Electricity has always been an important part of our lives. If such new power systems and loads are not taken seriously, and if electrical installations are not properly designed, constructed, and verified, and if appropriate protective equipment is not provided, they may pose risks to personal safety and property. Existing power installations should also be improved and updated in accordance with relevant standards.
Applications such as electric vehicle charging (EV), solar photovoltaic power generation (PV), and integrated photovoltaic-energy storage-charging systems are gradually emerging.
Adding it to the existing power structure is expected to exponentially increase the demand for related power applications by 2035.
1. Construct a new power system with new energy sources as the mainstay.
As shown in Figure 1, boost and buck circuits are widely used in such applications. The use of switch-mode power supplies (SMPS) has surged due to their compactness and energy efficiency. However, these and other types of switch-mode converters generate a large amount of high-frequency capacitive ground leakage current.
Switching power supplies—convert AC to DC to power electronic devices, as shown in Figure 2:
Figure 1. Constructing a new power system based on new energy sources.
Figure 2. Basic SMPS topologies: Buck, Boost, and Buck-boost
• Solar photovoltaic power generation;
• Electric vehicle charging;
• Various variable frequency home appliances;
• LED lighting;
• USB socket
• Smart homes and data networks, etc.
The common feature of the above applications is that they cause pulsating DC or high-frequency harmonics to enter the power supply side, which inevitably makes the AC side circuit less "clean".
In the applications of new energy vehicle charging, photovoltaic-storage-charging integration, and photovoltaic power generation, there are still a large number of AC/DC conversion (rectifier/inverter) circuits (as shown in Figure 3).
These AC-DC conversion circuits contain not only residual AC sinusoidal current at power frequency, but also residual DC current generated by two-phase/three-phase rectification and filtering, smoothing DC residual current, and residual current generated by some capacitors to ground. Figure 4 shows some typical complex residual current waveforms.
Figure 3. Photovoltaic-storage-charging integration
Figure 4 Residual current waveform
Therefore, the safety issue is that the RCD (residual current protection device) previously used in pure AC circuits is no longer as reliable (as shown in Figure 5).
Figure 5. The human body's response to electric current as shown in IEC 60479-1 "Effects of electric current on humans and livestock".
In Figure 5, the influence of alternating current is divided into four regions:
① Sensing threshold: The minimum value that can induce any contact current through the human body;
②Reaction threshold: The minimum contact current that can induce involuntary muscle contraction in the human body;
③ Let-go threshold: The maximum contact current at which a person can actively let go of the electrode;
④ Ventricular fibrillation threshold: The minimum contact current that can cause ventricular fibrillation through the human body.
(Note: Burns/cardiac arrest/respiratory arrest will occur if the curve is above c1.)
Note: The RCD's operating threshold must be below this threshold to ensure human protection, while being above the capacitive leakage current to avoid any accidental tripping.
The key technology of RCD is a high-sensitivity magnetic induction coil and magnetic core for detecting residual current. If there is a DC component in the circuit being detected, the DC component will cause the magnetic induction of the high-sensitivity magnetic induction coil to quickly enter the saturation region of the hysteresis curve. At this time, the magnetic induction coil is no longer sensitive to the residual current or even has no response. Therefore, RCD has the potential to cause blindness, which endangers personal safety and equipment safety (as shown in Figure 6).
Figure 6. Ordinary AC type RCD has blinding properties.
One of the major challenges in electrical equipment is to provide appropriate and adequate protection for these technologies to ensure their safety.
Residual current devices (RCDs) can be classified into several types according to their operating characteristics, which include a DC component in the residual current. The types shown in the figure below are in ascending order of inclusion.
• AC type
This is the RCD we've always used most often, suitable for AC circuits without a DC component. Its symbol is:
Type A
Applicable to AC class + pulsating DC or pulsating DC superimposed on a 6mA smooth DC, with the symbol:
• Type F
Applicable to Class A+ applications with high-frequency currents up to 1kHz, symbol:
• Type B
Applicable to Class F+ applications with high-frequency currents up to 1kHz, or two-phase or three-phase rectification, and purely smooth DC residual current, denoted as:
The leakage current detection type of RCD (Residual Current Device) is selected according to different circuits, as shown in Figure 7.
