What device is used to easily measure the resistance of ground lines in service?

The clamp-on ground-resistance tester is designed specifically to measure the resistance of ground lines without requiring physical disconnection from the system or needing to interrupt service. Its design allows it to measure the current flowing through the ground system and provides a direct reading of ground resistance. This capability makes it particularly useful for service applications where minimizing downtime and maintaining safety are critical.

Other devices mentioned in the options do have roles in measuring resistance but are not suited for this particular task. For instance, a high-voltage voltmeter is designed for measuring voltage levels in high-voltage applications and doesn't provide functionality for measuring ground resistance directly. An ohmmeter could measure resistance, but it typically requires the circuit to be de-energized and would not be practical in live service situations where ground resistance needs to be measured. A resistance bridge also measures resistance but usually in a laboratory setting rather than for field applications, as it typically requires more complex setups. Thus, the clamp-on ground-resistance tester stands out as the most appropriate device for this specific purpose.