MSCDN – MP2A Applications Guide
©2010 Siemens Protection Devices Limited
Chapter 5 Page 4 of 7
The final steady state thermal condition can be predicted for any steady state value of input current since
when t>>
τ
,
%
100
2
2
×
=
θ
θ
I
I
F
…(Eq. 4)
The thermal overload setting I
θ
is expressed as a fraction of the relay nominal current and is equivalent to
the factor k.I
B
as defined in the IEC60255-8 thermal operating characteristics. It is the value of current
above which 100% of thermal capacity will be reached after a period of time and it is therefore normally
set slightly above the full load current of the protected device.
0.1
1
10
100
1
1.5
2
2.5
Overload Level (Multiple of Setting I
θ
)
T
ime (secs)
3
tau = 100
tau = 50
tau = 30
tau = 20
tau = 15
tau = 10
tau = 5
tau = 3
tau = 2
tau = 1
Figure 2 – IEC60255-8 cold curve (tau in seconds)
