Current Transformer Installation Guide

Never Open The Secondary

An energised current transformer with an open secondary develops a high voltage across the terminals and can destroy its own insulation. Short the secondary before disconnecting anything. Bodies with double terminal entry, such as the DX range, exist precisely so a short circuit link can stay fitted while an instrument is changed.

Observe Polarity

P1 and P2 on the primary, S1 and S2 on the secondary. Reversed polarity on one phase of a three phase metering set produces readings that look plausible and are wrong, and it is the classic cause of a negative power reading on one phase.

Earth One Point Only

Earth the secondary circuit at one point, normally at the terminal block or the relay panel. Two earths create a circulating current through the secondary loop.

Close A Split Core Properly

Wipe the mating faces, close the core fully and check the latch. A partly closed core reads low and hums. Verify against a reference clamp before signing off.

Check The Burden At Commissioning

Confirm that every device in the loop was counted, including test blocks and transducers added late in the project. A burden calculation done at design stage is often no longer true by the time the panel is wired.

Commissioning Checklist

• Ratio and class on the rating plate match the drawing
• Polarity confirmed on every phase
• Secondary earthed at exactly one point
• No open secondary circuits anywhere in the loop
• Split cores fully closed and latched
• Reading compared against a reference clamp
• Terminal covers closed, and sealed where the way is billed