What Is A Split Core Current Transformer (CT)?

A split core current transformer, also known as a clamp-on CT, is a special type of instrument current transformer designed for measuring alternating current without disconnecting the primary conductor. Unlike conventional solid-core CTs, whose iron core forms a closed, unbreakable ring, the magnetic core of a split core CT is cut into two separable halves. Users can open the core, wrap it around an existing cable or busbar, then lock the two core segments together to reconstruct a complete magnetic loop. This unique mechanical structure is its most defining feature.

The basic working principle follows Faraday’s law of electromagnetic induction, identical to standard CTs. When alternating current flows through the primary cable passing through the CT window, it generates an alternating magnetic flux inside the iron core. The secondary winding wound around the core induces a proportional low-level secondary current. By transformation ratio, the large primary current is converted into a small, safe output current (typically 5A or 1A) for power meters, energy loggers, protection relays, and power quality monitoring devices. Insulation between the primary circuit and secondary winding provides electrical isolation, protecting low-voltage measuring equipment from high-voltage power systems.

The biggest advantage of split core current transformer lies in convenient, non-intrusive installation. Solid-core CTs require power shutdown, cable disconnection and threading work. In contrast, split core models can be mounted on live circuits during normal operation, drastically reducing construction downtime, labour costs and power outage risks. This makes them widely adopted in retrofit projects, building energy management systems, industrial machinery monitoring, and temporary power auditing. Common sizes cover small low-voltage cables and large rectangular busbars in distribution cabinets.

However, split core CTs also carry inherent limitations. A tiny air gap inevitably exists between the two joint surfaces of the split core. This gap increases magnetic reluctance, leading to slightly higher measurement error, greater magnetising current and minor flux leakage compared to solid-core alternatives. Measurement accuracy is sensitive to tightness of locking. If the two core halves are loosely closed or contaminated by dust, the ratio error and phase angle error will rise noticeably. In addition, split core designs generally cannot achieve ultra-high precision grades, so they are less suitable for high-standard metering scenarios that demand class 0.2S accuracy. They are more commonly used for monitoring, load analysis and secondary protection rather than official trade billing.

Safety rules must be strictly observed during operation. The secondary circuit of any CT must never be left open. An open secondary loop induces dangerously high voltage, threatening equipment and personnel. After installation, technicians should verify core closure, fix the CT firmly to avoid vibration-induced separation, and match the transformation ratio with receiving instruments.