Ratings of Circuit Breaker - Detaily Explained...

Short Circuit Breaking Current of Circuit Breaker :

    This is the maximum short circuit current which a circuit breaker can withstand before it. Finally cleared by opening its contacts. When a short circuit flows through a circuit breaker, there would be thermal and mechanical stresses in the current carrying parts of the breaker. If the contact area and cross-section of the conducting parts of the circuit breaker are not sufficiently large, there may be a chance of permanent damage in insulation as well as conducting parts of the CB.
    As per Joule’s law of heating, the rising temperature is directly proportional to square of short circuit current, contact resistance and duration of short circuit current. The short circuit current continuous to flow through circuit breaker until the short circuit is cleared by opening operation of the circuit breaker. As the thermal stress in the circuit breaker is proportional to the period of short circuit, the breaking capacity of electrical circuit breaker, depends upon the operating time.
    At 160°C aluminum becomes soft and losses its mechanical strength, this temperature may be taken as limit of temperature rise of breaker contacts during short circuit. Hence short circuit breaking capacity or short circuit breaking current of circuit breaker is defined as maximum current can flow through the breaker from time of occurring short circuit to the time of clearing the short circuit without any permanent damage in the CB. The value of short circuit breaking current is expressed in RMS. During short circuit, the CB is not only subjected to thermal stress, it also suffers seriously from mechanical stresses. So during determining short circuit capacity, the mechanical strength of the CB is also considered. So for choosing suitable circuit breaker it is obvious to determine the fault level at that point of the system where CB to be installed. Once the fault level of any part of electrical transmission is determined it is easy to choose the correct rated circuit breaker for this part of network. 

Rated Short Circuit Making Capacity 

    The short circuit making capacity of circuit breaker is expressed in peak value not in rms value like breaking capacity.Theoretically at the instant of fault occurrence in a system, the fault current can rise to twice of its symmetrical fault level. At the instant of switching on a circuit breaker in faulty condition, of system, the short circuit portion of the system connected to the source. The first cycle of the current during a circuit is closed by circuit breaker, has maximum amplitude. This is about twice of the amplitude of symmetrical fault current waveform. The breaker’s contacts have to withstand this highest value of current during the first cycle of waveform when breaker is closed under fault. On the basis of this above mentioned phenomenon, a selected breaker should be rated with short circuit making capacity. As the rated short circuit making current of circuit breaker is expressed in maximum peak value, it is always more than rated short circuit breaking current of circuit breaker. Normally value of short circuit making current is 2.5 times more than short circuit breaking current.

Rated Operating Sequence or Duty Cycle of Circuit Breaker:

    This is mechanical duty requirement of circuit breaker operating mechanism. The sequence of rated operating duty of a circuit breaker has been specified as  

Where, O indicates opening operation of CB. CO represents closing operation immediately followed by an opening operation without any intentional time delay. t' is time between two operations which is necessary to restore the initial conditions and / or to prevent undue heating of conducting parts of circuit breaker. t = 0.3 sec for circuit breaker intended for first auto re closing duty, if not otherwise specified. Suppose rated duty circle of a circuit breaker is

This means, an opening operation of circuit breaker is followed by a closing operation after a time interval of 0.3 sec, then the circuit breaker again opens without any intentional time delay. After this opening operation the CB is again closed after 3 minutes and then instantly trips without any intentional time delay. 

Rated Short Time Current 

    This is the current limit which a circuit breaker can carry safely for certain specific time without any damage in it. The circuit breakers do not clear the short circuit current as soon as any fault occurs in the system. There always some intentional and an intentional time delays present between the instant of occurrence of fault and instant of clearing the fault by CB. This delays are because of time of operation of protection relays, time of operation of circuit breaker and also there may be some intentional time delay imposed in relay for proper coordination of power system protection. Even a circuit breaker fails to trip, the fault will be cleared by next higher positioned circuit breaker. In this case the fault clearing time is longer. Hence, after fault, a circuit breaker has to carry the short circuit for certain time. The summation of all time delays should not be more than 3 seconds, hence a circuit breaker should be capable of carrying a maximum faulty current for at least this short period of time. The short circuit current may have two major affects inside a circuit breaker.

