Fusing requirements for our products.

Required fuse sizes are specified in the relevant owners and/or installation manual. This document explains why fuses are required. It is not comprehensive. Fusing is an extremely involved subject and could easily take up an entire book.

Fusing of electrical and electronic equipment is required for 2 basic reasons. A. To comply with legal and/or safety requirements which exist because of the necessity to B. Prevent overheating cables and/or fires in the event of a fault.

Assume a 2.5mm cable used to power a load from a 200 amp hour battery bank. If these cables become short circuited, the current that will flow as a result will, of course, cause immediate overheating of the cables, melting of the insulation, possible further short circuits with other cables and could finally start a fire. Not to mention the mess it will make of the installation.

A fuse is a deliberate "weak spot" put in the system so that the weak spot blows instead of the cabling. It is obvious therefore that the fuse must be rated so that it will blow before the cable is damaged. There must be a substantial margin between the rating of the fuse and the rating of the cable. For instance a cable of 2.5mm is generally rated to carry 30 amps. A fuse with a rating less than this must be used. There is little point fitting a 60 amp fuse in a 30 amp cable as the cable will blow first.

It is often thought that fuses are there to prevent further damage to electronic equipment in the event of a fault. This is totally incorrect. Electronic components act, to all intents and purposes, instantly. This also means that in the event of a gross fault, they fail instantly. A fuse takes somewhat longer. A fuse simply will not protect electronic equipment. By the time the fuse blows, the damage to the electronic compenents is already done.

So now we have clarified what a fuse is actually for i.e. to protect the cabling, it becomes clear why the fuses should be as close to the batteries as possible. There is little point in having a fuse at the end of a 10 metre cable if the short circuit happens 2 metres from the batteries.

SmartBank, SmartGauge and SmartGuard should be fused at 3 amps.

Fuses should never be put in the battery compartment unless the batteries are certified as being suitable for use in habitable spaces. Which means they will not give off explosive gasses during charging. Generally this means gel batteries and AGM batteries only. All other types give off explosive gasses during normal charging.

SmartBank fusing in the heavy duty split charge cables.

Fuses should not be fitted in these cables. It could be argued that all cables should be fused and if this is the case then starter motor cables should also be fused. This of course means enormous fuses. Perhaps in the several hundred amp rating. Possibly even thousand of amps.

On most installations fuses are not required in the starter circuits. SmartBank has an emergency parallel feature to enable engine starting from the auxiliary bank. Therefore all the battery paralleling cabling can be described as being part of the engine starting circuit and therefore is exempt from the normal fusing requirements.

In some types of installation fuses are still required in starter circuits (eg certain vehicle requirements). In these cases the same fuses should also be fitted in the SmartBank split charge cabling. i.e. the fuses fitted in the split charge system must be capable of handling the heaviest current whether that might be engine starter currents, bow thruster currents, windlass currents etc.

Fitting fuses of a smaller size in the SmartBank split charge cables can result in fuses continually blowing. Any fuses fitted must be capable of handling the full engine starter current, bow thruster currents, windlass currents etc.

It is the installer's responsibility to ensure compliance with the relevant legal requirements for any particular type of installation at the date of installation.

For instance, as the legal requirements stand today, it is not necessary to install fuses in the starter circuit of a narrowboat built to comply with the Recreational Craft Directive (RCD) however this may change in the future. There is no way of knowing.


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Page last updated 02/04/2008.
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