Switches, circuit breakers and switchboards

Learning Goals...are to understand:

• • switch gear types
• • protection types
• • the basics of power distribution

Introduction

To open or close an electrical circuit, a switch device is required. In addition, to protect an electrical circuit, a protection device is required.

Low-voltage switches with HRC ceramic enclosed fuses are used commonly in industries though the present trend is to use circuit breakers with overload and short circuit protection in place of the fuse switch. There are various types of switches and protection devices used in the industry for different applications. All these devices are generally called Switchgear.

Apart from switching on or off any section of an electrical installation, the switchgear must include the necessary protective devices. These protective devices automatically isolate a particular section of the installation under fault conditions.

The switchgear must withstand short-circuit faults without thermal or mechanical damage and are therefore given a short time withstand rating (usually 1 s in LV), Though this rating is mainly applicable to the bus bars and other conductors, control devices such as switches and circuit breakers must also have such rating (either short time or dynamic withstand for prospective short-circuit current) appropriate to their function.

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’s 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 630A with some vendors even offering switches of 800 or 1000A 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.

Miniature circuit breakers

Miniature circuit breakers (MCBs) are used widely, as protective devices for switching and protection in domestic, commercial, and industrial applications. They are popular because they replace the conventional fuse-and-switch and give more flexibility.

During a normal operation, it works as a switch; while during overload or short-circuit condition, it works as a protection device, isolating the faulty section.

Magnetic or thermal sensing devices provided within it actuate a tripping mechanism.

Typical voltage ratings: 240 V/415 V AC; 50 V/110 V DC Typical current rating: 1-55 A 5.2.4 Molded case circuit breakers

These are circuit breakers with tripping mechanisms and terminal contacts assembled together in a molded case.

This helps in getting high die-electric strength as well as mechanical strength to it. In addition, an arc chute is provided to increase the length of the arc and at the same time restricting hot gases that come in contact with the important parts of the breaker.

Molded case circuit breakers (MCCB) with ratings up to 3000 A are capable of interrupting currents up to 200 kA. These are used for the control of low-voltage networks.

Oil circuit breakers

The arc decomposes in dielectric oil. The gases formed due to the decomposition are channeled through a vent in the chamber. Oil circuit breakers are popular for high-voltage distribution systems, despite the perceived fire risk. These consist of an oil enclosure, in which the contacts and an arc control device are mounted. The arc is kept within the control device and the resultant gas pressure sweeps the arc, through the cooling vents in the side of the pot. There is a possibility of an explosion due to rise in pressure. In addition, these circuit breakers require regular replacement of oil as dielectric strength reduces during arcing. They are not suitable for applications where breakers operate repeatedly. The oil circuit breakers are commonly used up to the voltage level of 145 kV.

Air-circuit breakers

In this type of breakers, air (at atmospheric pressure) is used for arc extinguishing. It uses the high-resistance interruption principle. The length of an arc is increased by using arc chutes and arc runners.

They are used in AC and DC circuits up to 11 kV. They are generally of an indoor type for medium- and low-voltage applications. They are simple in construction, indoor type, panel-mounted and have current-limiting properties. They are particularly suitable for applications where repeated breaking is required. The operation can be manual as well as automatic.

The manual operating mechanisms can be either by spring or by motor charging, whereas in the automatic mode it can be through the solenoid coil.

The air-break circuit breaker for 3.3-11 kV has an arc control device, which is suitable for motor switching and is used mainly in power stations.

Typical characteristic curves of an air-circuit breaker.

++++1 Typical characteristics curves of an air-circuit breaker. If the ACB is provided with time-delayed short circuit trip, it will trip after the set time delay has elapsed; Magnetic release will take over at the short circuit setting, tripping the ACB instantaneously. Current Time 30ms 40ms 460ms S/C time delay trip S/C inst. trip

Vacuum circuit breakers are used for duties that require the following:

• Very high electrical and mechanical life

• Concealed current path

• Compactness.

The advantages of a vacuum circuit breaker are listed as follows:

• No exposed arc

• High operating safety due to reliable switching on high short-circuit faults

• Longer electrical life of up to 30 000 operating cycles at rated current

• High short time current withstanding capability

• Extremely short fault clearance time

• Integrated voltage suppressor

• Contact erosion indicator for erosion monitoring

• Maintenance-free current path.

