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The real name of the relay is the overVOLTAGE protection relay.
Internal is a zener diode that conducts straight to ground (through the fuse) everything over some voltage like say 16v (may be higher). It neatly clips all spikes with no problem but it won't handle a regularly overcharging alternator or as usually happens an improper jump start. Battery chargers can blow the fuse if the key is turned on while running.
The fuse blows because more than 10amps (I am guessing the fuse current rating) flows at above the zener cascade voltage. (a zener looks like an open circuit to normal voltage but becomes conductive above a voltage.)
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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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