Sunday, August 6, 2017

Short circuit characteristics of circuit breakers IS/IEC 60947-2:2003

Rated short-circuit making capacity (Icm)
The rated short-circuit making capacity of a circuit-breaker is the value of short-circuit capacity assigned to that circuit-breaker by the manufacturer for the rated operational making voltage at rated frequency and at a specified power factor for A.C., or time constant for D.C. It is expressed as the maximum prospective peak current. For a c the rated short-circuit making capacity of a circuit-breaker shall be not less than its rated ultimate short-circuit breaking capacity, multiplied by the factor n of table as below  .

Ratio of Making and breaking capacity of breakers

For d c the rated short-circuit making capacity of a circuit-breaker shall be not less than its rated ultimate short-circuit breaking capacity. A rated short-circuit making capacity implies that the circuit-breaker shall be able to make the current corresponding to that rated capacity at the appropriate applied voltage related to the rated operational voltage.

Rated short-circuit breaking capacities
In case of circuit breakers rated Short-Circuit capacity is the values of short-circuit breaking capacity of that circuit-breaker assigned by the manufacturer for the rated operational voltage, under specified conditions.

A rated short-circuit breaking capacity requires that the circuit-breaker shall be able to break any value of short-circuit current up to and including the value corresponding to the rated capacity at a power-frequency recovery voltage corresponding to the prescribed test voltage values and:
a)      For alternating current at any power factor not less than as per table below;


Value of Power factor and time constant corressponding to currents

b)    For direct current, with any time constant not greater than as per table given above.

For power-frequency recovery voltages in excess of the prescribed test voltage values , no short-circuit breaking capacity is guaranteed

Now what are Power frequency recovery voltages?
All tests shall be made at the rated frequency of the circuit-breaker. For all short-circuit tests, if the rated breaking capacity is essentially dependent on the value of the frequency, the tolerance shall not exceed  +-5%. If the manufacturer declares the rated breaking capacity to be substantially unaffected by the
value of the frequency, the tolerance shall not exceed +-25 %.

For alternating current the circuit-breaker shall be capable of breaking a prospective current corresponding to its rated short-circuit breaking capacity and the related power factor given in table above, irrespective of the value of the inherent D.C. component, on the assumption that the AC, component is constant.

Definitions:-
(a)   Rated service short-circuit breaking capacity of a circuit-breaker
A breaking capacity for which the prescribed conditions according to a specified test sequence include the capability of the circuit-breaker to carry its rated current continuously

(b)   Ultimate short circuit breaking capacity of circuit breaker:-
A breaking capacity for which the prescribed conditions according to a specified test sequence do not include the capability of the circuit-breaker to carry its rated current continuously

The rated short-circuit breaking capacities are stated as
(i)             Rated ultimate short-circuit breaking capacity;
(ii)            Rated service short-circuit breaking capacity.

(i)            Rated ultimate short-circuit breaking capacity (Icu)
The rated ultimate short-circuit breaking capacity of a circuit-breaker is the value of ultimate short-circuit breaking capacity assigned to that circuit-breaker by the manufacturer for the corresponding rated operational voltage. It is expressed as the value of the prospective breaking current, in kA (RMS. value of the a.c. component in the case of a,c. ),

(ii)           Rated service short-circuit breaking capacity (Ics.)
The rated service short-circuit breaking capacity of a circuit-breaker is the value of service short-circuit breaking capacity assigned to that circuit-breaker by the manufacturer for the corresponding rated operational voltage.  It is expressed as a value of prospective breaking current, in kA, corresponding to one of the specified percentages of the rated ultimate short-circuit breaking capacity, in accordance with table as given below, and rounded up to the nearest whole number. It may be expressed as a % of Icu (for example Ics = 25 % Icu).
Alternatively, when the rated service short-circuit breaking capacity is equal to the rated short-time withstand current , it may be stated as that value, in kA, provided that it is not less than the relevant minimum value of table as below.
Minimum Value of Rated Short time withstand current

Where Icu exceeds 200 kA for utilization category A, or 1000 kA for utilization category B, the manufacturer may declare a value Ics of 50 kA.
Standard ratios between making and breaking capacity of circuit breakers


Standard relationship between short-circuit making and breaking capacities and related power factor, for a.c. circuit-breakers
The standard relationship between short-circuit breaking capacity and short-circuit making capacity is as given in table below.
Short circuit breaking capacity and making capacity relation


