SWITCHGEAR ASSEMBLIES FOR NUCLEAR GENERATING PLANTS SERVICE CONDITIONS

The service conditions below are significant to the qualification of switchgear assemblies and included components.
 
Usual Service Conditions
The values given as usual service conditions represent the anticipated average conditions for switchgear assemblies in nuclear power generating stations. These values are recommended for use in generic qualification programs and do not imply a derating of the equipment.
 
Ambient Temperature
A yearly average ambient temperature of 30 °C with temperature excursions to 10 °C and 40 °C is considered usual.
 
Relative Humidity
Relative humidity variations between 10% and 90% are considered usual.
 
Altitude
Altitudes of up to 6600 ft (2000 m) above sea level for metal-enclosed low-voltage power circuit breaker switchgear and up to 3300 ft (1000 m) for metal-clad switchgear and metal-enclosed interrupter switchgear are considered usual.
 
Radiation
Radiation exposure of up to 104 rads equivalent gamma total integrated dose over the qualified life is considered usual.
 
Line Voltage
The line voltage shall be specified for the application. Voltage ratings as specified in ANSI/IEEE C37.20.1-1987 [12], ANSI/IEEE C37.20.2-1987 [13], ANSI/IEEE C37.20.3-1987 [14], and ANSI/IEEE C37.23-1987 [15] are considered usual. Switching surges of up to twice peak line-to neutral voltage may occur occasionally, but Class 1E switchgear installations are usually not exposed to lightning surges.
 
Frequency
A nominal frequency of 60 Hz is considered usual for ac equipment.
 
Control Voltages
The nominal control voltages shall be specified for the application. Control voltage variations within the ranges given in the standards below are considered usual:
ANSI C37.06-1979 [1] — high-voltage circuit breakers.
ANSI C37.16-1980 [2] — low-voltage circuit breakers.
ANSI C37.33-1970 [5] — high-voltage interrupter switches.
 
Control Currents
The maximum control auxiliary circuit currents to be made, carried, and interrupted by auxiliary contacts of breakers and other components shall be as specified for the application.
 
Continuous Current
The average loading shall be estimated for the application. The qualified life of switchgear components is dependent on the average loading. Considering duty and available power distribution options in nuclear Class 1E systems, the average loading rarely, if ever, approaches the continuous current rating of the equipment.
 
Short-Circuit Current
The maximum short-circuit current shall be specified for the application.
 
Mechanical Operations
In order to provide a basis for qualified life demonstration, circuit breaker mechanical operations shall be specified. A cumulative number of mechanical operations not greater than that corresponding to two maintenance intervals, as defined in applicable circuit breaker standards, is considered usual.
 
Mechanical Interface Loading
It is considered usual that the mechanical loads from incoming cables, conduits, other interfacing hardware, or equipment such as transformers are separately supported or isolated so as not to impose significant mechanical loading on the switchgear assembly structure.
 
Unusual Service Conditions
When switchgear assemblies are applied where the service conditions are not within the range given as usual service conditions, the applicable service conditions shall be specified and the switchgear assemblies shall be qualified for these conditions. Where qualification to unusual conditions becomes impractical, improvement of the service conditions may be necessary.
 
Design Basis Events (DBEs)
The DBE that usually applies to switchgear assemblies is a seismic event. Other DBEs, such as the severe environmental conditions associated with a loss-of-coolant accident and high-energy line break, are not normally applicable.
 
Seismic Excitation
Due to the seismic variations between sites and building structural differences, seismic loading in the form of required response spectra shall be specified for each switchgear assembly application. The switchgear assemblies shall be qualified in accordance with ANSI/IEEE Std 344-1975 [20].
 
Other Design Basis Events (DBEs)
If DBEs or unusual requirements in addition to the seismic event are applicable to switchgear assemblies, detailed conditions shall be specified and considered.

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