1. Because of the high electric current, there may be high thermal stress in the insulation and conducting parts of CB.
2. The high short circuit current, produces significant mechanical stresses in different current carrying parts of the circuit breaker. A circuit breaker is designed to withstand these stresses. But no circuit breaker has to carry a short circuit current not more than current for a specified short period. The rated short time current of a circuit breaker is at least equal to rated short circuit breaking current of the circuit breaker.
Rated Voltage of Circuit Breaker Rated voltage of circuit breaker depends upon its insulation system. For below 400 KV system, the circuit breaker is designed to withstand 10% above the normal system voltage. For above or equal 400 KV system the insulation of circuit breaker should be capable of withstanding 5% above the normal system voltage. That means, rated voltage of circuit breaker corresponds to the highest system voltage. This is because during no load or small load condition the voltage level of power system is allowed rise up to highest voltage rating of the system.

Nominal System Voltage	Highest System Voltage	Power Frequency Withstand Voltage	Impulse Voltage Level
11 KV	12 KV	-	-
33 KV	36 KV	70 KV	170 KV
132 KV	145 KV	275 KV	650 KV
220 KV	245 KV	460 KV	1050 KV
400 KV	420 KV	-	-

Source : Electrical4u

What is Operating Sequence in Circuit Breaker ? What does O-0.3- CO- 3-CO Means ???

This is the Rated Operating Sequence (Duty Cycle) of the circuit breaker. Which denotes the sequence of opening and closing operations which the circuit breaker can perform under specified conditions. The operating mechanism experinces sever stress during the auto-reclosure duty; however, the circuit breaker should be able to perform the operating sequence as follows:

O-t-CO-T-CO

where,

O = opening operation

t = time required for circuit breaker to be ready to receive closing order from auto-reclosure relay (0.3 s to be used for rapid reclosure), (3 min not to be used for rapid reclosure).

CO = close operation followed by open operations

T = time required by the circuit breaker, insulating media for regeneration and operating mechanism (3 min)

CO = close operation followed by open operations

What are the types of CT Cores... What does 5P20 Means...???

        CT's are categorized as Protection CT, Special Protection 
CT and Measuring CT. Based on this, the CT's are 
classified. Here is the meaning of the CT classes:

Class 5P20: 

The letter 'P' indicates it is a protection CT.

The number 5 indicates the accuracy of the CT. Most common 
accuracy numbers are 5 and 10.

The number 20 (called accuracy limit factor) indicates that 
the CT will sense the current with the specified accuracy 
even with 20 times of its secondary current flows in the 
secondary. This is required for protection CT, because the 
fault current is high and the CT should be able to sense 
the high fault current accurately to protect the system. 
The common numbers are 10, 15, 20 and 30.
This type of ct are used for over current protection  of feelers and transformer's.

Class PS:

PS is for 'Protection Special'. This class of CT's are used 
for special protection such as differential protection, 
distance protection etc.

Class 1M:

The letter 'M' indicates it is a measuring CT.

The number 1 indicated the accuracy of the CT. The 
measuring CT's should be more accurate than the protection 
CT. The most common accuracy numbers are 0.5 and 1.

Measuring CT's are intended to work in normal and it 
doesn't require working with accuracy beyond the CT 
secondary current rating, so the accuracy limit factor is 
not mentioned here.

Importance of Substation Earthings - Detaily Explained

Functions of an Earthing System

The two primary functions of a safe earthing system are i) ensure that person who is in the vicinity of earthed facilities during a fault is not exposed to the possibility of a fatal electrical shock, ii) provide a low impedance path to earth for currents occurring under normal and fault conditions

Earthing Standards

There are a variety of national and international standards available, which provide empirical formulae for the calculation of earthing design parameters and shock potential safety limits.

BS7354 - 1990 Code of Practice for Design of High Voltage Open Terminal Stations

IEEE Std 80-2000 IEEE Guide for Safety in AC Substation Grounding

EATS 41-24 - Guidelines for the Design, Installation, Testing & Maintenance of Main Earthing Systems in Substations

Ground Potential Rise (GPR)

The substation earth grid is used as an electrical connection to earth at zero potential reference. This connection is not ideal due to the resistivity of the soil within which the earth grid is buried. 

During typical earth fault conditions, the flow of current via the grid to earth will therefore result in the grid rising in potential relative to remote earth to which other system neutrals are also connected.

This produces potential gradients within and around the substation ground area - this is defined as ground potential rise or GPR.

