Low Voltage Switch Gear
Appendix
87
Glossary of standard terms
This Glossary offers brief expIanations of some of the standard
terms used in this catalogue. However, it must not be regarded as a
substitute for the actual text of the standard, especially where the
new terms used in IEC 947 are concerned.
Reference is therefore made aIongside each such term to the reIev-
ant section of the standard, e.g. IEC 947-1 in addition, IEV numbers
are given to enable you to find foreign language equivalents in the
International EIectrotechnicaI Vocabulary
(IEG 50), if required.
Rated conditional short-circuit current I
q
(IEC 947-1; 2.5.29/IEV 441-17-20)
The prospective current which a switching device, e.g. a circuit-
breaker, protected by a short-circuit protective device such as a
motor-protective circuit-breaker, can carry for the duration of the pro-
tective device tripping time.
Mininum command time
Minimum duration for a trip-initiating factor (controI puIse, short cir-
cuit) to effect the corresponding reaction, e.g. the short-circuit dura-
tion necessary to initiate tripping.
Rated breaking capacity
(IEC 947-1; 4.3.5.3)
The r.m.s. value of current which a switching device is capable of
breaking according to its utiIization category. The rated breaking
capacity is stated by reference to the rated operational voltage and
the rated operational current.
The equipment must be capable of breaking any value of current up
to and incIuding its rated breaking capacity stated.
Rated actuating voltage U
c
(rated control circuit voltage)
(IEC 947-1; 4.5.1)
The voltage which is applied to the actuating make contact in a con-
trol circuit. Due to the presence in the controI circuit of transformers
or resistors, this voltage may differ from the rated control supply volt-
age.
Rated service short-circuit breaking capacity I
cs
(IEC 947-2; 4.3.5.2.2)
The prospective short-circuit current which, depending on the rated
operationaI voltage, a circuit-breaker is capable of breaking repeat-
edly (test cycle: O - CO - CO; previousIy P-2). After interrupting this
current vaIue, the circuit-breaker must be capable, despite its own
thermal level having increased, of continuing to carry and disconnect
in the event of overIoading, the rated uninterrupted current.
Rating or rated power
(IEC 947-1; 4.3.2.3)
The rated operational power which an equipment is capable of
switching at the associated rated operational voItage in accordance
with the utilization category.
For example:
motor contactor utilization category AC-3: 37 kW at 400 V.
Rated operational voltage U
e
(IEC 947-1; 4.3.1.1)
The voItage to which the characteristics of an equipment are
referred. The rated operational current must not in any case exceed
the rated insulation voltage.
Rated operational current I
e
(IEC 947-1; 4.3.2.3)
The current which an equipment is capabIe of carrying taking into
account the rated operational current, duration of operation, utiliza-
tion category and ambient temperature.
Rated uninterrupted current I
u
(IEC 947-1; 4.3.2.4)
The vaIue of current which an equipment can carry in uninterrupted
duty (i.e. for weeks, months or years).
Rated making capacity
(IEC 947-1; 4.3.5.2)
The vaIue of current which an equipment is capabIe of switching On
in accordance with the utilization category and at the rated opera-
tional voltage.
Rated frequency
(IEC 847-1; 4,3.3)
The frequency for which an equipment is designed and to which the
other characteristic vaIues are referred.
Rated ultimate short-circuit breaking
capacity I
cu
(IEC 947-2; 4.3.5.2.1)
The maximum prospective fault current which a circuit-breaker is
capable of interrupting
(test cycIe: O - CO; previously P-1)
Rated insulation voltage U
i
(IEG 947-1; 4.3.1 .2)
The voltage to which insulation tests and creepage distances of an
equipment are referred. The maximum operationaI voltage must not
in any case exceed the rated insulation voltage.
Rated short-circuit breaking capacity I
cn
(IEC 947-1; 4.3.6.3)
The maximum value of current which an equipment is capable of
switching Off at rated operational voItage and rated frequency, and
without sustaining damage. It is expressed as r.m.s. value.
Motor rating
(IEC 947-1; 4.3.2.3)
Power output of a motor at the associated operational voltage.
Rated control supply voltage U
s
(IEC 947-1; 4.5.1)
The voltage applied to the input terminals of the control circuit of an
equipment. Due to the presence of transformers or resistors in the
control circuit, this may differ from the rated actuating (control circuit)
voltage.
Rated impulse withstand voltage U
imp
(IEC 947-1; 4.3.1 .3)
Measures the stability of the internal clearances of an equipment
against overvoltage peaks. The utilization of suitable switchgear can
ensure that overvoltages are prevented from transferring from the
mains to deenergized system sections within it.
Rated current I
n
(of a circuit-breaker)
(IEC 947-2; 4.3.2.3)
For circuit-breakers, this current value is equal to the uninterrupted
current and the conventional free air thermaI current.
