Low-voltage networks: Introduction; Air circuit breakers; Molded case circuit breakers [Industrial Electrical Power Systems]

Introduction

The low-voltage network is a very important component of a power system as it’s at this level that much of the power is distributed and utilized by the end consumer. Essential loads such as lighting, heating, ventilation, refrigeration, air-conditioning and so on are generally fed at voltages such as 380, 400, 415, 480, 500, 525 V, three-phase 3 wire and three-phase 4 wire.

In mining industry, heavy motor loads often require voltages as high as 1000 V. Because of the diverse nature of the loads coupled with the large number of items requiring power, it’s usual to find a bulk in-feed to an LV switchboard, followed by numerous outgoing circuits of varying current ratings, in contrast to the limited number of circuits at the medium-voltage level. Large frame (high current rated) air circuit breakers are therefore specified as incomers from the supply transformer and molded case circuit breakers (MCCBs) for all outgoing feeders. The downstream network generally consists of MCCBs of varying current ratings and as the current levels drop miniature circuit breakers (MCBs) are used for compactness and cost saving.

Air circuit breakers

These breakers are available in frame sizes ranging from 630 to 6000A in 3 and 4 pole versions and are generally insulated for 1000 V. Rated breaking capacities of up to 100 kA rms symmetric to IEC947-2 are claimed at rated voltage of 660 V. Fixed and draw-out models are available and each unit invariably comes complete with a protection device, which, in keeping with modern trends, is generally of the electronic type and will be discussed in more detail later.

Typical total breaking times are of the order of 40-50 ms for short circuit faults. Their operating speed is important as air circuit breakers (ACBs) are applied as the main incoming devices to the low-voltage network where they are subject to the highest fault levels determined by the supply transformer.

A typical construction of an ACB is shown on the next page. The spring charging can be manual and electrical. The operating and tripping mechanisms are similar to the ones used in high-voltage oil/vacuum/SF6 circuit breakers.

It’s common to note that most of the present day ACBs are fitted with solid-state built in overcurrent and ground fault relay. The same can be set for various current and time characteristics without the need for using external relays. These relays are provided with ample current and time setting ranges to achieve discrimination for various types of LV distribution systems.

++++ Typical internal construction of an air circuit breaker

Molded case circuit breakers

MCCBs are power switches with built-in protective functions used on circuits requiring lower current ratings. They include the following features:

• • Normal load current open and close switching functions
• • Protection functions to automatically disconnect excessive overloads and to interrupt short circuit currents as quickly as possible
• • Provide indication status of the MCCB either open, closed or tripped.

Although many different types are manufactured, they all consist of five main parts:

1. Molded case (frame)

2. Operating mechanism

3. Contacts and extinguishers

4. Tripping elements

5. Terminal connections.

Molded case

This is the external cover of the MCCB, which houses all the sensing and operating components. This is molded from resin/glass-fiber materials, which combine ruggedness with high dielectric strength.

The enclosure provides a frame on which to mount the components, but more importantly, it provides insulation between the live components and the operator. Different physical sizes of case are required by maximum rated voltage/current and interrupting capacity and are assigned a 'frame size'. It’s to be noted that the case is molded and as such, it’s not possible to access the internal components in case of any failures and it will be necessary to replace the complete MCCB under those circumstances.

Operating switch/mechanism:

The operating switch is accessible from outside for ON/OFF/RESET purposes. This is a handle, which connects the internal mechanism for the ON/OFF/RESET operations. In passing from ON to OFF (or vice versa), the handle tension spring passes through alignment with the toggle link and in doing so a positive rapid contact-operating action is produced to give a 'quick break' or a 'quick make' action. This makes it independent of the human element i.e. the force and speed of operating the handle.

The mechanism also has a 'trip free' feature, which means that it cannot be prevented from tripping by holding the operating handle in the ON position, during faults. In other words, the protective contact-opening function cannot be defeated.

In addition to indicating when the breaker is ON (in the up position) or OFF (in the down position), the TRIP condition is indicated by the handle occupying the positions midway between the extremes --- below. To restore service after the breaker has tripped, the handle must first be moved to the OFF position to reset the mechanism before being moved to the ON position.

ON-ON-ON-OFF-OFF-OFF; Handle-Handle-Handle-ON indication line--White line indicates ON--Handle centered:

ON indication line not visible (a) On; (b) Tripped; (c) Off

++++ Handle positions

Contacts and extinguishers

A pair of contacts comprises a moving contact and a fixed contact. The instants of opening and closing impose the most severe duty. Contact materials must therefore be selected with consideration to three criteria:

1. Minimum contact resistance

2. Maximum resistance to wear

3. Maximum resistance to welding

Silver or silver-alloy contacts are low in resistance but wear rather easily. Tungsten or tungsten-alloys are strong against wear due to arcing but rather high in contact resistance.