The GPR of a substation under earth fault conditions must be limited so that step and touch potential limits are not exceeded, and is controlled by keeping the earthing grid resistance as low as possible.

Step, Touch, Mesh & Transferred Potentials

In order to ensure the safety of people at a substation, it is necessary to ensure that step and touch potentials in and around the substation yard during earth-fault conditions are kept below set limits. 

These maximum permitted step and touch potentials are addressed within various national and international standards.

Step Potential

The step potential is defined as the potential difference between a persons outstretched feet, normally 1 metre apart, without the person touching any earthed structure.

Touch Potential

The touch potential is defined as the potential difference between a persons outstretched hand, touching an earthed structure, and his foot. A persons maximum reach is normally assumed to be 1 metre.

Mesh Potential

The mesh potential is defined as the potential difference between the centre of an earthing grid mesh and a structure earthed to the buried grid conductors.

This is effectively a worst-case touch potential - for a substation grid consisting of equal size meshes, it is the meshes at the corner of the earth grid that will have the highest mesh potential.

Transferred Potential

This is a special case of a touch potential in which a voltage is transferred into or out of a substation for some distances by means of an earth referenced metallic conductor. 

This can be a very high touch potential as, during fault conditions, the resulting potential to ground may equal the full GPR.

Earthing System Design Considerations

Conductors - a substation earthing grid will consist of a earthing system of bonded cross conductors.

The earthing conductors, composing the grid and connections to all equipment and structures, must possess sufficient thermal capacity to pass the highest fault current for the required time. 

Also, the earthing conductors must have sufficient mechanical strength and corrosion resistance. It is normal practice to bury horizontal earthing conductors at a depth of between 0.5m and 1m - this ensures that the earth conductor has the following properties :

1. Adequate mechanical protection
2. It is situated below the frost line
3. The surrounding earth will not dry out

Vertically Driven Earth Rods

Where there are low resistivity strata beneath the surface layer then it would be advantageous to drive vertical earth rods down into this layer - to be effective the earth rods should be on the periphery of the site. The length of the earth rod is chosen so as to reach the more stable layers of ground below. 

The earth rods would stabilise the earth grid resistance over seasonal resistivity changes at the grid burial depth.

Substation Fences

The earthing of metallic fences around a substation is of vital importance because dangerous touch potentials can be involved and the fence is often accessible to the general public. 

Fence earthing can be accomplished in two different ways :

1. Electrically connecting the fence to the earth grid, locating it within the grid area or alternatively just outside
2. Independently earthing the fence and locating it outside the earth grid area at a convenient place where the potential gradient from the grid edge is acceptably low.

Other Substation Earthing

The GPR at a substation is reduced by:

1. Overhead line earth wires which are connected to the substation earthing grid. This diverts part of the earth fault current to the tower footing earthing.
2. Cable entering and leaving the site. The armouring of such cables is usually earthed to the substation earthing grid at both ends. Part of the earth fault current will thus be diverted to a remote earthing grid via the cable armouring. 
3. 
   
4. Source : www.cablejoints.co.uk

What are the types of Class I, II, III, 0, 01 in Electrical Appliances ?

    Basically, a Portable appliance is given a Class rating by the manufacturer depending on how the user is protected from Electrical Shock. 

    Electrical appliances classes defined in IEC 61140, and are categorized into one of the five Classes - Class I, Class II, III, 0 or 01.

    If an appliance uses mains voltage, it has to provide  minimum two levels of protection, many  equipments have more, but 2 is the minimum. By having more levels of protection, this ensures the appliance remains safe even if the first level fails.Any portable appliances without a class rating should be treated as a Class 1 appliance.Lets see the different Classes.

Class I Appliances :

    The protection inside class 1 appliances combines together the protection of insulation of conductor and a means of connection to the Earth protective conductor (Earth wire).

    This plastic insulation of the conductor is known as basic insulation. If this basic insulation were to fail, say due to abrasion from excessive movement of the cable where it touches the metal case, then the user could receive an electric shock if  the Earth wire is absent.

    By connecting to the metal case of the equipment to the Earth wire , the Earth wire keeps all this metal at EARTH Potential.It is impossible to get an electric shock even when the metal case of the fire is connected directly to the LIVE voltage. In practice a fuse would blow either in the plug or the main fuse box to protect the user or a RCD would trip.

