What type of foam system uses an electrically operated device to inject foam concentrate into the water system?

The correct choice describes a foam system known as an injection system. This type of system is designed to directly inject foam concentrate into the water stream using an electrically operated device, which allows for precise control over the concentration of foam being produced. The electrically operated feature ensures that the application of foam is efficient and can be adjusted according to the specific firefighting needs.

The injection system is particularly beneficial because it can deliver consistent foam quality and allows for flexibility in the amount of foam concentrate used. This can be crucial in different firefighting scenarios, where varying amounts of foam may be required to effectively suppress fires, especially those involving flammable liquids.

Other types of systems like the balance-pressured system and the proportionate system do not utilize an electrically operated device for foam injection. The balance-pressured system relies on pressure differences to mix foam concentrate and water without the need for electrical components. Similarly, the proportionate system uses mechanical means to achieve the desired foam concentration but does not involve electrical injection methods. The self-educting system functions by using the venturi effect to draw foam concentrate into the water stream, which also does not involve electrical operation. Thus, the injection system uniquely fits the description provided in the question.