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37G - n
Enabled
Disabled
Inhibit 37G- n
Figure 3-20 Logic Diagram: Earth Current Inputs Undercurrent Detector (37G)
37 SEF- n
Enabled
Disabled
Inhibit 37SEF -n
Figure 3-21 Logic Diagram: Sensitive Earth Current Inputs Undercurrent Detector (37SEF)
3.4.8
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. The
operating time is a function of thermal time constant 49 Time Constant and previous current levels.
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 (recommended value, k = 1.05)
I
= Basic current, typically the same as In
B
= 49 Overload Setting (I
k.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
I
2
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,
I
2
θ
=
×
100%
F
2
I
θ
Where: θ
= final thermal state before disconnection of device
F
©2018 Siemens Protection Devices Limited
37G- n Setting
&
c
<
IL 4 (I
)
G
37 SEF - n Setting
&
c
IL4
(ISEF)
<
2
I
P
)
2
×
I
B
)
θ
)
B
7SR10 Description of Operation
37G- n Delay
37 SEF - n Delay
General Pickup
37G-n
General Pickup
37 SEF - n
Chapter 1 Page 43 of 77
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