Thermal Protection: Overload (49) - Siemens 7SR105 Rho User Manual

3.1.1

Thermal Protection: Overload (49)

Where Gn 49 NPS Weighting setting is selected to 'Average' the thermal protection uses the average of the three
RMS phase currents.
Where Gn 49 NPS Weighting setting is selected to 'Seq Comp' the relay calculates the positive and negative
phase sequence components from the three phase currents. These are then used to generate an equivalent
thermal current I which replaces the relay current in the IEC60255-8 operating characteristics. The equivalent
EQ
current is defined as follows:
2
I = I
EQ 1
Where K = Gn 49 NPS Weighting Factor setting.
Hot and Cold Operating Characteristics
When the thermal equivalent current is less than the thermal pickup level the relay uses the hot curve with which
to calculate the thermal capacity used. If the Thermal Equivalent current exceeds the thermal pickup setting, i.e.
when the motor is overloaded, then the relay reverts to cold curve.
'Cold' Operating Characteristic
In the 7SR105 Rho the 49 Overload setting (I
and the 'Cold' operating characteristic becomes:

2
I
τ
t = × ln

2
I −

'Hot' Operating Characteristic
The time to trip is defined as:-
 2
I −
t = τ × ln

I 2 −

Gn 49 Char (Thermal Characteristic)
The IEC thermal characteristic of the Rho conforms to IEC60255-8 (Thermal Electrical Relays).
A user definable thermal curve is available to allow matching of the relay thermal characteristic to all motor and
cooling system types.
'Starting' and 'cooling' constants modify the thermal heating time constant during motor run-up and stopped
conditions.
The thermal state may be reset from the fascia or externally via a binary input.
Thermal overload protection can be inhibited from:
Inhibit 49
©2018 Siemens Protection Devices
2
+ KI
2
) replaces k.I
θ


I 2

θ
2 
I
P

I 2

θ
A binary or virtual input,
7SR105 Rho Description of Operation
found in the expressions of the IEC255-8 standard
B
Chapter 1 Page 24 of 57