Areva MiCom P120, Technical Manual, Page: 253 / 668

Application Guide P12x/EN AP/A96
MiCOM P120/P121/P122/P123
Page 31/80
11.2 Setting Guidelines
The current setting is calculated as:
Thermal Trip (
θ
trip) = Permissible continuous loading of the plant equipment / CT ratio.
Typical time constant values are given in the following tables. The ‘Time Constant’
parameter is given in minutes.
Paper insulated lead sheathed cables or polyethylene insulated cables are placed above the
ground or in conduits. The table shows
τ
in minutes, for different cable rated voltages and
conductor cross-sectional areas:
CSA mm
2
6 -11 kV 22 kV 33 kV 66 kV
25 - 50 10 15 40 -
70 - 120 15 25 40 60
150 25 40 40 60
185 25 40 60 60
240 40 40 60 60
300 40 60 60 90
Time constant
τ
(minutes)
Other plant items:
Time constant
τ
(minutes)
Limits
Dry-type transformers 40
60 - 90
Rating < 400 kVA
Rating 400 - 800 kVA
Air-core reactors 40
Capacitor banks 10
Overhead lines 10
Cross section
≥
100 mm
2
Cu
or 150mm² Al
Busbars 60
An alarm can be raised when reaching a thermal state corresponding to a percentage of the
trip threshold. A typical setting might be ‘Thermal Trip’ = 70% of thermal capacity.