By Christopher W. Ferrone
The 50-DN alternator has been a workhorse for the motorcoach industry for over 40 years. It started out as a gear-driven, oil-cooled unit and in the late 1970s offered an air- cooled, belt-driven option. The 50-DN in any configuration has been a trusted engine accessory.
Nonetheless, since the early 1990s with the introduction of the DDC series 60 engine and a design change that ensued, this much trusted product suddenly became more vulnerable to fire causation.
The DDC series 60 as well as some Cummins and Caterpillar installations require an extended B+ cable from the bulkhead connection to the alternator B+ stud on the alternator. This section of cable is longer than what the earlier DDC series 71 and 92 engines used. The additional length is not directly the problem, however, the support of the B+ cable is a direct cause of most alternator-related fires.
Routed from the bulkhead connection to the alternator, the cable needs a support. [Figure 1] MCI and Prevost have come up with cable brackets mounted on the engine to support the B+ cable. [Photo 1] 
Operators can order these parts from MCI and Prevost.
The manufacturers also designed a cable support system in close proximity to the B+ stud on the alternator. [Photo 2]
Additionally, a small fabricated bracket mounts to the back of the alternator along with a plastic clamp system. [Photo 2] 
These properly installed supports protect the 50-DN from the stress of the weight of the cable on the B+ stud. Unsupported this force puts the stud into a constant state of bending at the base, which in turn puts stress on the internal insulator of the B+ stud — potentially creating a fire. [Photo 3a, 3b] 
Without proper support the insulation on the B+ cable may wear. If the bare B+ cable contacts the rear-mounted bracket on the back of the alternator housing a direct battery power to chassis ground occurs and leads to arcing. Arcing will cut through and allow the cable to drop from its mounting points and possibly contact the fuel supply lines. The alternator and the fuel lines are on the same side in a DDC series 60 engine.
Even with proper support of the B+ cable, some wearing of the insulation on the B+ cable can occur in the adjacent area where the cable comes over the top of the bracket at the end of the alternator. [Photo 5] 
For our American Sightseeing fleet, we have added an additional section of insulation the B+ cable section that goes over the rear bracket [Photo 6] to insulate the B+ cable from the small amount of wear created by the movement of the section of cable between the rear bracket and the engine support bracket.
As a final precaution, a rubber cover over the B+ stud eliminates any unwanted contact with a ground source.
As simple as it seems, a common forensic pattern I see in a great number of fire investigations is the absence of engine support bracket for the B+ cable. Perhaps when technicians remove the old engine in exchange for a rebuilt one, they fail to remove the bracket from the old motor for installation on the new engine setup.
Necessary attention to the alternator is straightforward and simple. After any engine repair remove the oil plug and drain the localized oil from the alternator. This basic chore assists in removing any engine debris from the alternator prior to starting the engine and circulating the contaminated oil.
Periodically inspect the front seal of the alternator for leaks. During any inspection of the coach make sure the alternator ground cable is secure, supported and in good condition.
Lastly, inspect the alignment of the alternator pulley with respect to the engine. Confirm that the belt is rotating in the same plane as the belt drive pulley on the engine. This will help ensure a longer life for the alternator front bearing.
Christopher W. Ferrone is president of Americoach Systems, Inc., Glenview, IL, an engineering firm specializing in transportation technology, analysis and safety.