The rated short-circuit making and breaking capacities are only valid when the circuit-breaker is operated in accordance with the requirements as given below in Operating conditions of circuit breakers.  For special requirements, the manufacturer may assign a value of rated short-circuit making capacity higher than that required by table above. Tests to verify these rated values shall be the subject of agreement between manufacturer and user.
Rated short-time withstand current (Icw)
The rated short-time withstand current of a circuit-breaker is the value of short-time withstand
current assigned to that circuit-breaker by the manufacturer under the test conditions
For A.C., the value of this current is the R.M.S value of the A.C. component of the prospective short-circuit current, assumed constant during the short-time delay.
The short-time delay associated with the rated short-time withstand current shall be at least 0.05 s, preferred values being as follows:
0.05 –0.1 –0.25–0.5–1 S
The rated short-time withstand current shall be not less than the appropriate values shown in table above


Operating Conditions of Circuit breakers
(a)   Closing
For a circuit-breaker to be closed safely on to the making current as a neutral pole, then all corresponding  to its rated short-circuit making capacity, it is essential that it should be operated with the same speed and the same firmness as during the type test for proving the short-circuit making capacity.

(i)            Dependent manual closing
For a circuit-breaker having a dependent manual closing mechanism, it is not possible to assign a short- circuit making capacity rating irrespective of the conditions of mechanical operation.
Such a circuit-breaker should not be used in circuits having a prospective peak making current exceeding 10 kA. However, this does not apply in the case of a circuit-breaker having a dependent manual closing mechanism and incorporating an integral fast-acting opening release which causes the circuit-breaker to break safely, irrespective of the speed and firmness with which it is closed on to prospective peak currents exceeding 10 kA; in this case, a rated short-circuit making capacity can be assigned,

(ii)           Independent manual closing
A circuit-breaker having an independent manual closing mechanism can be assigned a short-circuit making capacity rating irrespective of the conditions of mechanical operation.

(iii)           Dependent power closing
The power-operated closing mechanism, including intermediate control relays where necessary, shall be capable of securing the closing of the circuit-breaker in any condition between no-load and Its rated making capacity, when the supply voltage, measured during the closing operation, remains between the limits of 110 “A and 85 ‘A of the rated control supply voltage, and, when a c., at the rated frequency.
At 110 % of the rated control supply voltage, the closing operation performed on no-load shall not cause any damage to the circuit-breaker.
At 85 “A of the rated control supply voltage, the closing operation shall be performed when the current established by the circuit-breaker is equal to its rated making capacity within the limits allowed by the operation of its relays or releases and, if a maximum time limit is stated for the closing operation, !n a time not exceeding this maximum time limit.

(iv)           Independent power closing
A circuit-breaker having an independent power closing operation can be assigned a rated short-circuit making capacity irrespective of the conditions of power closing. Means for charging the operating mechanism, as well as the closing control components, shall be capable of operating In accordance with the manufacturer’s specification.

(v)            Stored energy closing
This type of closing mechanism shall be capable of ensuring closing of the circuit-breaker in any condition between no-load and its rated making capacity. When the stored energy is retained within the circuit-breaker, a device shall be provided which Indicates when the storing mechanism is fully charged, Means for charging the operating mechanism, as well as the closing control components, shall be capable of operating when the auxiliary supply voltage is between 85%. and 110 % of the rated control supply voltage. It shall not be possible for the moving contacts to move from the open position unless the charge is sufficient for satisfactory completion of the closing operation. When the energy storing mechanism is manually operated, the direction of operation shall be Indicated This last requirement does not apply to circuit-breakers with an independent manual closing operation.
(b) Opening
(i) General
Circuit-breakers which open automatically shall be trip-free and, unless otherwise agreed between manufacturer and user, shall have their energy for the tripping operation stored prior
to the completion of the closing operation,

(ii) Opening by over-current releases
Opening under short-circuit conditions The short-circuit release shall cause tripping of the circuit-breaker with an accuracy of +-20% of the tripping current value of the current setting for all values of the current setting of the short-circuit current release. Where necessary for over-current co-ordination , the manufacturer shall provide Information (usually curves) showing
àmaximum cut-off (let-through) peak current  as a function of prospective current (r, m.s. symmetrical); à  /2t characteristics for circuit-breakers of utilization category A and, if applicable, B for circuit-breakers with instantaneous override.
 Conformity with this information may be checked during the relevant type tests in test sequences II and Ill


(iii) Opening under overload conditions
1) Instantaneous or definite time-delay operation
The release shall cause tripping of the circuit-breaker with an accuracy of+-10% of the tripping current value of the current setting for all values of current setting of the overload release.

2) Inverse time-delay operation
Conventional values for inverse time-delay operation are given in table below.

Inverse time delay over current opening releases at different temperatures

At the reference temperature and at 1.05 times the current setting, i.e. with the conventional non-tripping current, the opening release being energized on all phase poles, tripping shall not occur in less than the conventional time from the cold state, i.e. with the circuit-breaker at the reference temperature. Moreover, when at the end of the conventional time the value of current is immediately raised to 1.30 times the current setting, i.e. with the conventional tripping current, tripping shall then occur in less than the conventional time later.
If a release is declared by the manufacturer as substantially independent of ambient temperature, the current values of table 6 shall apply within the temperature band declared by the manufacturer, within a tolerance of 0.3%/K.

