Current Protection: Thermal Overload (49) - Siemens 7SR224 Technical Manual

3.9

Current Protection: Thermal Overload (49)

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 2
I I
τ
= ×
P
t
⎨
ln
(
− ×
2
I k I
⎩
B
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 is expressed as a multiple of the relay nominal current and is equivalent to the factor k.I
θ
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:
A binary or virtual input.
Inhibit 49
©2010 Siemens Protection Devices Limited
⎫
⎬
)
2
⎭
)
θ
)
B
7SR224 Argus Description of Operation
Chapter 1 Page 33 of 70
B