Figure 7 Leakage detection
Therefore, when using a high proportion of new energy sources and diverse loads, different types of residual current operated protection devices should be selected correctly based on the presence of DC components and corresponding frequencies.
2. Application Scenarios: Electric Vehicle Charging
The electrification of automobiles is inevitable in the future. When charging electric vehicles, charging stations need to provide appropriate protection for all users.
Frequent plugging and unplugging by non-technical users, and with many charging points located outdoors, the risk of electric shock may be greater than indoors.
The standard IEC 60364-7-722 Part - Electric Vehicle Power Supply requires that a Type B or Type A 30mA RCD be selected as a DC ground fault protection measure (Clause 722.531.2) (as shown in Figure 8).
(Note: It is specifically noted that if Type A is used, an RDC-DD conforming to IEC 62955 must also be used.)
(A DC residual current detection device) is used to detect the pure, smooth DC residual current. Note: Relevant requirements are also mentioned in IEC 61851/IEC 62752.
Figure 8 shows IEC 60364-7-722, which is a standard for low-voltage electrical installations - power supply for electric vehicles.
The relevant requirements have also been updated in the Shanghai local standard DB31/T 1296-2021 "Technical Requirements for Intelligent Charging and Interactive Response of Intelligent Charging Piles for Electric Vehicles", which was implemented in 2021, and the relevant national standards are also being updated (as shown in Figure 9).
Figure 9 DB31/T 1296-2021
Technical Requirements for Intelligent Charging and Interactive Response of Smart Charging Piles for Electric Vehicles
3. Application Scenarios: Photovoltaic Power Generation
Photovoltaic (PV) power generation is a renewable energy source and one of the most promising technologies for addressing the challenges of global climate change and meeting the urgent needs of sustainable development. PV power generation offers several advantages:
Solar energy is unlimited and can be used globally.
It does not emit greenhouse gases (GHG) or other pollutants during operation, consumes very little, and does not produce wastewater.
Photovoltaic panels do not generate noise when generating electricity, and they are also easy to maintain.
Meanwhile, photovoltaic power generation reduces dependence on energy imports, which, in the long run, can improve energy supply security and stabilize power generation costs. Driven by national participation, sustainable energy development policies, technological development, and cost reduction, today's photovoltaic installed capacity is growing rapidly (as shown in Figure 10).
Figure 10. China's Photovoltaic Capacity Trend from 1990 to 2025
(Source: IEA, International Energy Agency)
According to IEC 62109-2:2011, in order to ensure personal safety and prevent building fires, inverters directly connected to the grid must have residual current (leakage current) protection.
The standard IEC 60364-7-712 – Solar Photovoltaic Power Systems requires that a Type B RCD be selected for the protection of the AC power supply circuit of the PV device (Clause 712.530.3.101) (as shown in Figure 11).
Figure 11. Requirements for Solar Photovoltaic Power Systems (Part 60364-7-712)
In China, in accordance with IEC standards, the corresponding standard GB/T 16895.32-2021 "Low-voltage electrical installations - Part 7-712: Requirements for special installations or locations - Solar photovoltaic (PV) power systems" (as shown in Figure 12) was just implemented on November 1st.
Figure 12 《GB/T 16895.32-2021 Part – Solar Photovoltaic (PV) Power Systems》
Figure 13 AC and DC leakage current sensors
Figure 14 shows that multiple installation methods are available.
In summary, if a similar environment uses a large number of DC loads or frequency converter loads, choosing an AC type RCD is no longer a safe option.
At the same time, a type B leakage current sensor detection solution is becoming more widespread. In electric vehicle charging, sensors that integrate A+6mA detection or conform to type B leakage current detection are already being used in portable charging guns of mode 2 and charging piles (wall boxes) of mode 3.
Similarly, in photovoltaic grid-connected inverters, sensors that meet Type B leakage current detection standards have been mass-produced and can meet Type B related residual current protection.
4 Magtron's AC/DC Leakage Current Detection Solutions
Zhejiang Juci Intelligent Technology Co., Ltd. is a high-tech company dedicated to the independent research and development and production of ICs, modules, and applications. Since 2014, its AC and DC leakage protection modules based on the mSafe leakage current detection chip have been widely used in the fields of new energy photovoltaic inverters (residential type) and electric vehicle charging protection, with a leading market share. AC and DC leakage current detection schemes are shown in Figures 13 and 14.