SF6 circuit breakers

SF6 gas is an inert gas with dielectric strength and arc-extinguishing qualities. In SF6 breakers, the rate of rise of dielectric strength is very high and the time constant is very small. This provides another type of oil-less circuit breaker. However, the life of the contacts is short when compared to that of the vacuum circuit breaker.

The SF6 circuit breaker has other advantages that make it equally acceptable for industrial use. All circuit breaker systems up to 36 kV are three-phase systems. However, for higher voltages of up to 420 kV, three separate single-phase breakers are sometimes used to facilitate the single-phase opening and the closing for transient faults.

The advantages of SF6 breakers are as follows:

• There is no danger of explosion or fire

• Excellent arc-extinguishing capabilities with minimum time

• Contacts wear and tear is lesser

• Outdoor SF6 breakers are simple, cheap, maintenance-free, and compact

• Suitable for voltage levels ranging from 3.6 to 760 kV

• Minimum maintenance

• No contamination of moisture or dust due to sealed construction.

High-voltage circuit breakers

High-voltage circuit breakers are either of the oil type, in which the contacts open under oil, or of the air-blast type. In this type of breakers, high-pressure air is forced on the arc through a nozzle at the instant of contact separation. The portion of ionized air between the contacts is blown away by the blast of high-pressure air. Therefore, they are called either air-blast circuit breaker or compressed air-circuit breaker.

A CT, on an inverse-time relay in which the time of closing the relay contacts is an inverse-time function of the current, initiates tripping of the high-voltage circuit breaker.

Therefore, the greater the current, the shorter is the time of closing. When the DC circuit is closed by a relay contact, a DC tripping coil trips the breaker.

The circuit breakers should open the circuit within 6 cycles from the time of closing of the relay contacts. Air-blast circuit breakers have received a wide acceptance in all fields for both indoor and outdoor applications. Indoor breakers are available up to 40 kV and interrupting capacities of up to 2.5 GVA. Outdoor three-pole breakers are available in extra-high-voltage ratings of up to 765 kV, capable of interrupting 55 GVA or 40 000 A of symmetrical current.

Motor circuit breakers

Motor circuit breakers provide overload, short-circuit, and single-phase protection for three-phase motors. The motor circuit breaker has a toggle switch for ease of operation and has auxiliary contacts, trip indicating contacts, and a U/V or shunt release. The three-pole circuit breaker can be connected in parallel to the fuses. In the event of one fuse blowing, the breaker actuated through its release gives a tripping signal through its auxiliary contacts to the motor control device to switch off the motor. Thus, the motor is not subjected to single phasing, and costly motor burnouts are prevented. The motor circuit breakers operate on the current-limiting principle. In the case of a short-circuit, the contacts are opened electro-dynamically by the short-circuit current. The instantaneous over-current releases, through the switching mechanism trips all the three poles of the breaker. A large arc voltage is quickly built up in the arc chamber, limiting the short-circuit. The breakers have a trip-free mechanism, and tripping cannot be prevented by the toggle switch position. After clearing the fault that caused the short-circuit, the limiter must be reset by hand before the circuit breaker can be switched on again.

Typical characteristic curves for overload and short-circuit release, and the current limiting feature of a motor circuit breaker.

++++2 Typical characteristic curves of a motor circuit breaker for overload and short-circuit release and current- limiting feature. Times set current Short circuit current IK (effective). Trapping time; Two-phase loading; Three-phase loading

Overloads and fault protection

Protection devices against electrical faults may be broadly divided into fuses or circuit breakers. In some applications, fuses are used with the circuit breakers to take over the interruption of higher short-circuit currents, particularly with the miniature or lower-rated MCCB.

Overload and fault protection in motor circuits

Often a motor is loaded beyond its rated capacity due to incorrect operating conditions.

This leads to a motor overload, an increase in current flowing through the winding and an increase in the temperature of the winding. This results in a permanent damage to the motor winding and the cables.

In a motor circuit, the starter overload relays, protect the motor, and the associated cables against overload and the fuses in the circuit provide the required degree of short-circuit protection. A short-circuit protection is required to protect motor conductors, overload relays, and motors from the short-circuit condition. It’s achieved by using the non-time delay fuse, instantaneous trip breaker, or the inverse time-breaker.

