Selection Guide
CT Selection Guide:
Important parameters for choosing your CT:
1. Operating Voltage
The actual operating voltage must NOT be greater than the CT’s rated voltage/insulation level.
BH-0.66 Series are suitable for system that have the operating voltage of 0.72kV or less.
BH-LZZB-10 Series are suitable for system that have the operating voltage of 10kV or less.
2. Ratio
The actual operating current must NOT be greater than the CT’s rated current. We recommend the actual operating current to be at 70%~90% of the CT’s rated current.
For example: If the actual operating current is 175A, CTs with rated primary current of 200A is recommended.
3 Accuracy Class
For metering CTs, the common accuracy classes are: 0.2S, 0.5S, 0.2, 05 and 1.0 ect.
For class 0.2S and 0.5S, the metering range is 1%~120% of the rated primary current.
For class 0.2, 0.5 and 1.0, the metering range is 5%~120% of the rated primary current.
For protection CTs, common accuracy classes are: 5P10, 10P20.
5P, 10P are composite error of 5% and 10% respectively.
10, 20 are accuracy limit factors (which means the composite error of the CT must be within specified limits at 10 or 20 times the rated primary current).
For example: 150:1, 5P10, 10VA means the CT’s composite error is less than 5% when the instantaneous primary current is 1500A with rated burden of 10VA or less.
4. CT Window type and size selection:
For busbar: Please take into consideration of the rated burden value and whether the spacing is suitable.
For cable: Please check cable diameters for suitable CT window sizes. The following table shows the reference between cable nominal cross-sectional area and cable diameter, with an example of suitable BH CT Model (for Single Core only).
|
Nominal Cross- sectional Area（mm2） |
Cable Diameter Φ（mm) |
Suitable BH Model Series (For Single Core) |
|||
|
Single Core |
2 Core | 3 Core | 4 Core | ||
| 1.0 | 6.3 | 10.0 | 10.4 | 11.3 | 30I、30×30I、30IF、Φ22 |
| 1.5 | 6.6 | 10.4 | 11.0 | 11.8 | 30I、30×30I、30IF、Φ22 |
| 2.5 | 7.0 | 11.2 | 11.8 | 12.5 | 30I、30×30I、30IF、Φ22 |
| 4.0 | 7.4 | 12.2 | 12.8 | 13.6 | 30I、30×30I、30IF、Φ22 |
| 6.0 | 8.0 | 13.2 | 13.9 | 14.6 | 30I、30×30I、30IF、Φ22 |
| 10 | 9.6 | 16.5 | 17.5 | 18.6 | 30I、30×30I、30IF、Φ22 |
| 16 | 10.7 | 19.1 | 20.5 | 21.5 | 30I、30×30I、30IF、Φ22 |
| 25 | 12.5 | 22.4 | 24.3 | 25.6 | 30I、30×30I、30IF、Φ22 |
| 35 | 14.0 | 25.1 | 26.7 | 27.3 | 30I、30×30I、30IF、Φ22 |
| 50 | 16.1 | 30.6 | 32.6 | 33.0 | 30I、30×30I、30IF、Φ22 |
| 70 | 17.5 | 33.6 | 36.5 | 37.5 | 30I、30×30I、30IF |
| 95 | 20.2 | 38.9 | 41.6 | 42.6 | 30IF、40I、40IF |
| 120 | 21.7 | 41.9 | 44.7 | 46.1 | 30IF、40I、40IF |
| 150 | 24.2 | 46.9 | 50.0 | 51.3 | 30IF、40I、40IF |
| 185 | 26.4 | 51.3 | 54.8 | 56.3 | 30IF、40I、40IF |
| 240 | 29.9 | 54.3 | 58.3 | 64.9 | 60I、80I、50IF |
5. Rated burden and secondary transmission distance.
The rated secondary current for current transformer is 5A or 1A. At the same burden, the transmission distance of 1A CT is 25 times of the 5A CT. So if the meter is at a far distance from the CT, 1A CT is recommended.
Secondary burden of CT include: metering units and lead wires
Metering units burden：5A Meter is about 1.0~1.5VA
1A Meter is about 0.3~0.5VA
See the table below for lead wire burden:
|
Lead wire cross-sectional area (Material: Copper) （mm2） |
At 1A secondary current, burden per meter of lead wire (50°C) （VA） |
At 5A secondary current, burden per meter of lead wire (50°C) （VA） |
| 1.5 | 0.013 | 0.33 |
| 2.5 | 0.008 | 0.2 |
| 4.0 | 0.005 | 0.125 |
Transmission distance at various rated burden (metering unit burden included)
|
Rated Burden （VA） |
Secondary Current |
Transmission Distance (in meters) with lead wire cross-sectional area below（m） |
||
| 1.5mm2 | 2.5mm2 | 4.0mm2 | ||
| 2.5 | 1A | 75 | 125 | 200 |
| 5A | 1.5 | 2.5 | 4 | |
| 5 | 1A | 168 | 281 | 450 |
| 5A | 5.2 | 8.8 | 14 | |
| 10 | 1A | 356 | 594 | 950 |
| 5A | 12.7 | 21.2 | 34 | |
| 15 | 1A | 543 | 906 | 1450 |
| 5A | 20.0 | 33.7 | 54 | |
