The first invention of a protective device was the fuse. The first fuses consisted of a thin silver wire in a holder to be plugged between two connection terminals. These re-wirable fuses are still used in many countries. The silver wire melts faster than the copper wire, or the insulation of the wire, and thus gives protection against overheating.
It depends on the user, however, whether he uses the right cross-section of the silver wire when he replaces it after a fault. Even the more advanced fuse cartridge of HRC fuses cannot ensure that after a fault, the right selection to replace the faulty unit will be made.
Another problem is the fact that after a fuse has tripped, the installation is no longer ready for immediate service because the fuse has to be changed, and perhaps there is no spare fuse available.
This fact has always been a disturbing problem. Before the invention of circuit breakers, many attempts were made to construct so-called “multiple fuses”.
The figure shows such multiple fuses. In this case, it is a nine-fold fuse. After melting the first fuse wire, it was possible to change over to the second one and so on, like in a revolving chamber. After the ninth wire, the end of the course was reached. These multiple fuses could not be introduced on a broader scale because they were expensive, and after all, not really satisfactory. This led to the invention of a circuit breaker.
Working principle of a circuit breaker
The working requirements of the circuit breaker can be listed as follows:
- Over currents shall be switched off only when the heat produced by them passes a pre-determined limit. For reason, the tripping time for smaller over currents is correspondingly longer than for heavier overloads.
- Short circuit currents shall be broken without any delay.
- After reducing the load, or removal of the fault, the installation must be ready for service again. The last claim, in particular, is fundamental and quite natural and yet the fuse cannot comply with this.
The circuit breaker consists of the following four basic functional components:
- A bi-metal overload trip
- An electromagnetic short circuit trip
- A switching mechanism with contacts
- An arc extinguishing system
In most designs, the overload trip is made as a thermal trip working with a bi-metal. The bi-metal consists of two metal strips of different temperature co-efficient of expansion which are rolled one on the other. The bi-metal is deflected when heated, for instance, by an electric current.
The figure shows a schematic drawing of the function of a bi-metal. The deflection of the bi-metal depends on the intensity and duration of the current. After a pre-determined deflection, which means after a certain time depending on the current intensity, it will activate the tripping mechanism.
In certain appliances, the deflecting bi-metal directly opens the contact. In the circuit breaker, however, the bi-metal only gives the signal to the switching mechanism which provides for the opening of the contacts.
It is a special advantage that the characteristic of the thermal trip can be widely influenced by the design of the material and the shape of the bi-metal.
Normally, the bi-metal is directly heated by the load current. With the low ratings of the circuit breakers, however, the bi-metal must be heated indirectly by a heater winding for low-rated currents in order to obtain enough energy.
The figure shows the current time dependence of this thermal tripping system. It shows a certain tolerance band which is allowed by the various standards, as will be discussed later on, and also necessary for production in large series. These tolerances mean that the apparatus has to trip within the envelope curves of the tolerance bands.
Furthermore, this diagram includes the critical temperature curve of a conductor. The curve shows how long currents of different intensities are admissible without damaging the insulation of the conductor. It can be seen that the current with an intensity of three times the rated current may continue for about 100 seconds. If the current is higher or continues longer, the insulation of the conductor will be damaged.
The diagram makes clear that the tripping curve of the protection device, for instance, a circuit breaker, must always have lower values than the critical temperature curve of the conductor of the apparatus to be protected, and this, under the consideration of the tolerance band and a certain reserve.
To support the bi-metal at high short circuit currents to bend faster, an iron core is attached to the bi-metal. This system causes the bi-metal to be overheated and also results in over bending. After the fault has been removed, the bi-metal does not return to its original position, which leads to a shift of the tripping characteristics. Therefore, the circuit breaker will trip much earlier than it should in accordance with the limits of the standards.
The electromagnetic trip consists essentially of a coil through which the load current flows. In this coil, there is a fixed and movable armature. If the current exceeds a predetermined intensity, the armature will be attracted against the force of the spring. The switching mechanism is actuated by the lever on the right-hand side and provides for an opening of the contacts. This is the classical method.
Besides this, for breaking short circuit currents, it is enormously important to separate the contacts within the shortest possible time. The so-called switch-off delay must be short as possible. The switching mechanism however has springs and masses, and it takes a rather long time to move these elements.
In our schematic diagram, you can see on the left side, a hammerhead that provides direction for the opening contacts avoiding any delay by spring and additional masses. Of course, the tripping of the switching mechanism must be done in parallel to release the spring force, to bring the lever into the “off” position, and to prepare the switching mechanism for a new closing of the circuit breaker. The hammer trip is one of two essential conditions for the limitation of short circuit currents.
The figure shows the current time characteristic of the electromagnetic trip. With lower overload currents, only the thermal trip is active. From a certain limit, in the picture shown it is 3.5 times the rated current, the electromagnetic trip has to operate within the tolerance band. In the given sample, the apparatus must not trip before 3.5 times the rated current, but it must trip at the latest 5 times the rated current. The tripping time is about 1/200th of a second.
With higher currents, the time becomes shorter, as shown in the diagram, but exact figures can no longer be given because, in this range, the duration of the arc with its non-sinusoidal current is predominant.
Tripping of a circuit breaker – 1/2 ms after short-circuit current is released
Tripping of a circuit breaker – 1 ms after short-circuit current is released
Tripping of a circuit breaker – 1 1/2 ms after short-circuit current is released
Tripping of a circuit breaker – 2 ms after short-circuit current is released
Tripping of a circuit breaker – 2 1/2 ms after short-circuit current is released
Tripping of a circuit breaker – 3 ms after short-circuit current is released