When Is A Fuse Not A Fuse ?

When a fuse blows, it's job is to break the circuit. However, if the fault current is too high for the fuse or mcb, the device may either melt, arc or even catch fire. 

Therefore it's vital to ensure that the fuse or mcb protecting your cables is up to the job. The danger is that many people assume that 'a fuse is just a fuse', and that their wiring is adequately protected, when in actual fact, it's not. 

All fuses and MCBs have a manufacturer's rated 'breaking capacity' expressed in kA (1000's of Amps).  This figure indicates how much current the fuse can safely interrupt under worst-case fault conditions.

Off-grid battery banks can easily have huge short-circuit potential of many thousands of amps. Conversely, many popular fuses, such as automotive blade fuses, Midi Fuses and Mega Fuses only have a relatively low short-circuit rating.

For example, a typical 100Ah, 12V battery has a short circuit current of between 1000 and 2000A, possibly even up to 4000A. Whereas, common blade-type fuses only have an interrupt rating of 1000A (1kA). Other popular fuses include Midi fuses and Mega fuses, rated at 1kA and 2kA respectively, making none of these fuses suitable for a battery bank of over approx 200Ah. 

At the very least, the main battery fuse or mcb interrupt rating needs to be higher than the battery bank can deliver in a short-circuit situation. Downstream fuses can be of a lower kA rating, as they will be protected by the main battery fuse.

Inadequate UK Safety Standards ?

We've recently asked the UK Boat Safety Standards to clarify why there is no mention of kA ratings in their fuse selection guidelines - especially as the American Boat and Yacht Council (ABYC) already specifies minimum kA ratings. When we get a reply, we'll update this blog.

Interestingly, the ABYC wiring standards are currently being upgraded due to the overall increase in high capacity battery banks aboard boats. 

In our experience, many self-build and even factory-built motorhomes are fitted with inadequate battery fusing. As far as we're aware there are currently no standards in place that cover this aspect.

Make Your System Safe

If you don't have a 'main' battery fuse or MCB on your power system, you need to add one as a matter of urgency. If you have more than a single battery, it's likely that you will need to use a fuse with a high kA rating. 

Here is a list of the popular fuses on the market and their normal DC interrupt ratings. We've added a selection of MCBs for comparison. We often use DC rated MCBs instead of fuses as they are re-settable, can also be used as an isolator and generally have higher kA ratings.

Always check the manufacturer’s latest datasheet to confirm specifications – we accept no liability for any inaccuracies.

Blade Fuse Warning

Under testing, a random selection of cheap un-branded blade fuses did not blow at their rated current - sometimes needing 3x more than their rated current to blow. This is an obvious danger, and we would strongly recommend that you replace all unbranded fuses on your system with ones from a reputable fuse manufacturer, such as Littlefuse or Cooper Bussmann.

We've also seen numerous examples of inadequate fuse holders, which don't make a good contact with the blade of the fuse, arc and eventually melt. If you're unlucky, they could catch fire. 

One such very recent occurrence saw a blade fuse melt, dripping molten plastic and metal onto the top of a battery, melting a hole through the top of the battery case ..... causing the battery to explode, showering everything in proximity with acid. Fortunately, no-one was harmed. 

It's not worth risking lives to save a few pennies. If you take one thing from this article, please, always buy fuses andamp; holders from a respected manufacturer.

Further Information

https://calculator.academy/short-circuit-current-calculator/

See also : "Make Your Off-grid Solar PV Installation 4X More Reliable"