Contacts are thus designed to have a rolling action, containing mostly silver at the closing current-carrying points, and mostly tungsten at the opening (arcing) point.

(b) Opening; (a) Closing

Majority of tungsten -- Majority of silver

++++ Dual function contacts

In order to interrupt high short-circuit currents, large amounts of energy must be dissipated within the molded case. This is achieved by using an arc-chute, which comprises a set of specially shaped steel grids, isolated from each other and supported by an insulated housing. When the contacts are opened and an arc is drawn, a magnetic field is induced in the grids, which draws the arc into the grids. The arc is thus lengthened and chopped up into a series of smaller arcs, which are cooled by the grids heat conduction.

Being longer it requires far more voltage to sustain it and being cooler tends to lose ionization and extinguish at first current zero.

Grid Induced flux; Arc; Supporting frame; Grids; Attraction force

++++ The arc chute; Tripping elements

The function of the trip elements is to detect the overload or short-circuit condition and trip the operating mechanism.

Thermal overload:

The thermal trip characteristic is required to be as close as possible to the thermal characteristics of cables, transformers, etc. To cover this overload condition two types of tripping methods are available, namely Bi-metallic and hydraulic.

Bi-metallic method:

The thermal trip action is achieved by using a bi-metallic element heated by the load current. The bi-metal consists of two strips of dissimilar metals bonded together. Heat due to excessive current will cause the bi-metal to bend because of the difference in the rate of expansion of the two metals. The bi-metal must deflect far enough to physically operate the trip bar. These thermal elements are factory-adjusted and are not adjustable in the field. A specific thermal element must be provided for each current rating.

A number of different variations on this theme are available. The bi-metal is temperature-sensitive and automatically rerates itself with variations in ambient temperature.

Deflection--Moving core-- Secondary CT core--Fixed core -- Heating resistor--Bi-metal (a) Direct heating (b) Indirect heating (c) Direct-indirect heating (d) CT heating

Thermal tripping methods--Hydraulic method

For its operation, this device depends on the electromagnetic force produced by the current flowing in a solenoid wound around a sealed non-magnetic tube. The tube, filled with a retarding fluid, contains an iron core, which is free to move against a carefully tensioned spring. For normal load current, the magnetic force is in equilibrium with the pressure of the spring.

When an overload occurs, the magnetic force exceeds that of the spring and the iron core begins to move reducing the air-gap in the tripping armature. Once the magnetic field is large enough, the armature closes to trip the mechanism. The time delay characteristic is controlled by the retarding action of the fluid.

(a) Normal operation; (c) Instantaneous tripping; (b) Long-delay tripping

Hydraulic tripping method--Short circuits

For short-circuit conditions, the response time of the thermal element is too slow and a faster type of protection is required to reduce damage, etc. For this reason, a magnetic trip action is used in addition to the thermal element. When a fault occurs, the short circuit current causes the electromagnet to attract an armature, which unlatches the trip mechanism. This is a fast action and the only delay is the time it takes for the contacts to physically open and extinguish the arc. This is normally of the order of 20 ms - typically 1 cycle.

In the hydraulic method, the current through the solenoid will be large enough to attract the armature instantaneously, irrespective of the position of the iron core. The interruption speeds for this type of breaker for short circuit currents are also less than 1 cycle (20 ms) similar to the bi-metallic type.

In both of the above methods, the thermal magnetic and the hydraulic magnetic, the tripping characteristics generally follow the same format.

Long delay (bi-metal) operation area Instantaneous (electromagnet) operation area; Overcurrent; Overcurrent 100%; 100% Ramp

Thermal/hydraulic magnetic; Electronic

Typical tripping characteristics -- Electronic protection MCCBs

Molded case circuit breakers of the conventional types mentioned above are increasingly being replaced by electronic trip units and current transformers, which are an integral part of the breaker frame. This modern trend in the technology results in increased accuracy, reliability and repeatability. However, the main advantage is the adoptability of the tripping characteristics compared to the above-mentioned electromechanical devices, which are generally factory pre-set and fixed for each current rating. Discrimination can then be improved. Furthermore, semi-conductor-controlled power equipment can be a source of harmonics which may cause mal-operations.

Electronic protective devices detect the true rms value of the current, thereby remaining unaffected by harmonics. A comparison of the thermal-magnetic and hydraulic-magnetic types is given.

