Comparison between the symbols in common electrical equipment system diagrams and the corresponding

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### Primary Equipment
- **QF**: Circuit breaker. It can connect and cut off the load current when the circuit is normal, and can automatically cut off the short-circuit current when a fault occurs. The physical object is a switching device with arc extinguishing ability. Common ones include vacuum circuit breakers, SF6 circuit breakers, etc. Its shape is usually a cuboid or a cylinder, with incoming and outgoing wire terminals.
- **QS**: Disconnector. It is mainly used to isolate the power source and ensure the safety of maintenance. The physical object is usually in the form of a knife switch, with obvious break points. It is composed of insulating pillars, conductive knife switches and operating mechanisms, etc., and is installed in switch cabinets or outdoor substations.
- **FU**: Fuse. When the circuit has an overload or short circuit, the fuse element melts and cuts off the circuit. Common ones include high-voltage fuses and low-voltage fuses. The physical object is a closed tube or box containing the fuse element, with different rated currents and fuse characteristics.
- **TA**: Current transformer. It is an instrument that converts the large current on the primary side into a small current on the secondary side for measurement based on the principle of electromagnetic induction. It consists of a closed iron core and windings, usually in a ring or column shape. The primary winding has a small number of turns and is connected in series in the measured circuit, while the secondary winding has a larger number of turns and is connected to measuring instruments or relays, etc.
- **TV**: Voltage transformer. It is a transformer with an iron core, mainly composed of primary and secondary coils, an iron core and insulation. It transforms the high voltage into a standard low voltage in proportion for measurement, protection and control. There are various types such as electromagnetic type and capacitive type. Its appearance is generally in the form of a box with oil-immersed or dry insulation structure.

### Secondary Equipment
- **KA**: Current relay. It is a relay that responds to changes in current and operates when the current exceeds the set value. The physical object is usually a small electromagnetic relay, with parts such as coils, iron cores, armatures and contacts, and is installed in the secondary circuit of the relay protection panel or switch cabinet.
- **KV**: Voltage relay. It is a relay that responds to changes in voltage and operates when the voltage exceeds or falls below the set value. Its structure is similar to that of the current relay and also works based on the electromagnetic principle. There are different models and specifications according to different voltage levels and application scenarios.
- **KT**: Time relay. It is used to achieve timing control in the circuit. After the input signal meets certain conditions, it outputs a signal after a set time delay. The physical object has various types such as electromagnetic type, air damping type and electronic type, and usually has a knob for adjusting time and multiple terminals.
- **KS**: Signal relay. It is used to send signals when the protection device operates so that the on-duty personnel can detect faults in a timely manner. It is generally an electromagnetic relay, with different colors and markings. After it operates, there will be obvious signal indications, such as the lighting of signal lights or the dropping of tags.
- **KM**: Intermediate relay. It plays the role of intermediate conversion and signal amplification in relay protection and automatic control systems, and generally has multiple normally open and normally closed contacts. The physical object is similar to a small electromagnetic relay, but has a larger contact capacity and can control multiple circuits simultaneously.

### Busbars and Lines
- **WB**: Busbar. It is a conductor for collecting and distributing electrical energy, usually made of copper or aluminum. In substations or distribution rooms, busbars are usually multiple metal bars arranged in parallel and installed in busbar bridges or the busbar compartments of switch cabinets, with different voltage levels and cross-sectional areas.
- **WL**: Line. It refers to the wires or cables that connect various electrical equipment and are used to transmit electrical energy. According to different voltage levels, current magnitudes and application environments, the lines can be overhead lines, cable lines, etc. Overhead lines are generally composed of multiple conductors and are erected through poles and towers; cable lines are insulated conductors and are usually laid underground or in cable trenches.

### Others
- **PE**: Protective grounding wire. It is a grounding wire set up to ensure the safety of people and equipment, which reliably connects the metal shells, frames, etc. of electrical equipment to the earth. The physical object is a wire with yellow and green stripes, usually connected between the grounding terminal of the equipment and the grounding electrode.
- **N**: Neutral wire. In a three-phase four-wire system, it is used to provide a path for unbalanced current and ensure the balance of three-phase voltages. It is generally a blue wire and is connected between the neutral point of the transformer and the neutral point of the load.

The above are just some comparisons between the symbols in common electrical equipment system diagrams and the corresponding physical objects. There may be differences in actual applications due to different systems and equipment.