Switches and circuit breakers
Switches
Knife switches are used for low-voltage circuits. These are mounted in front of the board or panel, and are operated by hand. Knife switches should be mounted for a vertical throw, with the blade side of the switch either dead or disconnected from the source of the power when open. This is to minimize the risk of an accidental contact.
Originally, all switchgears consisted of open knife switches. Protective devices such as fuses were mounted close to the switch. The use of high-voltage AC and the great increase of the total power in a system necessitated the use of oil-break, air-break, vacuum, air blast, or SF6 switchgear.
In LV installations knife type switches are of metal enclosed or cubicle mounted, double break type complete with arc chutes. The off-circuit LV isolators have been largely replaced by switches of either load break or load-break fault-make capabilities. In some applications, open-type boards are installed, but generally, most of the switchgear today is enclosed. Knife switches are usually spring controlled, giving a quick make and break with a free handle action. This makes the operation of the switch independent of the speed at which the handle is moved.
In all cases, it is impossible to open the cover with the switch in the on position. The rated current capacities of LV cubicle type switches with independent manual operation are limited to 630 A with some vendors even offering switches of 800 or 1000 A on request.
Copper-brush switches substitute a leaved copper brush with a wiping contact for the knife-blade contact, and make use of an auxiliary break, between the carbon blocks, to prevent burning of the copper leaves due to arcing. This type of a switch has been used as a circuit breaker, particularly in MV range with remote action by the addition of tripping coils, though closing is not done remotely. Switches with integral MV fuses also have the provision to open the switch on fuse blowing.
Circuit breakers
A circuit breaker works as a switching device as well as a current interrupting device. It does this by performing the following two functions:
1. Switching operation during normal working of operation and maintenance
2. Switching operation during abnormal conditions that may arise, such as over- current, short-circuit, etc.
Therefore, the need arises that it must withstand the abnormal current conditions, apart from the normal working current. All the switches discussed above, come equipped with a tripping device that constitutes an elementary load interrupter switch. The difference between a load interrupter switch and a circuit breaker lies in the current interrupting capacity.
A circuit breaker must open the circuit successfully under short-circuit conditions. The current through the contacts may be several orders of magnitude greater than the rated
current. As the circuit is opened, the device must withstand the accompanying mechanical forces and the heat of the ensuing arc, until the current is permanently reduced to zero.
When any high-voltage circuit is interrupted, there is a tendency towards an arc formation between the two separating contacts.
If the action takes place in air, the air is ionized and plasma is formed by the passage of current. When ionized, the air becomes an electric conductor. The space between the separating contacts thus has a relatively low impedance and the region close to the surface of the contacts has a relatively high voltage drop. The thermal input to the contact surface is therefore relatively large and can be highly destructive.
Therefore, the major aim in a circuit breaker design is to quench the arc rapidly enough, to keep the contacts in a reusable state by one of the following methods:
1. High-resistance interruption
In this method, arc resistance is increased. This method is generally used in DC circuit breakers and low-medium-voltage AC circuit breakers. The increase of arc resistance is caused by elongation of the arc against an arc chute which contains arc-splitting plates.
The arc is driven outwards using a combination of contact profile, air movement and in some cases by a magnetic blow out device.
2. Low-resistance of zero-point extinction
In this method, the arc is interrupted at a current zero instance. At that instance, the air between the separating contacts is unionized by introducing fresh air, SF6 gas, or oil between the contacts. Naturally, this method is used for an AC arc interruption.
By using a combination of shunt and series coils, the circuit breaker can be made to trip when the energy reverses. Circuit breakers may trip, when a local breaker or fuse immediately clears the difficulty.
To ensure that the service is uninterrupted, automatic re-closing schemes are often used for circuit breakers feeing to overhead lines where self clearing faults (e.g. a bird fault) can occur. After tripping, an automatic scheme operates to re-close the breaker with a short delay giving an opportunity for the fault to clear. If a short-circuit still exists, the breaker trips once again. The breaker attempts to re-close two to three times and if the short-circuit persists it remains permanently locked out.