Connecting a current transformer correctly – step by step
Most installation mistakes come down to three things: wrong mounting direction, swapped secondary leads and an open secondary circuit. Primary terminals are marked P1 and P2 (older: K and L), secondary terminals S1 and S2 (older: k and l). Current flows from P1 to P2 in the conductor; the secondary current flows from S1 through the meter back to S2.
Step 1: Mounting direction: P1 towards the supply
Mount the CT so that P1 faces the supply and P2 the load; mount all phases the same way round. Energy meters count negatively if a CT is reversed.
Step 2: S1 and S2 to the meter
Two leads per CT go to the current path of the meter: S1 to the input, S2 to the output of the same phase. Lead length and cross-section determine the burden.
Step 3: Earth at one point only
If the secondary circuit is earthed, use exactly one terminal, the same on all CTs of the metering point – which one is set by the grid operator’s connection conditions or the design.
Step 4: Short-circuit before disconnecting
Never operate the secondary circuit open while current flows in the conductor. Short-circuit S1 and S2 before removing the meter.
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FAQ
Is k or l earthed?
Exactly one secondary terminal is earthed, the same on all CTs of a metering point; which one is set by the grid operator’s connection conditions or the design.
Which way round is the CT installed?
P1 (K) towards the supply, P2 (L) towards the load.