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Old 09-08-2006, 12:32 PM
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Don Moyer Don Moyer is offline
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Fire from unfused electrical circuits

The MMI Service and Overhaul Manual lists 4 primary safety concerns surrounding operation of the Atomic 4 in Chapter 1-2: (1) Burns from touching hot parts of the engine, (2) Injury from getting caught in the alternator belt, (3) Electrical shock, and (4) Fire potential from inflammable fluids such as gasoline, oil, etc.

I would like now to add a 5th very critical safety item to our earlier list of concerns: Fire from unfused electrical circuits.

The attached table lists all of the fusing requirements with which we are familiar that apply specifically to the Atomic 4, and it represents our current understanding of the technical background provided in the popular text by Nigel Calder “Boatowner’s Mechanical and Electrical Manual” and the recommendations of ABYC. Reports (mostly from our own customers' experience) over the past couple years points to two of these circuits as being especially critical from a SAFETY standpoint:

1) The primary ignition circuit leading from the IGN terminal of the ignition switch to the positive terminal of the coil (20 amp protection recommended).

In the event of a short circuit in one of the electrical systems on the engine itself, this circuit receives more than enough electrical energy from the ignition switch to melt the purple ignition wire and any wire running next to it in a wiring harness.

Examples of short circuits in this category include shorting of the field exciting circuit to the alternator, shorts in the distributor, shorts in an electric fuel pump circuit, etc. A few of the boat builders of the late 1970's installed 20 amp fuses in this circuit, but most did not. In general, therefore, it is not possible to rely on your boat builder to have supplied fusing in this circuit.

2) The circuit between the positive terminal of the coil and an electric fuel pump (5 amp protection recommended)

Most electric fuel pump manufactures today recommend 5 to 10 amp fuse protection in this circuit, but many engines supplied by Universal in the late 1970’s and early 1980’s (probably several thousand!) were shipped without fuses in this circuit. The Facet Pump Company currently recommends 5 amp fuse protection in this circuit, but many of the earlier Facet pumps had stickers allowing up to 10 amp fuse protection.

This circuit is particularly hazardous due to the “soldering iron” effect. If this circuit is only protected by the 20 amp fuse between the coil and the ignition switch, a partially shorted fuel pump can create considerable heat (equivalent to a soldering iron, hence the name of this process) while only drawing 10 to 18 amps – thereby never blowing the 20 amp fuse. Considerable heat can be generated even at 8 to 10 amps, which is probably why Facet is now recommending 5 amp fusing.

3) Main positive battery cable leading to the big battery cable terminal on the starter solenoid

ABYC standards do not require a large fuse to protect the main battery cables running to the starter itself, as long as a cut off switch is installed to disconnect the batteries when not in use. However, although we have only heard of one case where a major short circuit occurred at the starter itself, the consequences were so devastating (everything from the starter to the batteries turned to ash) that we’re including it in our discussion of fuse protection.

For what it's worth, Nigel Calder and the people at West Marine also recommend fuse protection within the main positive battery cable, pointing to the devastating consequences resulting from a direct short (e.g., if a wrench were to fall on the main battery terminal of the starter solenoid). If you wish to pursue such protection, we believe that a 200 amp fuse would be a good choice to provide for starter operation, and to protect the circuit.

You’ll find fusing kits in the electrical section of our online catalog that are packaged to facilitate the installation of the critical fuses in your ignition and fuel pump circuits (OVEL_11_392 and OVEL_11_397), and we’ll defer to your West Marine catalog for several options in the event that you wish to install a high amperage fuse in your main battery cable circuit.


Don Moyer
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Last edited by Don Moyer; 09-08-2006 at 12:37 PM.
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