Current Protection: Thermal Overload (49) - Siemens 7SR224 Description & Operation

(1) Description of Operation 7SR224
3.9 C P
URRENT
The relay provides a thermal overload suitable for the protection of static plant. Phase segregated
elements are provided. The temperature of the protected equipment is not measured directly. Instead,
thermal overload conditions are calculated using the measure 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
I
τ
= ×
t
⎨
ln
(
− ×
2
I k
⎩
Where
T = Time in minutes
τ
= 49 Time Constant setting (minutes)
In = Log Natural
I = measured current
I = Previous steady state current level
P
k = Constant
I = Basic current, typically the same as In
B
k.I = 49 Overload Setting (I
B
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:
2
I
−
t
= ⋅ − ×
θ (1 e ) 100%
τ
2
I
θ
Where: θ = thermal state at time t
I = measured thermal current
I = 49 Overload setting (or k.I
θ
The final steady state thermal condition can be predicted for any steady state value of input current
where t >τ,
2
I
= ×
θ 100%
F
2
I
θ
Where: θ
= final thermal state before disconnection of device
F
49 Overload Setting I
factor k.I as defined in the IEC255-8 thermal operating characteristics. It is the value of current above
B
Page 34 of 66
: T
ROTECTION HERMAL
⎫
2
I
P
⎬
)
2
I
⎭
B
)
θ
)
B
is expressed as a multiple of the relay nominal current and is equivalent to the
θ
O (49)
VERLOAD
©2009 Siemens Protection Devices Limited