Driving and fog lamps use a relay and dual power source. DOT restricts driving lamp operation to coincide with high beam use and fog lamp operation to coincide with low beam use. Each relay is controlled by a manual switch that is connected to the high beam circuit (driving lamps) and low beam circuit (fog lamps). For driving lamps to illuminate, the headlamp switch must be ON, the dimmer switch must be selected to high beam, and the driving lamp switch must be ON. For fog lamps to illuminate, the headlamp switch must be ON, the dimmer switch must be selected to low beam, and the fog lamp switch must be ON. So, both control circuits are protected by the circuit breaker that is integral to the headlamp switch. The actual driving lamps and fog lamps are separately protected usually by inline fuses that connect directly to battery positive, using the firewall junction block for convenience. Of course, you can make the installation more complex if you wish.
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