Which device is essential when testing ground resistance for ensuring safety?

The clamp-on resistance tester is essential for testing ground resistance because it allows for accurate measurement of ground resistance without needing to disconnect the grounding system. This device uses a technique known as the clamp-on method, which involves clamping around the ground rod or conductor and measuring resistance by inducing a current and measuring the voltage drop. This non-invasive approach is crucial for ensuring safety, as it prevents any disruption to the electrical system while also providing reliable readings.

Using a multimeter, while possible for measuring resistance, is typically not suited for ground resistance testing because it requires that the circuit be broken, which can introduce safety hazards and disrupt normal operations. A circuit breaker, while critical to electrical safety, does not measure resistance; instead, it protects circuits from overloads. Similarly, a transformer is used to change voltage levels and does not play a role in resistance measurement. Therefore, the clamp-on resistance tester is the most appropriate device for this specific application.