Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 165)

6 Considerations when building control systems and switchgear
assemblies
6.1 Temperature rise
The temperature of the devices in the switchboard cabinet and that of touchable parts are
important factors with respect to operational reliability, life span and personal safety. It depends
among other things on the ambient temperature of the switchgear assembly, the heat flow via
enclosures, if any, and/or air conditioning, the method of installation of devices (mutual heating,
heat abduction, formation of hotspots), wiring (heat-flow via conductors ) and last but not least
the dissipated heat (load losses) of the devices.
6.1.1 Temperature rise limit values
The relevant standards such as IEC 60947-1 (low-voltage switchgear) and IEC 60439-1
(Low-voltage switchgear assemblies) define upper limits for the temperatures of the relevant
constructive parts. IEC 60204 (Safety of machinery - Electrical equipment of machines) refers to
IEC 60439-1.
Part Temperature-rise
limits
1)
Reference
(ambient)
temperature
Temperature-
limit
(absolute) 1)
Metallic manual
operating means 15 … 25 K 40 °C 55 … 65 °C
Non-metallic manual
operating means 25 … 35 K 40 °C 65 … 75 °C
Metallic parts intended to
be touched but not hand-
held 30 … 40 K 40 °C 70 … 80 °C
Non-metallic parts
intended to be touched
but not hand-held 40 … 50 K 40 °C 80 … 90 °C
Metallic surfaces not
intended to be touched 40 … 50 K 40 °C 80 … 90 °C
Non-metallic surfaces not
intended to be touched 50 … 60 K 40 °C 90 … 100 °C
Terminals for external
connections
(Cu silver- or nickel-
plated) 70 … 80 K 40 °C 110 … 120 °C
1) The higher values apply for used devices and for parts which are not intended to be
touched or operated frequently respectively.
Tab. 6.1-1
Temperature-rise limit values in accordance with IEC 60947 and IEC 60439
Copyright © IEC, Geneva, Switzerland. www.iec.ch
The permissible temperature-rises or temperatures appear in some cases to be high, for
example those for the temperature-rise of terminals. This limit value is based on the connection
of conductor material with a permissible continuous insulation temperature of 70 °C. The high
temperature of the connection point itself is permissible as after only a short distance, the
conductor temperature starts to decrease due to the heat flow from the terminal point via the
connected line. The conductor material (cable, busbar etc) acts as a thermal aerial assisting in
the heat dissipation process. Experience gained over many decades and with billions of terminal
points confirms the correct choice of the limit values.
LVSAM-WP001A-EN-P - April 2009 6-1