There has been a lot of talk about fuses and breakers in vans and “professional advice”, but it would transpire that none of them are professionals and are basing the information on hear say.
Lets actually look at FACTS.
What a Fuse Does:
- A fuse is a safety device designed to protect an electrical circuit from overcurrent.
- It’s a sacrificial component; it’s meant to break the circuit by melting when the current exceeds a safe level.
- This prevents damage to equipment and reduces the risk of fires.
How a Fuse Works:
- Basic Structure:
- A fuse typically consists of a thin wire or metal strip (the “fuse element”) enclosed within a protective housing (often glass, ceramic, or filled with a granular material).
- Normal Operation:
- When the current flowing through the circuit is within its normal range, the fuse element remains intact.
- Overcurrent Condition:
- If an overcurrent occurs (due to a short circuit, overload, or other fault), the increased current generates excessive heat in the fuse element.
- This heat causes the element to melt (or “fuse”).
- Circuit Interruption:
- The melting of the fuse element breaks the circuit, stopping the flow of current.
Arc Gaps and Their Effect:
The size of the arc gap required is dependant on the voltage of the circuit. Higher voltages require larger arc gaps.
Arcing:
When a fuse blows, especially in high-voltage or high-current circuits, an electrical arc can form across the gap created by the melted fuse element.
This arc is a sustained electrical discharge that can continue to conduct current, even after the fuse element has melted, which is very dangerous.
Arc Gaps and Arc Quenching:
To prevent this, fuses are designed with features to “quench” or extinguish the arc.
Arc gaps are a part of this process. The distance between the broken ends of the fuse element after it blows is the arc gap. A larger arc gap makes it harder for the arc to sustain itself.
Arc quenching mediums are also used. These mediums help to cool and extinguish the arc. Common methods include:
Granular fillers (e.g., sand): These materials absorb the heat of the arc and help to cool it.
Gas-filled fuses: Some fuses contain gases that help to extinguish the arc.
Specialized contact geometry that stretch and cool the arc.
Effect on Fuse Performance:
The design of the arc gap and the quenching medium significantly affects the fuse’s ability to interrupt high currents safely. A well-designed fuse will quickly and reliably extinguish the arc, preventing damage to the circuit and equipment.
Arcing –
Focus on interrupting rating:
- Instead of a precise arc gap, fuse specifications emphasize the “interrupting rating.” This rating indicates the maximum fault current that a fuse can safely interrupt.
A 12V 600 amp arc gap would be extremely small, likely only a few millimeters in length, due to the low voltage involved; even with a high current like 600 amps, the voltage needed to create a significant arc is not present at 12V, making it difficult to establish a sustained arc at this voltage level.
The Mega Fuse has a 50mm arc gap. To arc a 50mm gap, you would typically need a voltage in the range of several thousand volts depending on the air conditions and electrode geometry, with a current that can vary depending on the application, but generally in the range of tens to hundreds of amps to sustain the arc once initiated; a simple rule of thumb is that a larger gap requires a higher voltage to jump across it.
The math here is 3 × 10^6 V/m
So you can understand I took this as a base line and divided 3,000,000 by 100 then multiplied it by 10 to arrive at 300,000 volts for every 10cm gap. The design of the fuse (and they can be different) will alter the gap that has occurred when the fuse blows.
We offer mega fuses and we offer breakers. Inside the breaker looks like this:
The two green strips and the vertical bars is the part which stops the arc, it can be celled the arc arrester.
The break we sell is manufactured to IEC/EN608947-2 GB14048.2 which has a 6ka rating, This means it will break a fault current up to 6000amps.
What are breakers for?
A lot of people say they are for fault currents and they are, but when designing systems I tend to air on the “tolerance” side of things. By this I mean when things are not working correctly which may not be a fault.
If you have a 24v system, and a 3000w inverter you would expect a 125a current as the nominal current, there will be surges too, and both fuses and breakers have this tolerance. Therefore you fuse for 125 amps.
The breaker rather than a fuse is the choice as if you over run ( go out of spec) then the fuse will blow where as a breaker can be reset, where as the fuse has to be replaced. What you would take from a blown fuse or breaker tripping is that the system is undersized. – you need to upgrade.
We use breakers and fuses to protect the system parts. You may have heard people say ” to protect the cables” and this is one of the big important factors. a 125 amp breaker on a 125 amp cable will be pointless as the cable will likely fail before the you trip off. effectively using the cable as the fuse is never good.
We would suggest using a higher current cable that the breaker. In our world we would be using our 370 amp cable (35mm) as we account for voltage drop or cable resistance. Our 25mm cable is rated for 230 amps. but we do sell as range of cables and their ratings differ for example we sell as 60amp 25mm cable and a 230amp. So do take this into consideration – ALWAYS READ the description.
We are NOT just protecting the cables, but the devices attached to them. While the cables and battery may be able to push a high current, we need to ensure that they are working within spec and we can do this by putting in fuses and breakers.
Using Mega Fuses.
When using a fuse such as the mega fuses, you should be aware that they need to be kept in free air, not wrapped or covered. You may also want to use them as “last resort”. This means that you would have them as an internal fuse for when or if everything else fails.
For example:
In most batteries there are breakers as well as a BMS, the BMS may be rated for 500 amps the internal fuse may be 500 amps and you may have 200 amps as your system breaker therefore you would want a “last resort” below 500 amps but above 250 amps. This protects the battery and BMS but does not interfere with the battery operation and higher currents, you would have your system breaker that protects against abnormal conditions.
Where you are protecting.
This is often misplaced. The fuse/breaker should be near to the source. The idea is that should a fault occur on a line (phone talk) than the protection cuts off the source. its not good having power making a long run before the breaker, as you could snag a cable, or it could be cut within a crash scenario.
Questions answered?
Can you protect the battery with a fuse?
No. The battery is a source and therefore it cannot be protected itself, you have to protect other things from the battery. if you are building your own battery, you can fuse cells.
How do I account for surges?
You should not need to account for surges as breakers work by heat, so as they heat up they trip, the speed of this allows for short surges in current, but not for long. If you’re asking this question then you system may be undersized.
I have multiple batteries how do I fuse them?
Where there is a fault in any single battery, you will run into issues with them all. So you should fuse each battery. Think of it like having 4 batteries in a group, one fails and the other 3 will push power into the one battery that has failed. Fuse between batteries.
If I am using parallel batteries do I split the current?
Yes and no. with multiple batteries and the tech used, load sharing is more ideological than practical use.
You can reduce the rating, but this is likely to cause issues, and therefore you should should x1.5
For example a 8000w inverter on 48v will draw a total nominal current of 166 amps
/4 batteries is 41 amps each. You would see around 62amps on each battery unless there are problems.
Is it true than a slow short is worse than a big short?
Yes, the way fuses blow is by heat and if you have a low and slow burn, the fuse could not break fully, as the fuse melts, it can be nearly a bridge. usually you can hear or smell it but sometimes it can be unnoticed for a long time.
Is there anything else I could use?
Yes. arc faults can be detected with an AFD ( Arc Fault Detection) this will “shunt” the system to ground, which will blow fuses and trip breakers.
Note this video is AC AFDD:
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