If the manufacturer indicates a few values for the rated voltage, then the greatest represents the
maximum rated voltage; a2) Rated insulation level;
a3) Rated frequency fr.
The standard values for the rated frequency of high voltage circuit-breakers are 50 Hz and 60 Hz;
a4) Rated normal current Ir:
Current which the main circuit of a circuit-breaker is capable of carrying continuously under specified conditions of use and behavior;
a5) Rated short-time withstand current Icw.
The rated short-time withstand current Icw of a CB, disconector or swich-disconector means the rms
value of a rated, admited, short-time current, indicated by a manufacturer, which the equipment can support without any damages. The testing determination of this current for a concret equipment is made in standard conditions [CEI 60947-1].
The rated short-time withstand current must be greater than twelve times the rated maximum operation current and, without other manufacturer’s indication, the current duration must be 1 s:
s
T
.
pt
,
I
I
cw≥12⋅
e cw=1
, 89A complete determination of the rated short-time withstand current is made, on the base of the mentioned standard, as follows:
(
)
{
I
;
kA},
pt.I
,
kA
Max
I
cw=
12⋅
e5
e≤25
, 90kA
,
I
.
pt
,
kA
I
cw=30
e>25
, 91InAC the rated short-time withstand current is compearing with the rms value of the periodical short- circuit current component. It is necessary that the last mentioned value to be lower than the product between the short duration acceptable rated current and the factor n, indicated in table 3, in accordance with CEI 60947-1:
cw k
n
I
I
≤
⋅
, 92Table 8: n factor based on PF and short circuit level Short-circuit current, kA Power Factor Time constant, ms n factor ≤1,5 0,95 5 1,41 (1,5, 3] 0,9 1,42 (3, 4,5] 0,8 1,47 (4,5, 6] 0,7 1,53 (6, 10] 0,5 1,70 (10, 20] 0,3 10 2,00 (20, 50] 0,25 15 2,10 50≤ 0,2 2,20
At the same time, the short duration acceptable rated current represents the upper limit value of the rms value of the short-circuit current periodical component which is presumed constant during the short timing τ, for which the following normalized values are recommended:
{0,05;0,1;0,25;0,5;1}
s
∈
τ
1. The rated short-time withstand current is equal to the rated short-circuit breaking current [5, p.33] - EN 60947-3:1999 Low-voltage switchgear and controlgear – Part 3: Switches, disconnectors,
switch-disconnectors and fuse-combination units.
a6) Rated peak withstand current (Ip):
It is equal to the rated short-circuit making current;
a7) Rated duration of a short-circuit tk.
A rated duration of a short-circuit need not be assigned to a self-tripping circuit-breaker provided that the following applies. When connected in a circuit the prospective breaking current of which is equal to its rated short-circuit breaking current, the circuit-breaker shall be capable of carrying the resulting current for the break-time required. This break time is that required by the circuit-breaker with the over current release set for the maximum time lag when operating in accordance with its rated operating sequence. Direct over current releases include integrated tripping systems.
a8) Rated supply voltage
of closing and opening devices and of auxiliary circuits Ua; a9) Rated supply frequency
of closing and opening devices and of auxiliary circuits;
a10) Rated pressures
of compressed gas supply and/or of hydraulic supply for operation, interruption and insulation, as applicable;
a11) Rated short-circuit breaking current Icn.
The rated short-circuit breaking current is the highest short-circuit current which the circuit breaker shall be capable of breaking under the conditions of use and behavior prescribed in standards. Such a current is found in a circuit having a power-frequency recovery voltage corresponding to the rated voltage of the circuit-breaker and having a transient recovery voltage equal to a specified value. For three-pole circuit-breakers, the AC component relates to a three-phases short-circuit.
The rated short-circuit breaking current is characterized by two values: – the rms value of its AC component;
– the percentage DC component. If the DC component does not exceed 20%, the rated short- circuit breaking current is characterized only by the rms value of its AC component.
The circuit-breaker shall be capable of breaking any short-circuit current up to its rated short-circuit breaking current containing any AC component up to the rated value and, associated with it, any percentage DC component up to that specified, under the conditions mentioned above.
The following applies to a standard circuit-breaker:
- at voltages below and equal to the rated voltage, it shall be capable of breaking its rated short- circuit breaking current
- at voltages above the rated voltage, no short-circuit breaking current is guaranteed. The standard value of the AC component of the rated short-circuit breaking current shall be selected from the R10 series specified in IEC 60059. The R10 series comprises the numbers
{1 – 1,25 – 1,6 – 2 – 2,5 – 3,15 – 4 – 5 – 6,3 – 8}
and their products by 10n.
The value of the percentage DC component shall be determined as follows:
- for a self-tripping circuit-breaker, the percentage DC component shall correspond to a time interval equal to the minimum opening time of the first opening pole Top of the circuit breaker. Time Tr in the
formula (6) is to be set to 0 ms
- for a circuit-breaker which is tripped solely by any form of auxiliary power, the percentage DC component shall correspond to a time interval equal to the minimum opening time of the first opening pole Top of the circuit-breaker plus one half-cycle of rated frequency Tr.
The minimum opening time mentioned above is that specified by the manufacturer. The minimum opening time is the shortest opening time, which is expected by the manufacturer to cover the entire population of the circuit-breaker concerned under any operational conditions when breaking asymmetrical currents.
