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There are ongoing reports of 25 – 30 amp blade fuses and holders used in RV power circuits suffering severe burning. In some cases both blade holder and fuse are totally destroyed.
The most recent I came across was unusual: two such 30 amp fuses in separate circuits in a converted coach. The fuses were located within a centimetre or two of a big (unsealed) battery bank. It would be exaggerating to say the installation was well done – but I’ve seen worse.
One fuse was in a cable between the battery and solar modules, but that cable proved not to be the cause. Nor was the failure caused by excess current: the solar regulator limited that to a tad over 20 amps.
The second fuse was in a battery charging circuit. There was no fault with the charger, or cabling. Again there could have been no excess current: the charger’s maximum output also was 20 amps.
Despite the above, and as the accompanying pix shows, both fuse holders were totally destroyed. The fuse holders had substantially melted. The blade fuses appeared to have eventually melted through radiated and conducted heat – not through excess current. Supporting this, similarly melted fuse holders, but with the fuses still electrically intact, have been found.
Most (but not all) such failures are where the fuses are located close to unsealed lead-acid batteries and/or exposed to water and dirt. Then blade contacts and fuse holders may corrode fast. The resultantly introduced inter-contact resistance generates heat sufficient to melt the fuse holder and eventually the fuse element itself. It seems likely this takes place over some length of time as most owners report they had been aware of progressively increasing problems with charging, or with appliance operation.
The reason such failures happen primarily with the 25 – 30 amp sizes is probably because fuses of this capacity are mostly used to protect high current carrying cables and are more likely to be located in electrically adverse environments. And of course, the high currents typically flowing have greater potential for heat generation. The contacts on these fuses are also very small indeed to carry currents of 20 amps or more.
The damage typically done is such that the blade holder is a total write-off. Even if only marginally burnt it is usually impossible to remove the blade fuse. Both fuse holder and fuse must be replaced – and this is not always feasible in remote areas.
The contact surfaces of most blade fuses and holders begin to corrode after a year or three. Where such corrosion (including low-current blade fuses and holders) is in its early stages, the resistance progressively introduced may result in voltage drops of typically 0.5 volt and more: sufficient to play havoc with a fridge or solar regulator circuit. Progressively poor solar charging showed something going wrong in the coach in question – but here the problem escalated before the cause was found.
In any fuse or connector that handles more than an amp or so, contact corrosion is likely to cause that device to become warm. In some instances the introduced resistance may be so high that not enough current can flow to generate heat –but that’s rare. Heat generated in fuse holders and connectors is however always a warning sign that something is seriously wrong. Fixing The Problem Whilst it is good to locate cable-protecting fuses close to the battery terminals, keep them at least 300 mm away or so, and not above the terminals of any unsealed battery – and protected from the weather. In fact unless the battery enclosure is exceptionally well ventilated, it is best to have any possibly spark producing device located outside that enclosure.
I strongly recommend replacing conventional blade fuses and holders (the trade know these as ATO or ATC) of 20 - 30 amps by the much larger Maxi blade fuse (illustrated). These are available from 20 - 80 amps. Maxi fuse melting time is longer. A 30 amp Maxi fuse takes 1.0 - 2.0 seconds at 350% of its rated current value, and 0.2 – 1.0 seconds at 600%. This is fine for cable protection of correctly specified cabling - but don’t use a Maxi fuse larger than necessary.
Better still is to replace large amperage fuses with manually re-settable circuit breakers. Those of totally reliable quality (found mainly at marine electrical suppliers) cost upwards of $50 each. Far cheaper versions are available but they tend to be very temperature sensitive.
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