Terminal connections:

These connect the MCCB to a power source and a load. There are several methods of connection such as busbars, straps, studs, plug-in adapters, etc. Up to 250-300 A whenever cables are used, compression type terminals are used to connect the conductor to the breaker. Above 300 A, stubs, busbars or straps are recommended to ensure reliable connections, particularly when using aluminum cables. Please refer to the subsequent pages regarding the cautions to be taken while connecting the MCCB in a power circuit.

Miniature circuit breakers The miniature circuit breakers are similar to molded case circuit breakers but as their name implies, these are smaller in size and are mostly used for current ratings below 100 A. These are normally available in single pole (SP), single pole neutral (SPN), double pole (DP), triple pole (TP), triple pole neutral (TPN) and four pole (FP) versions.

The MCCBs and MCBs are available for both AC and DC ratings for the various standard voltages. However, it must be remembered that a 220 V AC MCB may not be suitable for a 110 V DC application, unless it’s tested and approved by the manufacturer. Hence, proper care must be taken while using MCBs and MCCBs in DC circuits.

===

Current, Low temperature, Low frequency, Low temperature, Low frequency, Horizontal, High temperature, High frequency, High temperature, High frequency, Ceiling

Operating current is affected by ambient temperature (bi-metal responds to absolute temperature not temperature rise).

Negligible effect up to several hundred Hz; above that the instantaneous trip is affected due to increased iron losses.

Bi-metal must provide adequate deflection force and desired temperature characteristic. Operating time range is Negligible effect.

Affected only to the extent that the damping-oil viscosity is affected.

Trip current increases with frequency, due to increased iron losses.

Oil viscosity, cylinder, core and spring design, etc., allow a wide choice of operating times.

Mounting attitude changes the effective weight of the magnetic core.

Ambient temperature; Frequency; Flexibility of operating characteristics; Mounting attitude; Item; Thermal-magnetic type; Hydraulic-magnetic type; ON—ON—OFF--OFF

++++ Comparison of thermal-magnetic and hydraulic-magnetic types.

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Current-limiting MCCBs

Current-limiting MCCBs are essentially extremely fast-acting breakers that interrupt the short circuit fault current before it reaches the first peak, so reducing the current or energy let-through in the same manner as a fuse. They are therefore required to operate in the first quarter of a cycle, i.e. 5ms or less and limit the peak short circuit current to a much lower value, after which can be switched on again, if necessary, without replacement of any parts or elements.

Prospective short-circuit current; Let-through current; Current

Inductive (magnetic stress) energy =1/2 Li

Thermal (resistive heating) energy =R? 0i

++++ Limited short circuit let-through current

This high contact speed of separation is achieved by using a reverse loop stationary contact. When a fault develops, the current flowing in the specially designed contacts causes an electro-dynamic repulsion between them. The forces between the contact arms increase exponentially rather than linearly. As the contact gap widens, the arc is quickly extinguished by a high-performance arc-chute.

By limiting the let-through current, the thermal and magnetic stresses on protected equipment such as cables and busbars is reduced in case of a short circuit. Provided combination series tests have been done and certified, this also permits the use of MCCBs with lower short-circuit capacities to be used at downstream locations from the current limiting MCCB. This is known as cascading and results in a more economical system, but additional care must be taken to preserve discrimination between breakers. This will be discussed in more detail in the next Section 11.4.

Accessories:

The following accessories are available with all different makes of MCCB:

• Shunt trip coils

• Under-voltage release coils

• Auxiliary switches

• Mechanical interlocks

• Residual current devices ( ground fault protection). A typical molded case circuit breaker (MCCB) is shown. When selecting an MCCB for an application it’s important to ensure that the following ratings are correct:

1. Voltage rating (AC/DC)

2. Current rating

3. Breaking capacity rating.

++++ Typical molded case circuit breaker

Installation:

++++ indicates the standard precautions to be followed while installing the MCCB.

Connection --indicates the standard precautions to be followed while connecting the MCCB.

Change ratio of the rated current value according to the installation angle Don’t remove the rear cover Don’t remove the compound inserted into the screw part of the base rear or the rear cover Cautions for installation

++++ Cautions for installation

1. Take sufficient insulation distance

2. Don’t apply oil to the threaded parts

1. Parallel conductors for all poles

Take care, as the insulation distance may be insufficient according to the installation position of the connection conductor.

As some type are provided with insulation barriers.

Don’t apply lubrication oil to the threaded parts.

Application of lubrication oil reduces the friction of the threaded part, so that loosening and overheat can be caused. In case of lubrication, even the standard tightening torque can produce excessive stress in the threaded part and thus breaking of the screw.

Install the connection conductors in parallel for all poles.

++++ Cautions for connections.