Which group of flame effects should not be left unattended while connected to a fuel source?

The correct choice highlights the importance of safety regarding certain groups of flame effects, especially those designated as Group III, VI, or VII, which typically involve more intense and potentially hazardous flame effects. These groups may consist of effects that use larger flame sizes, more volatile fuels, or complex ignition systems.

Leaving these effects unattended while connected to a fuel source poses significant risks, including the potential for uncontrolled flames, spillage of flammable materials, or failure of safety mechanisms. Ensuring that trained personnel are always in attendance mitigates these dangers, allowing for immediate response in the case of an emergency or unforeseen incident.

In contrast, the other groups may either involve less hazardous configurations or are designed with safety measures that allow for short-term unattended operations during certain conditions. Hence, they may not require constant monitoring in the same manner as the specified groups. Understanding the classifications of flame effects under the NFPA 160 standard is crucial for safe deployment in any public performance context.