Low Voltage Current Transformer
The Ip/5 A ratio low voltage current transformer delivers at the secondary a current (Is) of 0 to 5 A that is proportional to the current measured at the primary (Ip). This allows them to be used in combination with measurement equipment:
- Ammeters.
- Kilowatt-hour meters.
- Measurement units.
- Control relays etc.
When the primary is energized, the measurement equipment nearly acts as a short circuit which keeps the secondary voltage very low. This voltage will increases significantly if the short circuit is removed.
Low Voltage Current Transformer selection – conductor rating aspects
The choice depends on the conductor profile and the maximum intensity of the primary circuit.
- We recommend that you choose the ratio immediately higher than the maximum measured current (In). Example: In = 1103 A; ratio chosen = 1250/5.
- For small ratings: From 40/5 to 75/5 and for an application with digital devices, we recommend that you choose a higher rating, for example 100/5.
- This is because small ratings are less accurate and the 40 A measurement, for example, will be more accurate with a 100/5 CT than with a 40/5 CT.
- Specific case of the motor starter: to measure motor starter current, you must choose a CT with primary current Ip = Id/2 (Id = motor starting current).
