HFD Circuit Breaker

"HFD appears to be nothing more than a series designation for a model of circuit breaker by Cutler Hammer (Westinghouse)."

Actually, Siemens (US) uses the same designations for the old ITE breakers, but they mean different things.

In Siemens / ITE, the "F" means it is a 250A frame size, the D is manufacturing series, i.e. it is newer than the "C" series of breakers they stopped selling 30+ years ago. In Cutler-Hammer / Westinghouse, the "F" is the Series (age) of breaker, in other words it is older than the current "G" line of breakers, the D means it is a 150A frame.

But in BOTH cases, the "H" in the front happens to mean the same thing; High interrupting capacity. So in C-H / West., the FD is rated 25kAIC, the HFD is rated 65kAIC. In Siemens / ITE, the FD is rated 35kAIC and the HFD is rated 65kAIC.

So a designer would select a High interrupting breaker if they know that the Available Fault Current (AFC) in the system they will be connected to is greater than the next lowest rated breaker. In other words if you were applying this breaker to a system with 38,000A AFC, you cannot use the FD in either brand, you must use the HFD.