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Protection against direct contact
Design measures incorporated into equipment in order to prevent
direct contact, i.e. without tools, with live parts of a system (finger
proof, back-of-hand proof).
Control circuit reliability
Measures the probability of switching states arising during the
lifespan of a contact, which would be interpreted as faults by down-
stream electronic controllers (PLCs). Control circuit reliability is
expressed in values based on tests using standard limit values for
signal inputs.
Damp heat, constant
This test subjects the equipment to an ambient temperature of 40°C
at a constant humidity of 93%. At set intervals during the test, the
electrical and mechanical function of the equipment are examined.
Damp heat, cyclic
This test subjects the equipment to cyclically changing climatic con-
ditions: a cycle applies 40°C ambient temperature at 93% relative
humidity for 12 hours, followed by 12 hours of 25°C at 95% relative
humidity. At set intervals during the test, the electrical and mechani-
cal function of the equipment are examined.
88
Low Voltage Switch Gear
Finger proof
An equipment whose live parts cannot be touched by the operator
during actuation is termed finger proof. This also affects operator
activity on neighbouring switching devices. The finger proof area of a
push-actuated operating medium is a circular area of at least 30 mm
radius around the actuating element, and vertical to the direction of
actuation.
Within this circular area, touch-critical parts must be located at not
less than 80 mm depth under the actuating level.
Utilization category
(IEC 947-1; 2.1 .18/IEV 441-17-19)
A combination of specified requirements relating to the condition in
which the switching device or the fuse fulfills its purpose, selected to
represent a characteristic group of practical applications. The speci-
fied requirements may concern, e.g. the values of making capacities,
breaking capacities and other characteristic values, data concerning
associated circuits, and the relevant conditions of use and behaviour.
(IEC 947-2; 4.4)
For circuit-breakers, the utilization category denotes whether the
equipment is designed for selectivity using time delay (category B) or
not (category A).
Back-of-hand proof
An equipment whose live parts cannot be touched by a sphere of
50 mm diameter, is regarded as back-of-hand proof.
Altitude
The density of air decreases with increasing altitude, and this
reduces its insulating capacity as well as its heat transfer capability.
The rated operational voltage and current of switching devices, con-
ductors and motors as well as the tripping behaviour of thermal over-
load relays are affected by this.
Upon request, OMRON ELECTRONICS will supply information as to
the suitability or otherwise of switchgear for operation at altitudes
above the 2000 m limit specified by the standard.
Conventional free air thermal current
(IEC 947-1; 4.3.2.1)
The maximum value of current which an equipment is capable of car-
rying for a minimum of eight hours without thermal overloading. As a
rule, it corresponds to the maximum operational current.
Creepage path
(IEC 947-1; 2.5.51/IEV 151-03-37)
The shortest distance along the surface of the insulating material
between two conductive parts. The creepage distance is determined
by the rated insulation voltage, the pollution degree and the creep-
age current resistance of the material used.
Clearance
(IEC 947-1; 2.5.46/IEV 441-17-31)
The distance between two conductive parts along a string stretched
the shortest way between these conductive parts. The clearance in
air is determined by the rated impulse withstand voltage, overvoltage
category and pollution degree.
Appendix
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