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Circuit protection in high-current wiring is vital.


Today’s camping vehicle current demands are much greater than those in the ‘good ol’ days’ and traditional blade-fuse circuit protection isn’t adequate. Circuit breakers are an alternative, but larger, screw-in fuses are probably a better bet for off-road vehicles, where vibration is a constant connection hazard.



We’ve had a basic Redarc battery isolator in our 75 Series bush travel vehicle for many years. Being a 1993 model, ‘Harry HJ’ doesn’t have a variable-output alternator and the second, under-bonnet battery is adjacent to the starting battery that receives the alternator’s primary charge.

All has worked well for the best part of 500,000 kilometres, but over the last few months we’ve been having some weird electrical issues. The main power supply from the auxiliary battery into the cargo box on Harry’s tray had been happily supplying a 550W inverter – mainly for small battery charging purposes – a 12V power outlet and our Engel 20-litre back-up fridge.

Then we noticed that sometimes the fridge didn’t seem to be working optimally, or the inverter wouldn’t fire up. Checking the auxiliary battery voltage showed that it was holding full charge and, when the inverter and fridge were working, current flow was as expected.

We checked out the fridge and power outlet terminal block and its blade fuses and we bench-tested the fridge and the inverter to ensure they were functioning properly.

We suspected the battery isolator assembly might be the problem and called for professional assistance.  Moss Vale (NSW) is home to the auto-electrical Cross Roads Group – also Ironman agents – and the boys and girls there are well used to fixing 4WD and caravan electric problems.



They listened to our tale of woe and then got to work. Two hours later, the checking was done and the culprits were disconnected and replaced. The problem lay in the original circuit breakers that had gradually deteriorated over the years and were providing erratic current delivery.


The solution was set of ‘Midi’ fuses.

In one way that seemed like a backward step, because if there’s a circuit problem a fuse will ‘blow’ and current will cease to flow, rather than a circuit breaker’s ability to reset and keep current flowing.

However, that fuse failure will point to a circuit that has a problem, forcing a diagnosis, rather than a problem being masked.

Since this incident we’ve heard of other ‘weird’ electrical problems, including the situation where a fuse holder melted, but without blowing the fuse.


Redarc’s analysis of this situation is that the fuse holder was making poor contact with the fuse.


Loose connections are common and can allow dust to get between the contact points, resulting in voltage drop.

Heat in an electrical circuit is always caused by current flowing through a resistance. If there is significant resistance in the contact between fuse and fuse holder, a current well below the fuse rating can cause enough heat to melt an in-line fuse holder.

Redarc points out that this heating of the electrical fuse may not always be immediate. The initial heating may not be enough to reach melting point, but that heat can cause oxidation of the metal connections, leading to increased contact resistance.

This accelerates the heating effect until there is rapidly increasing heating/increasing resistance, leading to what may appear to be a sudden failure.



When installing battery management systems and battery chargers, Redarc and Narva say it’s essential to use good quality battery fuses and fuse holders that ensure an enduring, solid connection.



Popular, automotive blade-type fuses and holders are usually fine for lower currents, but are getting close to their limit at 20A current levels. For higher currents it’s essential to use heavier-duty fuses and fuse holders that are ‘bolt down’ types, not push-in blades. Redarc’s 40A Fuse Kit and 60A Fuse Kit are examples and Narva has ANS fuses in the 50A to 150A range.



These fuses have screw holes at each end and are designed to be screwed into into flip-top terminal blocks that keep the joints insulated and protected from dust ingress.

Although these heavy-duty fuses are becoming more popular they’re not necessarily available in remote areas, so it’s a good idea to carry replacements when you travel.





























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