Siemens 7SR242 - Current Protection: Thermal Overload (49)

7SR242 Duobias Description Of Operation
©2010 Siemens Protection Devices Limited Chapter 1 Page 35 of 52
3.9 Current Protection: Thermal Overload (49)
Optionally a phase segregated thermal overload element is provided, this can be selected to either winding 1 or
winding 2. The thermal state is calculated using the measured True RMS current.
Should the current rise above the 49 Overload Setting for a defined time an output signal will be initiated.
Operate Time (t):-
()
⎭
⎬
⎫
⎩
⎨
⎧
×−
−
×=
2
B
2
2
P
2
IkI
II
t ln
τ
Where
T = Time in minutes
τ = 49 Time Constant setting (minutes)
In = Log Natural
I = measured current
I
P
= Previous steady state current level
k = Constant
I
B
= Basic current, typically the same as In
k.I
B
= 49 Overload Setting (I
θ
)
Additionally, an alarm can be given if the thermal state of the system exceeds a specified percentage of the
protected equipment’s thermal capacity 49 Capacity Alarm setting.
For the heating curve:
100%)e(1
I
I
θ
τ
t
2
θ
2
×−⋅=
−
Where: θ = thermal state at time t
I = measured thermal current
I
θ
= 49 Overload setting (or k.I
B
)
The final steady state thermal condition can be predicted for any steady state value of input current where t >τ,
100%
I
I
θ
2
θ
2
F
×=
Where: θ
F
= final thermal state before disconnection of device
49 Overload Setting I
θ
is expressed as a multiple of the relay nominal current and is equivalent to the factor k.I
B
as defined in the IEC255-8 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.
The thermal state may be reset from the fascia or externally via a binary input.
Thermal overload protection can be inhibited from:
Inhibit 49 A binary or virtual input.