NOTES ON PT AND CT

Current Transformers (CT) operate on the principle of electromagnetic induction, serving as devices that step down high currents to low currents for measurement and protection purposes. Key parameters of a CT include: the current transformation ratio (e.g., 200/5, 200/1...); capacity (typically low, only a few tens of VA since CTs only serve measurement and protection); accuracy class (including classes for protection such as 5P10, 5P20..., and classes for measurement such as Cl 0.5 or 0.2...). Current transformers are connected in series with the main electrical circuit or configured as ring-type CTs (where the main circuit passes through without a direct connection). The fundamental rule for connecting a CT is that the secondary circuit must never be left open—if left open, the secondary voltage can reach thousands of volts if the CT has a large ratio, which may cause electrical discharge.

Voltage Transformers (PT/VT) also operate on the principle of electromagnetic induction. They step down high voltages to low voltages primarily for measurement, protection, and occasionally to supply power to certain control devices in the panel. The capacity of a PT typically ranges from a few tens to a few hundred VA (depending on the design)—unlike distribution transformers, which have capacities ranging from several hundred kVA to MVA. The transformation ratio of a PT is generally the primary phase voltage / secondary phase voltage (e.g., $22/\sqrt{3} / 0.4/\sqrt{3} \text{ kV}$). The accuracy class of a PT is also classified into two types: one for measurement and one for protection. When connecting a Voltage Transformer, it must be connected in parallel with the electrical circuit. The secondary circuit of a PT is allowed to be open, but must NEVER be connected directly between the phase wire and neutral (e.g., phase 'a' must NOT be directly connected to neutral 'n'); otherwise, the PT secondary circuit will fall into a short-circuit state when energized, causing the current to surge significantly.