What rating must disconnectors have to ensure safety isolation?

The requirement for disconnectors to have a rating that ensures safety isolation is crucial for protecting electrical circuits and personnel. The correct answer specifies that disconnectors must be rated for design current and short-circuit fault.

Design current refers to the maximum continuous current that the disconnector is designed to carry under normal operating conditions. This rating ensures that the disconnector can handle the regular electrical load without overheating or failing during typical usage.

Short-circuit fault rating is also vital because it indicates the disconnector's ability to interrupt a fault current that may occur during a short-circuit condition. In such cases, the disconnector needs to be capable of safely breaking the circuit without causing additional hazards, such as fire or equipment damage. This attribute of the disconnector is necessary to ensure that, in the event of a fault, the electrical system can be safely isolated.

In contrast, other options like equipment failure and overload current, voltage and maximum load current, or design current and operational current do not fully encompass the critical safety requirements. While those factors are important for different aspects of electrical safety and functionality, they do not specifically and comprehensively address the necessity for disconnectors to handle both regular operational loads and the extreme conditions of short-circuit scenarios.