The width of the temperature band shall be at least 10K on either side of reference temperature.

Saturday, August 5, 2017

Working and functions of alternator in cars and starting current of cars

Alternator in Cars

Alternators are used in cars to generate current for running of cars. We quite often get confused that why alternator is required when there is battery available in cars.
Battery in cars is only used for starting the cars, but for lights operation and functioning of other parts current is required now from where that power will come???
Working and functions of alternators in cars; Starting current of cars

For fulfilling the above purpose alternator is required, alternator has following functions in cars:-
(i)            Running of lights, heater, Air conditioners and operation of other electrical accessories
(ii)          Recharging of battery as it get discharged during starting.
Now how this alternator works in cars and who provides mechanical power to alternator so that power output can be generated at alternator output??
When you switched off your car engine than radio of car will work on car battery. For engines running following are required:-
(i)                    Air
(ii)                  Fuel
(iii)                 Spark
Last part is supplied by alternator because spark is generated through electricity,  although battery is available for supplying the spark but not for keep running the car. Battery electricity is sufficient for keep running the vehicle for few KMs  but vehicle required  much more than that , so to serve the purpose alternator will comes into picture.  Alternator in vehicle has output of 13.5 - 14.8 volts.
In past generators were used in cars, these generators are very inefficient in comparison to alternators, also at that time charging of battery and keeping accessories lighting up was different from present scenario. There are following components of alternators:-
(i)            Stator
(ii)          Rotor
(iii)         Voltage regulator you can also say automatic voltage regulator
(iv)         Direct current circuitry consisting of diodes
Now with rotation of rotor electricity get generated and output is used for charging batteries and keeping the auxiliaries ON.
DC circuitry is used to convert the alternating current generated by alternator to DC. Voltage regulator is used to keeping the voltage generated by alternator within limits.  Feedback is received by voltage regulator and accordingly output gets controlled.  Now day’s voltage regulators are integral part of alternators. But in past voltage regulators were big boxes and they were kept in hood and wired into the system. Voltage regulator functions are as below:-
(a)  Cut off the power to battery when battery get fully charged i.e. when battery voltage reached certain level around 14.0-14.5  volts
(b)  To keep voltage level within limits as per requirement of auxiliaries.

How to get noticed that Alternator is faulty??

It can be easily detected as you will observe reduced illumination from head lights , sometimes reduced illumination may not get detected while driving as during that time battery will provide necessary electricity . But as battery power get used for illumination of headlights now when you tried again to restart the vehicle then vehicle doesn’t start as battery get drained up.

Starting current of a car:-

When you tried to start a car your battery should be strong enough to provide crank to engines so that car get started easily. You may often see that very heavy leads are connected at battery terminals as starting current is very heavy for starting the car. By Ohms law we know that Voltage = Current X Resistance and now Voltage of battery is 12 V and starter motor resistance is approximately 0.12 Ohm so
Current = 12/ (0.12 + Internal resistance of battery)
Now lower is the resistance of battery higher will be the starting current. Now it is general practice that more expensive is the battery lower will be the resistance.
So higher will be the current and faster will be the starting of vehicle.
From above equation we will also find that if we neglect battery resistance then starting current will be 100A. So staring current will be also less than 100A always. If your battery is fully charged then it will be good enough to overcome the problem of poor internal resistance of battery. Battery is considered to be fully charged at 12.6 volt levels and low at 12.6 Volts and completely discharged at 11.9 Volts. Good quality batteries will able to start vehicle even when it is completely discharged so much but a poor quality battery may not be able to start vehicle even at 80% charged position.

In general when your car is new then you can buy any battery and don’t worry about anything but when your get older always buy a high quality battery, as resistance of older wires may get increased which may leads to lower starting current. So we have also seen that during starting current is very high approx. 100 A so it is always advisable to use strong leads capable of taking that 100A load.

Friday, July 28, 2017

Designing a plant; Reduction factors while laying cables in bunches and layers

There are following procedure to be adopted for correct dimensioning of a plant

(i)            Load Analysis:-
First step in dimensioning of a plant is to check for connected load and their location

Now check for location of power distribution panels
Now we can calculate cable requirement i.e. length of cables and path of cable laying
Now we will do calculations of total power consumption while taking account utilization factors and demand factors


(ii)          Transformer and generator size calculations:-
Transformer and generator size are usually selected 15-30% more in comparison to total connected load considering future prospectus.

