality maybe having an AC outlet for occasional use, I never know if I'll need to plug something with an AC adapter, even just for an emergency, might be useful. So in that case I guess I do need a breaker appropriate for the wires. It's usually 10A breaker for 1.5mm2 and 16A for 2.5mm2 at least here in Italy and I guess most of Europe. I'll probably go with 2.5mm2 wires anyway and was thinking of having a single main RCCB that also has ground fault protection, which is usually standard here, but I'm unsure of how it can work on a chassis grounded system.

 

[robbob2112]

Yes that was the idea. I will have separate victron mppt, AC charger and dc-dc and for the inverter I'm planning on getting a Meanwell NTS-2200 (good compromise of quality and size/weight, as Victron inverters are almost 3 times as heavy) and it will be used only for the induction cooktop (limited to 2000W), everything else will be on DC.
I haven't found if the meanwell inverter has neutral and ground tied together, but it's not an all in one inverter and it has a dedicated grounding lug.

Using the chassis as a ground is inviting corrosion at any point where the wires connect to the chassis...

Something Henry ford did to save a few pennies over a hundred years ago and we still do it... but unless the connection is tight and protected with some sort of grease at every point you can end up with a bad negative connection ....

I had an old ford pickup that was forever having the brake lights not work... the cause was always a loose wire or a bit of rust...

My recommendation is always run both poles to everything if you are running the wires or have the option and you will have fever problems.

I didn't see it mentioned - but the time/current curve on a fast class T is nearly 10 minutes at 2 x current... Most smaller fuses have a faster time/current curve than that...

But we all are supposed to use fuses and wires that are 125% of the expect/rated current. This prevents false blows for no reason other than surge.


Breakers all have trip curves that are time verse current and usually they are at a specific temperature and they will trip faster at higher temp than lower temps...


Summary - I wouldn't expect a small fuse to ever blow after a larger main one even if your total of fuses are way more than the class T. Where you get into trouble is if you have multiple fuses and breakers of the same size in line - in that case - which one blows has a lot of factors and takes an engineer to figure an educated guess at it.

 

[PVGeezer]

Yes I'll be using the EU 220V version. That's great to know, I didn't know a breaker wasn't required if only a circuit was connected, but it makes sense.
But in reality maybe having an AC outlet for occasional use, I never know if I'll need to plug something with an AC adapter, even just for an emergency, might be useful. So in that case I guess I do need a breaker appropriate for the wires. It's usually 10A breaker for 1.5mm2 and 16A for 2.5mm2 at least here in Italy and I guess most of Europe. I'll probably go with 2.5mm2 wires anyway and was thinking of having a single main RCCB that also has ground fault protection, which is usually standard here, but I'm unsure of how it can work on a chassis grounded system.

To be clear, I'm not suggesting not having a circuit breaker for your AC inverter output. The Meanwell unit already has one internally, you don't need a second one. Breakers are meant to protect the conductors, so you can connect a second set of wires to the inverter output and that won't need a separate breaker as long as both sets of wires are rated for 15A.

 

[filippomasoni]

Using the chassis as a ground is inviting corrosion at any point where the wires connect to the chassis...

There are a couple of grounding bolts on the chassis by factory on my Ford Ranger probably for light etc, but I plan on using the factory recommended negative for a dc-dc by Ford in the BEMM which is in the body of the engine bay, just under the battery.
That will go to a negative busbar that will be connected to the dc-dc and essentially be my connection to chassis and also the negative of the engine battery to run the dc-dc charger. It's also recommended here: https://diysolarforum.com/resources/grounding-made-simpler-part-4-mobile-systems.159/

I didn't see it mentioned - but the time/current curve on a fast class T is nearly 10 minutes at 2 x current... Most smaller fuses have a faster time/current curve than that...

But we all are supposed to use fuses and wires that are 125% of the expect/rated current. This prevents false blows for no reason other than surge.


