Engineering notes
Current transformers for energy meters
Choose a CT for main and branch meters: ratio, accuracy class, never leave the secondary open.
A branch meter on a board almost never sees the phase directly. The cable goes through the CT window; the meter only sees 5 A or 1 A on the secondary. If the CT says 200/5 and someone sets the meter to 400/5, the kWh on the BMS doubles or halves. The meeting is about “bad software”. The goods are right. The ratio is wrong.
On the schedule, pick a primary a step above running load — about 20% spare. The window has to take the cable; a clamp that will not fit 240 mm² is found on the day. Accuracy class as the file states, often 0.5 or 1 — do not put class 3 on a billing check. A live primary with an open secondary is a hazard, not “not finished yet”.
Run CT leads on their own, not in the same closed slot as power. Energy products · Modbus meters