The percentage value of the dc component (iDC%) is based on the time interval (Top + Tr) and the time
constant τ using the formula:
⎞
⎛
T
+T
The graphs of the DC component against time given in figure 1 below are based on: a) standard time constant of 45 ms
b) special case time constants, related to the rated voltage of the circuit-breaker: - 120 ms for rated voltages up to and including 52 kV
- 60 ms for rated voltages from 72,5 kV up to and including 420 kV - 75 ms for rated voltages 550 kV and above
Figure 3: Percentage D.C. current component in relation to the time interval from initiation of short-circuit current, for different time constantτ.
These special case time constant values recognize that the standard value may be inadequate in some systems. They are provided as unified values for such special system needs, taking into account the characteristics of the different ranges of rated voltage, for example their particular system structures, design of lines, etc.
In addition, some applications may require even higher values, for example if a circuit-breaker is close to a generator. In these circumstances, the required DC component and any additional test requirements should be specified in the inquiry.
a12) Rated ultimate short-circuit breaking capacity Icu
The rated ultimate short-circuit breaking capacity Icu represents the highest rms value of the current
The magnitude of this current, determined under the conditions specified in the product standard, is the upper (superior/higher) limit of the short-circuit ac component (the dc component is considered null):
k cu
I
I
≥
93a13) Transient recovery voltage - TRV
related to the rated short-circuit breaking current is the reference voltage which constitutes the limit of the prospective transient recovery voltage of circuits which the circuit-breaker shall be capable of withstanding under fault conditions.
In some cases, particularly in systems with a voltage 100 kV and above, and where the short-circuit currents are relatively large in relation to the maximum short-circuit current at the point under consideration, the transient recovery voltage contains first a period of high rate of rise, followed by a later period of lower rate of rise. This waveform is generally adequately represented by an envelope consisting of three line segments defined by means of four parameters.
In other cases, particularly in systems with a voltage less than 100 kV, or in systems with a voltage greater than 100 kV in conditions where the short-circuit currents are relatively small in relation to the maximum short-circuit currents and fed through transformers, the transient recovery voltage approximates to a damped single frequency oscillation. This waveform is adequately represented by an envelope consisting of two line segments defined by means of two parameters.
The influence of local capacitance on the source side of the circuit-breaker produces a slower rate of rise of the voltage during the first few microseconds of the TRV. This is taken into account by introducing a time delay.
The transient recovery voltage corresponding to the rated short-circuit breaking current when a terminal fault occurs, is used for testing at short-circuit breaking currents equal to the rated value.
a14) The rated short-circuit making current Icm
of a circuit-breaker having simultaneity of poles is that which corresponds to the rated voltage and the rated frequency.
The rated short-circuit making capacity Icm of a circuit-breaker or switch represents the value of the short-circuit closing capacity, expressed by the highest instantaneous value of the current that the device can connect at the rated voltage and frequency and at a specified power factor. This parameter is indicated by the equipment manufacturer in the device catalogue data.
According to IEC 60947-1 the rated short-circuit making capacity is established in comparison with the limit value of the short-circuit rated breaking capacity Icu, by multiplying it to the factor k, given in the table 2, for LVCB, in accordance with the relationship:
I
k
I
=
⋅
Values of the multiplying factor k when fixing the rated short-circuit making capacity Icmof the LVCB
Table 9: Icu and k factor
Icu, kA cosϕ k (6, 10] 0.50 1.7 (10, 20] 0.30 2.0 (20, 50] 0.25 2.1 50≤ 0.20 2.2
The rated short-circuit making capacity implies that the breaker is able to connect the current suitable to this rated capacity at an applied voltage corresponding to the use rated voltage.
Being an instantaneous value, the rated short-circuit making capacity Icm of an CB is compared with
the peak current ip and the next inequality have to be fulfilled in order that the device withstands the
short-circuit action:
p cm
i
I
≥
95The following values apply for the high voltage CB (Ur >1 kV):
• for a rated frequency of 50 Hz and the standard value of the time constant of 45 ms it is equal to 2,5 times the rms value of the AC component of its rated short-circuit breaking current (Icn), so a following relationship can be written:
cn H
cm
k
I
I
=
⋅
, 96where the multiplying factor kH was introduced. In this case, kH=2,5;
• for a rated frequency of 60 Hz and the standard value of the time constant of 45 ms it is equal to 2,6 times the rms value of the AC component of its rated short-circuit breaking current
• for all special case time constants it is equal to 2,7 times the rms value of the AC component of its rated short-circuit breaking current, independent of the rated frequency of the circuit- breaker
a15) Rated operating sequence; a16) Rated time quantities:
- opening time of a circuit-breaker defined according to the tripping method as stated below and with any time delay device forming an integral part of the circuit-breaker adjusted to its minimum setting - for a circuit-breaker tripped by any form of auxiliary power, the opening time is the interval of time
between the instant of energizing the opening release, the circuit-breaker being in the closed position, and the instant when the arcing contacts have separated in all poles
- for a self-tripping circuit-breaker, the opening time is the interval of time between the instant at which, the circuit-breaker being in the closed position, the current in the main circuit reaches the operating value of the over current release and the instant when the arcing contacts have separated in all poles
The opening time may vary with the breaking current. For circuit-breakers with more than one interrupting unit per pole, the instant when the arcing contacts have separated in all poles is determined as the instant of contact separation in the first unit of the last pole. The opening time includes the operating time of any auxiliary equipment necessary to open the circuit breaker and forming an integral part of the circuit-breaker.
- arcing time (of a multipole switching device): interval of time between the instant of the first initiation of an arc and the instant of final arc extinction in all poles
- break time: interval of time between the beginning of the opening time of a mechanical switching device and the end of the arcing time