Breakers all have trip curves that are time verse current and usually they are at a specific temperature and they will trip faster at higher temp than lower temps...


Summary - I wouldn't expect a small fuse to ever blow after a larger main one even if your total of fuses are way more than the class T. Where you get into trouble is if you have multiple fuses and breakers of the same size in line - in that case - which one blows has a lot of factors and takes an engineer to figure an educated guess at it.

Thanks, that was also my theory. Looking at the trip curve of the airpax breaker a 5A version with 1000% current, ie 50A will trip in 0.01 to 0.2 sec, so well before the class T will start to get hot, even if I'm already at max load, and I've sized the system with a big margin.

 

[PVGeezer]

Using the chassis as a ground is inviting corrosion at any point where the wires connect to the chassis..

I'm not suggesting using the chassis as a return current carrying path, just as the protective ground for the inverter and appliances.

I didn't see it mentioned - but the time/current curve on a fast class T is nearly 10 minutes at 2 x current... Most smaller fuses have a faster time/current curve than that...

But we all are supposed to use fuses and wires that are 125% of the expect/rated current. This prevents false blows for no reason other than surge.

Yes. Size the OCPD for 125% of the load current and the conductors for the OCPD rating or higher. If it's a solar circuit then size the conductors for 1.56 x Isc. No need for fuses in the PV circuits unless you are combining more than 2 strings of modules in parallel.

Breakers all have trip curves that are time verse current and usually they are at a specific temperature and they will trip faster at higher temp than lower temps...


Summary - I wouldn't expect a small fuse to ever blow after a larger main one even if your total of fuses are way more than the class T. Where you get into trouble is if you have multiple fuses and breakers of the same size in line - in that case - which one blows has a lot of factors and takes an engineer to figure an educated guess at it.

It is very possible to have a higher rated OCPD trip before a lower rated one if the higher rated one has a faster response time. Look to see if the trip curves cross over each other.

 

[robbob2112]

For a class T to blow in 0.01 seconds the current needs to be around 10x rated current at that point... the higher the current the faster the blow....

I'm not suggesting using the chassis as a return current carrying path, just as the protective ground for the inverter and appliances.


Yes. Size the OCPD for 125% of the load current and the conductors for the OCPD rating or higher. If it's a solar circuit them size the conductors for 1.56 x Isc. No need for fuses in the PV circuits unless you are combining more than 2 strings of modules in parallel.

It is very possible to have a higher rated OCPD trip before a lower rated one if the higher rated one has a faster response time. Look to see if the trip curves cross over each other.

I read a really good article that Tim linked the other day on OCPD in lines.... in that case talking about breakers in an industrial setting -- and how a breaker at a machine could stay on while the whole building breaker could trip and all about trip curves...

This is some stuff I copied into my answer thread for reference on the topic. Now I wish I had the link to that article.

Post in thread 'Robbob2112 Answer thread'

Jan 16, 2025

Breaker trip curves and meanings

Tripping Curves of Circuit Breaker. B, C, D, K & Z Trip Curve

Types of Circuit Breaker Based on its Tripping Curves. What is a Trip Curve & How to Read it? Why We Need Different Tripping Curves for MCB?

www.electricaltechnology.org

What does B,C,D curve mean for Acti 9 MCB? | Schneider Electric Australia

And this (copy/pasted because things disappear and links go dead)

Circuit breakers curves

Circuit breakers are critical components of electrical systems. They are designed to protect electrical equipment from damage due to overcurrent or short circuits.

www.linkedin.com

• robbob2112

• 

 

[PVGeezer]

It's bad practice in an industrial setting to have an upstream breaker trip before a downstream one but on a small vehicle (or even a residential) system it's really more of a nuisance than anything. What's critical is that something trips so your conductors don't cause a fire.

This is where going through the fuse coordination study really helps. In doing that you might find that the OCPD combinations could cause an upstream one to trip first in some cases but you accept that because it is cost effective and everything is safe. Then later if it happens you'll know what to look for and how to correct it.