Protection for Motor-Feeders
Structure of a Motor-Feeder
Direct-on-line starting is the most widely used type of motor-feeder.
The direct-on-line starting motor-feeder can comprise up to four different items of switchgear providing one or more functions. It must also incorporate the specific characteristics of the application.
A Switchgear for short-circuit protection
B Control gear
C Switchgear for overload protection
D Switchgear for ground-fault protection
E Characteristic t = f(I) of an asynchronous D.O.L. starting motor
F Starting phase
G Current peak on activation
Characteristics Defined by Standard IEC/EN 60947-4-1
A motor-feeder must satisfy the general rules of standard IEC/EN 60947-4-1, in particular the rules concerning the protection of contactors and motor-feeders.
In the matter of protection, this standard defines:
-
Coordination of motor-feeder protections
-
Thermal relay trip classes
-
Insulation coordination
Coordination According to Standard IEC/EN 60947-4-1
Two types of coordination are allowed: type 1 coordination or type 2 coordination.
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In type 1 coordination, deterioration of the contactor and relay is accepted under the two following conditions:
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The contactor or starter does not represent a danger to people or installations.
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The starter can operate correctly when parts have been repaired or replaced.
-
-
In type 2 coordination, some slight welding of the contactor or starter contacts is allowed if, following type 2 coordination tests:
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They are easy to separate
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The control and protection switchgear functions then work without the need for repair
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To ensure type 2 coordination, standard IEC/EN 60947-4-1 lays down three Id fault current tests intended to check that the equipment behaves correctly in overload and short-circuit conditions.
1 Overload zone Id < 10 In
2 Impedant short-circuit zone 10 In < Id < 50 In
3 Short-circuit zone Id > 50 In
Thermal Relay Trip Classes
Standard trip time values
|
Class |
1.05 In |
1.2 In |
1.5 In |
7.2 In |
|---|---|---|---|---|
|
5 |
t > 2 hours |
t > 2 hours |
t < 2 minutes |
0.5 s ≤ t ≤ 5 s |
|
10 |
t > 2 hours |
t > 2 hours |
t < 4 minutes |
4 s ≤ t ≤ 10 s |
|
20 |
t > 2 hours |
t > 2 hours |
t < 8 minutes |
6 s ≤ t ≤ 20 s |
|
30 |
t > 2 hours |
t > 2 hours |
t < 12 minutes |
9 s ≤ t ≤ 30 s |
Classes 5 and 10 are most commonly used. Classes 20 and 30 concern applications in which motor starting conditions are difficult.
Additional Protection
Depending on the application and the operating constraints, additional protection may be required concerning:
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Phase unbalance or phase loss
-
Locked rotor
-
Undercurrent
-
Long starts
ComPact NSX Motor Circuit Breakers
ComPact NSX motor circuit breakers incorporate MA thermal-magnetic trip units and MicroLogic type M electronic trip units.
Protection functions by trip unit type
|
Protection |
Trip unit type |
|||
|---|---|---|---|---|
|
MA |
MicroLogic 1.3 M |
MicroLogic 2 M |
MicroLogic 6 E-M |
|
|
Overloads |
– |
– |
✔ |
✔ |
|
Short-circuits |
✔ |
✔ |
✔ |
✔ |
|
Insulation faults (ground-fault protection) |
– |
– |
– |
✔ |
|
Phase unbalance or phase loss |
– |
– |
✔ |
✔ |
|
Locked rotor |
– |
– |
– |
✔ |
|
Undercurrent |
– |
– |
– |
✔ |
|
Long starts |
– |
– |
– |
✔ |
The protection against insulation faults in the MicroLogic 6 E-M trip unit is the ground-fault protection type.
Type 1 and 2 coordination tests have been carried out with motor-feeder components in respect of all ComPact NSX motor circuit breakers.
Trip classes by trip unit type
|
Class |
Trip unit type |
|||
|---|---|---|---|---|
|
MA |
MicroLogic 1.3 M |
MicroLogic 2 M |
MicroLogic 6 E-M |
|
|
5 |
– |
– |
✔ |
✔ |
|
10 |
– |
– |
✔ |
✔ |
|
20 |
– |
– |
✔ |
✔ |
|
30 |
– |
– |
– |
✔ |
Trip Unit Long-Time Protection
The pickup setting Ir for trip unit long-time protection is expressed in amperes:
-
This value corresponds to the operating current used in the motor application
-
The maximum Ir setting corresponds to the trip unit rating In