What characteristic is essential for a good insulator under load?

A characteristic essential for a good insulator under load is high dielectric strength. This property refers to the maximum electric field that a material can withstand without breaking down or becoming conductive. When an insulator is under electrical load, it must maintain its insulating properties to prevent unintended current flow, which can lead to failures or hazards in electrical systems. High dielectric strength ensures that the material can handle high voltages without failing, making it crucial for safe and effective insulation in various applications.

In contrast, the other options do not contribute positively to the effectiveness of an insulator. High electrical conductivity is undesirable in insulators because it would allow current to flow through the material, undermining its insulating abilities. Low thermal stability is also problematic since it indicates that the material could degrade or lose its insulating properties when exposed to heat generated by electrical loads. Lastly, while low material cost is often a consideration in material selection, it does not relate to the insulating properties needed under load conditions and cannot guarantee performance or safety. Thus, high dielectric strength remains the key attribute for effective insulation under load.