To Midi, or not to Midi

Properly labelled and mounted Midi fuses make finding, and fixing, 12V problems much easier.

Wiring up of accessories of any description needs to be done carefully and well but we need to make sure our wiring is always fused.

I have spoken about this a lot in the past but one thing I haven’t spoken about much is the different types of fuses. In particular, the Automatic fuse or  Automatic Resettable Circuit Breaker vs the Midi type fuse.

The red-capped fuses here are Automatic Resettable Circuit Breakers. They work great, but I feel they introduce some issues that are better solved by Midi fuses.

I have fitted the Automatic Circuit Breakers in my 200 series. In the past, it was common to find them fitted to brake controllers and other accessories. They certainly do the job, but I’ve moved away from them recently.

The way these fuses work is via two different metals touching each other. If there is a fault in the circuit or it gets overloaded, the circuit breaker heats up and the metals pull away from each other, breaking the connection.

As they cool down, the metals will slowly move back together, reestablishing the connection. Pretty ingenious, really, and fully automatic.

I feel like the beauty of these devices is their biggest potential weakness. Opening and closing the circuit like that, until whatever causes the fault is rectified, still lets heat build up throughout the system, and potentially hides problems, or makes them appear to be intermittent (a sparky’s worst nightmare).

A Narva fuse box handling lower amperage fuses running off both the main battery and auxiliary. It’s easy to spot any problems at a glance.

I really would prefer a fuse to leave the connection broken until I can find and fix the problem. Some accessories are probably better off staying off until the problem is fixed, rather than switching on and off with no control or timing.

With Midi fuses, once the fuse is blown it is blown, marking potential problems in a way that is easy to find and saving that information until the problem is fixed. Add in some nifty labelling to your Midi fuses and you have a very quick and easy troubleshooting methodology for most 12V problems.

Midi fuses tend to be rated for higher amperage applications, from 30A right up to 250A in the ANL range.

If you’re fusing something smaller, i.e. 10A, an inline fuse holder, or a fuse box, is the best solution.

I like fuse boxes for the same reason car manufacturers use them: they are a one-stop shop for troubleshooting electrical issues.

Every 12V build requires careful planning as well as careful implementation. And Midi fuses provide me with peace of mind and serviceability in all my builds.