1. EE4503 Electrical Systems Design
Protective Devices
and
Protective Devices Cooordination

2. Topics LV protective systems Protective Devices
Protective Devices Coordination

3. Low Voltage Protective Systems
Fully Rated Protective System
Selective Protective System
Cascade Protective System

4. Protective Systems Fully Rated Protective System
All devices are able to withstand the maximum short circuit current. (Interrupting Capacity)
Highest Cost
No need to prioritize the devices

5. Protective Systems Fully Rated Protective System
If short circuit at F2 is 30kA and F1 is 10kA
A, B, C: 𝐼 𝑐𝑢 =30𝑘𝐴
D, E, F: 𝐼 𝑐𝑢 =10𝑘𝐴
Electrical Systems Design: ประสิทธิ์ พิทยพัฒน์

6. Protective Systems Selective Protective System
All devices are able to withstand the maximum short circuit current. (Interrupting Capacity)
Tripping Characteristic Curve of each devices are “selected” in the way that the device that is closer to the fault trips first.

7. Protective Systems Selective Protective System
A, B, C: 𝐼 𝑐𝑢 =10𝑘𝐴
D, E, F: 𝐼 𝑐𝑢 =5𝑘𝐴
There are two kinds
Fully
Partially
Electrical Systems Design: ประสิทธิ์ พิทยพัฒน์

8. Protective Systems Fully Selective Protective System
The selectivity is on the full range of short current.
A & B are selective more than 10𝑘𝐴
Electrical Systems Design: ประสิทธิ์ พิทยพัฒน์

9. Protective Systems Partially Selective Protective System
The selectivity is only on a certain range of short current.
A & B are selective up to 5𝑘𝐴
Electrical Systems Design: ประสิทธิ์ พิทยพัฒน์

10. Protective Systems Cascade Protective System
The upstream device has the faster tripping rate at the high current than the lower one.
The upstream device is able to withstand the maximum short circuit current while the downstream is lower capacity.
In case of high current short circuit the upstream handle the breaking function.

11. Protective Devices Coordination
Overlay the plot of tripping curve of each protective devices
In general, no crossing of tripping curve
Lower current (downstream devices) branch will be tripped faster.
Higher current (upstream devices) branch will be tripped slower.

12. Protective Devices Coordination

13. Reference Electrical Systems Design: ประสิทธิ์ พิทยพัฒน์
Device_Coordination.pdf
Coordination of LV protection devices: Schneider Electric
Depend on the area, end-user can connect to the “meter” of the authority in three ways.