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16
Electronic Motor Protection Relays
Technical Data
Operational Characteristics
Thermal Curve
In accordance with IEC 60255-8 Thermal Electrical Relays specifications, protection functions activate based on hot curves and cold
curves depending on the thermal characteristics of the load current before an overload can occur. The operating characteristic curves
are as shown on the graph, represented as functions of current and time from class 1 to 60.
The relay takes real-time measurement of motor heating accumulation, and in the event of an overload activates the protection function
that is optimized to the motor's operational conditions based on the cumulative heat data.
The formula for each operating characteristic curve can be represented as follows:
Cold curve indicates that the load current before the overload, I
10,000
1,000
100
10
1
0.1
0.01
1
Hot Curve
t = τln
Item
Operating Time
t
Time Constant
τ
Rated Load Current
I
B
Fault Current (Input Current)
I
Load Current Before Overload
I
P
Overload Factor
k
is zero in the hot curve formula.
P
2
2
I
- I
p
[sec]
2
2
I
- (kI
)
B
Duration of the operation of the relay.
According to the above OC formula, the values for time constant τ for time T under 600 % of rated current and
k = 1.1 are as follows.
T
1
10
τ
29.25
292.49
Rated current set value
Fault current value where a trip occurs. The reference value is six times (600 %) the rated current.
Load current recorded by the relay prior to the overload.
Represents the motor's service factor value. K = 1.1
Current
Cold Curve
t = τln
2
I
- (kI
Details
20
30
40
584.98
877.48
1,169.97
Cold Class 1
Cold Class 10
Cold Class 20
Cold Class 30
Cold Class 40
Cold Class 50
Cold Class 60
Hot Class 1
Hot Class 10
Hot Class 20
Hot Class 30
Hot Class 40
Hot Class 50
Hot Class 60
10
2
I
[sec]
2
)
B
50
60
1,462.46
1,754.95
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