    Usually Power Cord of Class I appliances is having three core cable with three Pin Plug - One Pin for Live Wire, One for Neutral and another for Earth Wire.

    For Class I items, you just need to remember they offer two levels of protection. Remember:

• Basic Insulation
• Earth wire

Symbol :

Examples :

• Toasters,
• Kettles,
• Washing machines,
• Iron Box,
• Water Heater,
• Welding Machines

Class II Appliances:

    These appliances are known as double insulated due to the presence of at least two layers of insulation. Often they’re constructed with insulated wiring inside, as well as extra insulation because of the device’s plastic case.

    The earth connection present in Class II appliances is not required for safety.Hence usually the Power Cord having two core wire - One for Phase and another for Neutral.

    For Class II items, you just need to remember they offer two levels of protection. Remember:

• Basic Insulation
• Device Plastic Case

Examples:

• Drillers,
• Angle Grinders

Symbol:

CLASS III

    Equipment built to the Class III standard is designed to be supplied from a special safety isolating transformer whose output is known as Separated Extra-Low Voltage or SELV. This must not exceed 50 V AC and is normally below 24V or 12V. 

    There is no use of an Earth in Class III appliances. The electrical safety of Class III appliances is taken care of in the safety isolating transformer design where the separation between the windings is equivalent to double insulation.

Symbol:

The transformer SELV  is marked as shown here 

Class 0 :

    A much rarer form of equipment, these kinds of appliances are generally not found with business and residential environments.

    Class 0 appliances depend only on basic insulation without a provision for earth. If it fails, it is entirely dependent on the environment around it to remain safe.

     Hence it is usually wired with Two Pin Plug without provision for Earth Connection.

Examples:

• A two Pin plug Soldering Iron without provision for earthing.

Class 01 :

    Class 01 appliances do have room for an earth connection, in addition to class 0 equipments.

Examples:

• A two Pin plug Brass Lamp with Provision for Earthing at it Case.

    Class 0 and 01 appliances have been effectively banned since 1975.

Summary:

• If the rating plate has a double box then it is Class II;
• If the rating plate does not have a double box then assume Class I;
• Class II appliances are double insulated and inherently safer than Class I appliances;
• Class I appliances depend on the Earth provided by the installation wiring for their safety.
• Class 0 and 01 are not used for Commercial applications.

Why crest factor is so important in UPS ???

          The crest factor of an AC current waveform is the ratio of waveform’s peak value to its rms value:

          crest factor = |peak current| / rms current

The crest factor for a sinusoidal current waveform is 1.414 since the peak value of a true sinusoid is 1.414 times the rms value. Current waveforms for purely resistive loads are sinusoidal, so the crest factor will be 1.414.

Some loads, such as switching power supplies or lamp ballasts, have current waveforms that are not sinusoidal. They draw a high current for a short period of time, and their crest factors, therefore, can be quite a bit higher than 1.414.

Figure 1 shows current waveforms for two different loads, one sinusoidal (the blue trace) and one non-sinusoidal (the red trace). Both have an rms current of 5A, but as you can see the crest factor is quite different.

These two waveforms both have an rms current of 5 A, but their crest factors are very different.

As you would expect, the sinusoidal current waveform has a crest factor of 1.414:

crest factor = |peak current| / rms current = 7.07 A / 5 A = 1.414

The non-sinusoidal current waveform, on the other hand, has a peak value of 21.21 A. The crest factor for this waveform is then:

crest factor = |peak current| / rms current = 21.21 A / 5 A = 4.24

Why is crest factor important?

In the examples shown in Figure 1, both loads draw the same amount of true power (assuming that the input voltage is the same for both). This means that a power source selected to feed the loads at 120VAC would need to provide the 600VA that both loads require.

A power source with a power output rating of 600VA may not, however, be able to provide the required peak currents that the non-sinusoidal load demands. When selecting an AC power source to power this load, you would need to choose a source possible of supply more than 21 A of peak current. In order to determine whether or not an AC source can handle high crest factor peak currents, look for the “peak repetitive current” or “crest factor” specifications in a power source’s spec sheet.

The California Instruments i-iX Series II is a good example of an AC power source that is designed to handle high crest factor loads. The i/iX Series II AC sources can drive difficult nonlinear loads, such as switching power supplies, with a crest factor of up to 5:1, with ease.

SOURCE : ProgramablePower