What marking is required on a switchboard supplied by an ungrounded 240-volt system?

The marking required on a switchboard supplied by an ungrounded 240-volt system is "Caution: Ungrounded System Operating 240V between conductors." This is correct because it directly informs users about the specific characteristics and potential risks associated with the ungrounded system.

In an ungrounded system, the absence of a direct ground connection increases the risk of voltage appearing on the equipment casings, which can lead to hazardous conditions. This marking warns operators and maintenance personnel that the system operates at a potentially dangerous voltage and has unique safety considerations compared to grounded systems. Proper identification and warning systems are critical for the safe operation and maintenance of electrical equipment.

Markings such as "Warning: High Voltage" or "Alert: Potential Shock Hazard" may not provide the necessary clarity about the specific conditions of the ungrounded system and its operating voltage. The option "Notice: Equipment Under Testing" does not address the safety concerns needed for those working around an ungrounded system. Therefore, the selected marking emphasizes both the voltage and the ungrounded nature of the system, which is essential for safety awareness.