Usually, manufacturers give recommendations regarding the fuse ratings required to cope with the motor starting surges and indicate the minimum cable sizes required to achieve a short-circuit protection. In a well-designed combination, the starter itself interrupts all the overloads up to the stalled rotor condition. The fuses should only operate in the event of an electrical fault. The starter manufacturers indicate the maximum fuse rating, which may be used with a given starter to ensure satisfactory protection.

Bimetal relay with single-phasing protection

This is an overload protection provided externally to the motor. It’s connected in series with the motor supply. A bimetallic strip operates once the temperature exceeds predetermined limits, causing the contacts to open. After the relay has tripped and the contacts are open, the problem should be solved before pressing the reset button. The bimetal relays provide an accurate overload and an accelerated single-phasing protection for the three-phase motors. It incorporates a dual slider principle for accelerated tripping under the single-phasing protection.

The bimetal relay also provides protection against severe unbalanced voltages. The bimetal relays protect themselves against overloads of up to 10 times the maximum setting. Beyond this limit, they have to be protected from short-circuits. It’s mandatory to use backup fuses. I-t characteristics for three-phase operations and single-phasing conditions.

Phase failure relays

This protection interrupts power in all phases upon failure of any one phase. Normal overload relays or fuses may not protect the motor from damage due to single phasing.

Phase-failure relay senses the negative-sequence voltage component of the supply and offers protection against phase failure, unbalanced phases, phase reversal, and under-and over voltage faults.

++++… characteristics for three-phase operation and single-phasing condition. Multiples of set current; Time in seconds; Cold; Multiples of set current; Time in seconds; I-t characteristics on 3-f operation I-t characteristics on 3-f operation

Winding-protection relays

Winding-protection relays provide protection against overheating of the windings of motors, alternators, transformers, etc. Temperature is sensed with the help of a PTC thermistor embedded in the winding that gives a tripping signal when the temperature exceeds the response temperature of the thermistor.

In some cases, thermocouples or RTD (resistance temperature detectors) are fitted inside the winding to accurately indicate the temperature of the winding.

Switchboards

A switchboard is a distribution board (DB) that receives a large amount of power and dispatches it in small packets to various electrical equipments.

It has power-controlling devices such as breakers, switches along with protection devices such as fuses, etc.

Switchboards in general are divided into the following four classes:

• • Direct-control panel-type
• • Remote mechanical-control panel-type
• • Direct-control truck-type
• • Electrical-operated

Direct-control panel-type switchboards

With the direct-control panel-type, switches, rheostats, bus bars, meters, and other apparatus are mounted on the board or near the board and the switches and rheostats are operated directly by operating handles if they are mounted on the back of the board. For both AC and DC, voltages are limited to 600 V or lower, but with oil circuit breakers, they may operate up to 2500 V. Such panels are not recommended for capacities of more than 3000 kVA. 5.4.2 Remote mechanical-control panel-type switchboards

Remote mechanical-control panel-type boards are the AC switchboards with the bus bars and connections removed from the panels and mounted separately away from the load.

The oil circuit breakers are operated by levers and rods. This type of board is designed for a heavier duty than the direct-control type switchboards and is used up to 25 000 kVA. 5.4.3 Direct-control truck-type switchboards Direct-control truck-type switchboards are used for 15 000 V or lower and consist of equipment enclosed in steel compartments completely assembled. The high-voltage parts are enclosed and the equipment is interlocked to prevent any operational mistakes. This type of a switchboard is designed for low- and medium-capacity plants and for auxiliary power in large generating stations.

Electrically operated switchboards

Electrically operated switchboards employ solenoid or motor-operated circuit breakers.

Rheostats, etc. are controlled by small switches mounted on panels. Electrically operated switchboards make it possible to locate high-voltage and other equipment independent of the location of the switchboard.

Switchboards should be erected at least 1-2 m (3-4 ft) from the walls. Switchboard frames and structures should be grounded. For low-potential equipment, the conductors on the rear of the switchboard are usually made of a flat copper strip known as a copper bus bar. Aluminum bus bars are also used due to its low cost. Switchboards must be individually adapted for each specific electrical equipment/system.

Motor control center

In large plants, a number of electrical motors are placed offsite. To conveniently locate all supply cables, control circuitry, and various protections at one location, there is a Motor Control Center (MCC). The size of an MCC depends on the number of electrical circuits and motors it controls.

An MCC consists of a number of cubicles or compartments in a compact, floor-mounted assembly. The cubicles are sized differently for starters depending on the rating of the motor it controls.