IEC 60947-8 pdf free download
IEC 60947-8 pdf free download.Low-voltage switchgear and controlgear- Part 8: Control units for built-in thermal protection (PTC) for rotating electrical machines.
5.2.3 Rated system operating temperature
When the PTC thermistor and the control unit are supplied as a combined unit, the TFS shall
be declared under item 0) of 6.1.
The tolerance on the declared value of the TFS shall be ±6 K unless otherwise specified.
NOTE The tolerance is the sum of the tolerances of the PTC thermistor and the control unit.
Routine tests shall be carried out to verify correct operation under normal operating conditions
in accordance with 8.2.1.1.
5.2.4 Maximum permissible rated operating temperature for the system
The maximum permissible value of the TFS depends on both the particular PTC thermistor and the control unit. It shall be declared under item 0) of 6.1.
NOTE For any particular device, the maximum value of the TFS will be dependent on the characteristics and the materials used in the manufacture of the PTC thermistor, or by the limits on the characteristics of the PTC thermistor which can be modified by the range of settings available with the control unit design.
5.2.5 Control unit with reset temperature
When the reset function is provided, the reset temperature value and tolerances shall be provided under item o) of 6.1.
The reset temperature is verified in accordance with 9.3.3.8.
To restart the machine (electric motor) after the tripping of the control unit (switched off), it is important for the machine winding and the PTC thermistor to cool sufficiently to permit normal machine acceleration without nuisance tripping, especially with a high inertia load. The temperature value for restarting depends on installation and service conditions. The control unit can be designed in order to permit a selection of different temperature values.
For a manual restarting system, the maximum temperature should be considered. For automatic restarting systems, the machine manufacturer should consider the minimum and maximum differential temperatures which result from choices of TNF or TFS and reset temperature with the declared tolerance values. Differential values which are too narrow might not permit sufficient cool-down for restarting without nuisance tripping. Differential temperatures that are too wide can result in an excessively long machine cooling down time or resetting can be prevented in high ambient temperatures.
5.2.6 Control unit with sensor short-circuit detection
Control unit can be declared with sensor short-circuit detection. PTC thermistors have a low resistance and therefore a special measure is necessary to recognize a reduction of the resistance to nearly zero by a short-circuit. For safety applications, or to increase the lifetime of a rotating electrical machine, it is recommended to establish a short-circuit detection system within the PTC thermistor circuit. The safety of the thermal protection, in particular, is increased by such a short-circuit detection.
Such a short-circuit detection only identifies a short-circuit but it does not automatically cover a defined action. All following actions depend on the configuration of the control unit and the application.
5.2.7 Control unit with sensor wire break detection
Control unit can be declared with sensor wire break detection. The wire break detection shall monitor the PTC thermistor circuit, detect any interruption and give a corresponding signal.IEC 60947-8 pdf download.
