Have you ever carefully looked at the nameplate of an electricity meter? Do you know what the “回” symbol on it represents?
In fact, this symbol is not only found on electricity meters. Take a look at your mobile phone charger — you will likely see the same “回” mark on it as well.
In simple terms, the “回” symbol indicates Class II equipment with double insulation. Such appliances have high insulation performance and do not require a protective earth (ground) connection.
Four Classes of Protection Against Electric Shock
Class 0: Basic insulation only
Equipment that relies solely on basic insulation for protection against electric shock. When the equipment has accessible conductive parts, these parts are not connected to the protective earth (ground) wire of the fixed wiring. If the basic insulation fails, safety depends entirely on the environment.
Because basic insulation failure can lead to electric shock, Class 0 equipment has gradually been phased out.
Examples: Some household appliances with metal enclosures but power plugs without an earth pin, such as table lamps and electric fans, fall into Class 0.
Class I: Relies on reliable earthing
The basic protection of Class I equipment is basic insulation, but the key feature is that it must be connected to protective earth (PE). When basic insulation fails, the fault current is conducted to the ground through the earthing conductor, triggering an upstream protection device (such as a circuit breaker) to disconnect the supply, thereby preventing electric shock to the user.
Identification: Uses a three-pin plug (including an earth terminal).
Common equipment: Refrigerators, washing machines, air conditioners (with metal enclosures), as well as industrial automation equipment and control cabinets.
Class II: Relies on double insulation
Class II equipment does not rely on earthing. Its safety is ensured by double insulation or reinforced insulation. Even if one layer of insulation fails, another layer or reinforced insulation provides protection, so no earthing is required.
Identification: A clear “回” (double-square) symbol on the nameplate.
Common equipment: Hand-held power tools, electricity meters, chargers, and most household appliances.
Class III: Relies on safety extra-low voltage (SELV)
Class III equipment is powered by safety extra-low voltage (SELV), with voltages below AC 50V (e.g., 12V, 24V). Since the voltage itself does not pose a hazard to the human body, no additional protection against electric shock is required inside the equipment.
Identification: Usually powered by batteries, an external power adapter, or a safety isolating transformer.
Common equipment: Electric shavers, low-voltage sensors, mobile phones, IoT terminals, etc.
Why Are Most Electricity Meters Class II?
The vast majority of household and industrial electricity meters are designed as Class II for the following reasons:
Complex installation environments with unreliable earthing: Electricity meters are installed in millions of households and various industrial sites, with diverse installation conditions. Class II design does not rely on external earthing, avoiding potential hazards from poor earthing and enabling wider adaptability.
Double insulation for inherent safety: With a “回” construction using double or reinforced insulation, electricity meters are designed so that even if internal insulation fails, the externally accessible parts remain non-live. Safety does not depend on external factors.
Compliance with national standards: Chinese standards for electricity meters (such as GB/T 17215 series) explicitly require them to be Class II insulated and encapsulated instruments. This is a mandatory requirement for product compliance and market sale.
Class II protection for electricity meters is a mature design that balances safety, environmental adaptability, and cost. The “回” symbol is a testament to their independent and complete protection system against electric shock.
