Look at a capacitor definition on the web. It is plates with a dielectric between them that can collect and retain a charge. You need a positive and a negative terminal. The case is the negative terminal. Otherwise, it could not collect a charge as current would not flow into it. Several have made this error.
Don't believe me. Take some insulated material and a nylon washer and isolate the condenser when you mount it on the breaker plate. See what happens. No return path to ground. You can't have a single lead condenser. Just like the transistor emitter on the original electronic box is the case of the transistor that shows. It attaches to the box case which is grounded. Some devices are made that way. Same as the diodes in an alternator. A single lead, but the metal diode case is the second lead. No return path. No worky