DCC Circuit Breaker History
The purpose of Layout circuit breakers is to manage power going to different parts of the layout such that there is not a total loss of power when a short develops someplace on the layout. It is not unlike the circuit breaker panel on your house. It something shorts out, you do not lose all the lights and power in the house. Circuit breakers, unlike a fuse, do not require any parts to reset them. Hence it is easy to recover from a fault.
The circuit breaking element be it a bulb, semiconductor switch or relay is wired in series with the track. Normally it will have a very low resistance state when it is ON and a high resistance state when it is OFF.
The track current runs through a monitor or sensor of some kind so it can sense the current. When there is a short circuit, the sensor will then activate itself after a small delay and put the circuit breaking element into the OFF state. The full DCC track voltage now appears across the circuit breaking element.
True On-Off switches have an extremely low resistance measured in Milli-ohms (0.001 ohms) and almost an infinite resistance measured in Giga-ohms (1,000,000,000 ohms).
Light Bulbs have a low resistance measured in ohms (1 ohm) when the filament is cold or not generating light and a high resistance that is 10 times higher (10 ohms) when the filament is hot and glowing. That actual bulb resistance value varies with the watt rating of the bulb for a given voltage. The watt rating only applies to the bulb when it is hot running at the rated voltage.
So the light bulb starts to emulates an on-off switch but does not have the dynamic range or ratio in resistance between on and off that a true on-off switch has. The limited ratio between on and off means the current never really stops flowing when using the bulb unlike a true on-off switch. So the light bulb is a poor man's circuit breaker. They are very inexpensive.
To learn more about Light bulbs as circuit breakers, go here: Light Bulbs
DC layout have been using light bulb as a circuit breaker protector for 50 years or more. But light bulbs have limitation especially when you are attempting to run multiple trains in the same block the light bulb is intended to protect. It will start to drop the voltage. However with DCC, it opened up the possibility of making improvements in the circuit breaker concept for layouts approaching an ideal function.
The first generation of DCC circuit breakers were simple in design and concept. Look for a short circuit and kill power. A short circuit was any current that exceeded a pre-determined current level for any brief period of time.
Today, DCC "sound decoders" have become so common in engines today that they now present a challenge to First Generation DCC circuit breakers. The Sound decoders, with their large onboard capacitors confuse these DCC circuit breakers into thinking there is always a short circuit when power is restored. This prevents them from recovering properly from an actual short circuit EVEN AFTER the actual short circuit itself has been cleared. The problem is called "High Inrush Current" issue the sound decoders present as a load to a DCC circuit breaker. If you want to learn more about this problem is go here: DCC Inrush Current Problem
Next generation DCC circuit breaker can now tell the difference between a true short and one created by a charging capacitor. They can recover from a true short circuit regardless of how many sound equipped engines are located on the track.