The invention is generally associated with fault detection and circuit protection devices (circuit breakers) in AC power electrical systems. It could also be used in DC power electrical protection systems.
Circuit breakers are used to protect electrical circuits from a short circuit current and overloads. These devices in conjunction with the fault detection relay, in general, are designed to sense the existence of the fault current in the protected circuit and then to disconnect the faulty circuit from the source of power. In some devices the trip operation setting (the level of the current which leads to the operation of the protective device) can be adjusted and in some devices it is fixed during the manufacture; but all circuit breakers require a device or means to first sense the existence of an abnormally high current and then disconnect the circuit in a time interval necessary for the physical operation of the circuit breaker contact mechanism (if intentional delay is not provided).
The time interval between the beginning of the short circuit current and the disconnection of the circuit is crucial. A fast disconnection helps to reduce damage to equipment due to high current flow through the circuit and circuit components.
In many cases another issue is also very important. This is a protection selectivity (coordination) problem. When two or more circuit breakers are connected in series in a distribution system, between the source of power and the consumer, it is a requirement of standards and codes to have an arrangement in which only the device closest to the fault point is disconnected during the fault, and all other devices remain unaffected so that the other parts of the distribution system are not disconnected from their source of power. When circuit breakers connected in series are similar in size and the short circuit available in the circuit is more than the maximum instantaneous trip setting of all of them, the probability is high that more than one circuit breaker will trip as a result of a fault. In some distribution systems, such as emergency systems, this situation is not acceptable and actions have to be taken to avoid it. A possible solution is to replace the up-stream circuit breaker with another of a larger frame size and very often with a solid state trip unit. It allows the provision of an instantaneous trip setting exceeding the fault current level in the circuit. This is usually expensive and this solution increases the thermal stress on the system during a fault. Sometimes this solution is not enough, and the next option is to install devices like a current limiting reactor in the circuit in order to reduce the circuit fault current and shift the circuit characteristics. This assumes that the up-stream circuit breaker protection characteristics are no longer in the instantaneous trip operation zone. Depending on the characteristics of the circuit, such current limiting devices can be expensive and heavy pieces of equipment.
Faster operation of the down-stream circuit breaker could be helpful, however, in most cases, when up-stream and down-stream circuit breakers see the fault current exceeding their instantaneous settings, even if one of them, usually the down-stream breaker, begins to operate first, the other circuit breaker will trip as well.
At the same time, if the down-stream circuit breaker operates very fast and its contacts start to open before the fault current reaches the instantaneous trip setting of the up stream-breaker, the dynamic impedance of the down stream breaker prevents the up-stream breaker from tripping.
Another approach to this problem is to detect the rate-of-change of the circuit current and develop a control signal to open the breaker even before the fault current reaches an abnormal level. Protection relays with this control philosophy exist, but the use of these devices is limited to special applications.
The presence of a motor load or a step load makes this type of protection unreliable. The conditions of the operation depend on the parameters of the circuit and very often there is no way to distinguish between the initial current of an electric motor starting or initial load current and a fault current.
There are other numerous approaches including traditional fuses, semi-conductors and superconductors.