Friday, August 18, 2017

Utilization categories and Releases in circuit breakers

Circuit Breakers Utilization Categories:-

The utilization category of a circuit-breaker shall be stated with reference to whether or not it is specifically intended for selectivity by means of an intentional time delay with respect to other circuit-breakers in series on the load side, under short-circuit conditions ,
Attention is drawn to the differences of the tests applying to the two utilization categories

There are following types of utilization categories in Circuit breakers:-

(i)            Category A

These types of Circuit-breakers not specifically intended for selectivity under short- circuit conditions with respect to other short-circuit protective devices in series on the load side, i.e. without an intentional short-time delay provided for selectivity under short-circuit conditions, and therefore without a short-time withstand current rating.
(ii)          Category B

Circuit-breakers specifically intended for selectivity under short-circuit conditions with respect to other short-circuit protective devices in series on the load side, i.e. with an intentional short-time delay (which may be B adjustable), provided for selectivity under short-circuit conditions. Such circuit-breakers have a short-time withstand current rating as defined by manufacturer.


Releases in Circuit breakers

Releases referred to type of tripping applications requirement in circuit breakers, there are following types of releases available in circuit breakers.
(i)            Shunt release
(ii)           Over current release
There are following types of over current releases
(a)  Instantaneous
(b)  Definite time delay
(c)  Inverse time delay – This are two types of inverse time delay
    àIndependent of previous load
   à Dependent on previous load
(iii)          Under voltage releases
(iv)         Other releases

There is terms used more often that are Overload release and Short circuit releases.
Overload release- This term referred to for protection against overloads and
Short circuit release- This term is used to identify over-current releases intended for protection against Short circuits.

 There are following Characteristics associated with Different types of releases:-
(i)            Shut Release and Under-voltage release have similar types of characteristics that are:-
(a)  Rated control circuit voltage
(b)  Type of current
(c)  Frequency rating in case of alternating current
(ii)          Over current release have following characteristics:-
(a)  Rated current settings
(b)  Type of current
(c)  Frequency rating in case of alternating current
(d)  Current settings on front of MCCB or Circuit breaker
(e)  Time settings on front of MCCB or circuit breaker
Shunt release or under-voltage releases are used in Circuit breakers for breaking or opening the breaker in case of no supply or voltage below a certain level.

Rated current of Circuit breaker:- 
Rated current of a circuit breaker and its release is the RMS value of current corresponding to the maximum current setting which it shall be capable of carrying under the test conditions, without the temperature-rise exceeding the values specified in table below:-
Temperature rise limits of circuit breakers Terminals



Current setting of over-current releases
(i)            With Adjustable release:-
For circuit-breakers fitted with adjustable releases the current setting range shall be marked on the release or on its scale, the marking may be either directly in amperes, or as a multiple of the current value marked on the release.

(ii)          Without Adjustable release:-
In these breakers the marking may be done on the circuit-breaker. If the operating characteristics of the overload release comply with the requirements as per table below-,
Characteristics of Overload release


It will be sufficient to mark the circuit-breaker with its rated current. In the case of indirect releases operated by current transformers, the marking may refer either to the primary current of the current transformer through which they are supplied, or to the current setting of the overload release. In either case, the ratio of the current transformer shall be stated. Unless otherwise specified
à The operating value of overload releases other than those of the thermal type is independent of the ambient air temperature within the limits of -5 “C to +40 ‘C;
à For releases of the thermal type, the values stated are for a reference temperature of +30 “C +- 2 “C. The manufacturer shall be prepared to state the influence of variations in the ambient air temperature.

Tripping time setting of over-current releases

1) Definite time-delay over-current releases

The time-delay of such releases is independent of the over-current. The tripping time setting shall be stated as the duration in seconds of the opening time of the circuit-breaker, if the time-delay is not adjustable, or the extreme values of the opening time~ if the time-delay is adjustable.

2) Inverse time-delay over-current releases
The time-delay of such releases is dependent on the over-current.
The time/current characteristics shall be given in the form of curves supplied by the manufacturer. These shall indicate how the opening time, starting from the cold state, varies with current within the range of operation of the release. The manufacturer shall indicate, by suitable means, the tolerances applicable to these curves.
These curves shall be given for each extreme value of the current setting and, if the time setting for a given current setting is adjustable, it is recommended that they be given in addition for each extreme value of the time setting.

Marking On Circuit Breakers
Each circuit-breaker shall be marked in a durable manner.

a)  The following data shall be marked on the circuit-breaker itself or on a nameplate or nameplates attached to the circuit-breaker, and located in a place such that they are visible and legible when the circuit-breaker is installed;
           (i)  Rated current (In);
     (ii) Suitability for isolation, if applicable, with the symbol
(iii) Indication of the open and closed positions, with O and I respectively, if symbols are used.

b) The following data shall also be marked externally on the circuit-breaker, except that they need not be visible when the circuit-breaker is installed;

      (i)       Manufacturer’s name or trade mark;
      (ii)      Type designation or serial number;
(iii)          IEC 60947-2 if the manufacturer claims compliance with this standard;
(iv)         Utilization category;
(v)          Rated operational voltage
(vi)         Rated impulse withstand voltage;
(vii)        Value (or range) of the rated frequency
(viii)       Rated service short-circuit breaking capacity at the corresponding rated voltage;
(ix)         Rated ultimate short-circuit breaking capacity at the corresponding rated voltage
(x)          Rated short-time withstand current, and associated short-time delay, for utilization category B;
(xi)         Line and load terminals, unless their connection is immaterial;
(xii)        Neutral pole terminals, if applicable, by the letter N;
(xiii)       Protective earth terminal, where applicable, by the symbol
(xiv)       Reference temperature for non-compensated thermal release, if different from 30 ‘C.

c) The following data shall either be marked on the circuit-breaker as specified in item b), or shall be made available in the manufacturer’s published information:

(i)       Rated short-circuit making capacity,
      (ii)       Rated insulation voltage, if higher than the maximum rated operational voltage,
(iii)          Pollution degree if other than 3;
(iv)         Conventional enclosed thermal current if different from the rated current,
(v)          IP Code, where applicable
(vi)         Minimum enclosure size and ventilation data (if any) to which marked ratings apply;
(vii)        Details of minimum distance between circuit-breaker and earthed metal parts for circuit-breakers intended for use without enclosures;
(viii)       Suitability for environment A or environment B, as applicable,
(ix)         R.M.S. sensing, if applicable

d) The following data concerning the opening and closing devices of the circuit-breaker shall be placed either on their own nameplates or on the nameplate of the circuit-breaker; alternatively, if space available is insufficient, they shall be made available in the manufacturer’s published information:
 (i)     Rated control circuit voltage of the closing device and rated frequency for alternating current
(ii)       Rated control circuit voltage of the shunt release and/or of the under-voltage release, and rated frequency for Alternating current;
(iii)         Rated current of indirect over-current releases;
(iv)         Number and type of auxiliary contacts and kind of current, rated frequency and rated voltages of the auxiliary switches, if different from those of the main circuit.
(v)          Terminal marking


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.