Which of the following is an acceptable form for a porcelain or glass insulator?

Porcelain or glass insulators are essential components used in electrical systems to prevent the flow of electricity to unwanted paths, thereby ensuring safety and efficiency. An acceptable form for these insulators includes various configurations depending on the application and mounting requirements.

A knob, which is typically used in overhead lines, provides a means of attachment and insulation. A rack serves as a support structure that can hold multiple insulators, making it suitable for installations requiring the support of several lines at once. Finally, brackets or strain forms can also be used to secure the insulator in place, especially in tension applications where wires need to be held taut.

Each of these forms has specific applications and advantages, offering flexibility in design and installation. This versatility allows electrical contractors to select the most appropriate insulator type for their specific needs, conforming to safety standards and ensuring uninterrupted electrical service. Thus, all options provided are acceptable forms for porcelain or glass insulators, making the answer comprehensive and inclusive.