(iii)         Conductor size selection-
Now we calculate cable size according to load requirement of various connected loads. Cable selected may be copper or aluminum. Cables selection must also consider voltage drop at load current under specific reference conditions.

(iv)         Selection of Protective circuit breaker:-
Short circuit calculations can be done and accordingly switchgear busbar and switchgear should be selected. It is always considered to select circuit breaker with breaking capacity higher than short circuit current. Rating of circuit breaker should be higher than rated current of load connected to circuit breaker. Characteristics of circuit breaker should be according to connected to load.

(v)          Protection of conductors:-
For protection against overload circuit breaker rating should be higher than the load current but should be lower than Rated current carrying capacity of conductor.
In case of short circuit protection circuit breaker setting should be lower than short circuit current withstand by conductor.

(vi)         Protection of Load:-
For protection of load such as motors which constitute 70% of total load of any industrial and commercial establishment overload relays and other protections must be provided after breaker so that tripping of relays leads to protection of load. For protection of human beings from electrical shocks it is always recommended to install RCCB or ELCB.


Selection of the cable
For installation and calculation of current carrying capacity of cables in Industrial, commercial and houses cable selection should be as per International standard IEC 60364-5-52 i.e. “Electrical installations of buildings Part 5-52 for “Selection and Erection of Electrical Equipment- Wiring systems”.

There are following ways and parameters are used to select the cable type:
a)    Conductivity of Material:-
The foremost parameters to be considered while selection of cables is conductivity of material. Copper is costlier then aluminum but selection depends upon cost of material, size of material , weight of material, resistivity of material and resistivity to corrosive environment. Generally copper is having higher current carrying capacity i.e. 30% higher than aluminum conductors for same cross-sectional area, this is due to fact that aluminum is having higher resistivity than copper i.e. 60% higher than copper conductor.

b)     Insulating Material used for conductors:-
There are so many insulating materials used for copper or aluminum conductors. Insulating material may or may not be used for conductors. Materials used for conductors may be PVC, XLPE. Insulating material will affects maximum temperature that a cable able to carry under normal and short circuit conditions.

c)     Type of conductor:-
            There are following types of conductors:-
a)    Bare conductor
b)    Single core cable without sheath
c)    Single core cable with sheath
d)    Multicore cable with sheath and armored
e)    Flexible multicore cable
Cable can be selected according to mechanical resistance, degree of insulation and difficulty of installation required by the method of installation.

Conductors reduction factor while laying the cables in different arrangement of laying the cables:-
It has been observed that with presence of other cables laid around the cable , cable current carrying capacity is influenced significantly. This happens because heat dissipation of single cable get affected due to presence of other cables nearby.
Cables in layers and bunches


Below we will discuss effect of other cables on current carrying capacity of single cable. For same there is factor K2 comes into picture according to installation of  cables laid close together in layers or bunches.

The value of correction factor K2= 1 when:
Distance between two single core cables of different circuits is more than twice that of external diameter of the cable with larger cross section.
Adjacent cables are loaded less than 30% of current carrying capacity.

The correction factors for cables which are either bunched or laid in layers is usually calculated by assuming that  all cables laid in bunches are similar cables and also load on cables is same. The calculation of the reduction factors for bunched cables with different crosssections depends on the number of cables and on their cross sections. These factors have not been tabled, but must be calculated for each bunch or layer.
The reduction factor for a group containing different cross sections of insulated conductors or cables in conduits, or cable ducting is:
where:


K2= 1/(n)1/2

• K2 is the group reduction factor;
• n is the number of circuits of the bunch.

The reduction factor obtained by this equation reduces the danger of overloading of cables with a smaller cross section, but may lead to under utilization of cables with a larger cross section.  Such under utilization can be avoided if large and small cables are not mixed in the same group.
The following tables show the reduction factor (k2).

Reduction Factor for grouped cables:-
Item
Arrangement (Cables Touching)
1
2
3
4
5
6
7
8
9
12
16
20
To be used with current- carrying capacities Reference
1
Bunched in air/ on a Surface/ enclosed
1.00
0.80
0.70
0.65
0.60
0.57
0.54
0.52
0.50
0.45
0.41
0.38
Method A to F
2
Single layer on wall, floor or flat tray
1.00
0.85
0.79
0.75
0.73
0.72
0.72
0.71
0.70
No further  reduction factor for more than  nine circuits or multicore cables
Method C
3
Single layer fixed directly under a wooden ceiling
0.95
0.81
0.72
0.68
0.66
0.64
0.63
0.62
0.61
4
Single layer on perforated tray or vertical tray
1.00
0.88
0.82
0.77
0.75
0.73
0.73
0.72
0.72
Method E & F
5
Single layer on ladder support
1.00
0.87
0.82
0.80
0.80
0.79
0.79
0.78
0.78