The necessity and regulatory requirements of N-PE connection in off grid state of hybrid inverters

This article first introduces the basic concepts and functions of N-wire (neutral wire) and PE wire (protective grounding wire) in electrical systems, and then explores in detail the reasons why N-PE must be reliably connected in off grid states of hybrid inverters, including safety, system stability, and regulatory requirements. Finally, relevant regulations from some countries were listed, and the design of the inventronics hybrid inverter was taken as an example to illustrate how its product meets the regulatory requirements for N-PE connection in off grid conditions, providing reference for related research and practice.

1. Electrical System and N-PE Concept

（1） Grounding method of electrical system

Electrical system grounding is an important measure taken to ensure the safe operation of electrical equipment and personal safety. According to the terminology and concepts of various protective grounding methods specified by the International Electrotechnical Commission (IEC), common grounding methods include TN systems, TT systems, and IT systems. Among them, the TN system is divided into three forms: TN-C, TN-S, and TN-C-S. In the TN-C system, the N line and PE line are shared and called PEN line; In the TN-S system, the N line and PE line are completely separated; The TN-C-S system is a hybrid form that converts from TN-C to TN-S.

The meaning represented by the letters:

(1) The first part of the letter represents the relationship between the neutral point of the distribution system and the ground

T: The neutral point of the power supply is directly grounded at one point; I: The neutral point of the power supply is insulated from ground or grounded through a high impedance point.

(2) The second part of the letter represents the relationship between the exposed conductive parts of the electrical device and the ground

T: The exposed conductive parts are directly grounded, regardless of the grounding point of the distribution system;

N: The exposed conductive parts are directly electrically connected to the neutral point of the distribution system (also known as the neutral system);

(3) The letters after the "-" sign are expansion instructions

C: The protective neutral wire and the working neutral wire use the same wire;

S: Completely separate the protective neutral wire from the working neutral wire, and use two independent wires for each;

C-S: Use the same wire for the front part of the protective neutral wire and the working neutral wire, and completely separate the rear part of the protective neutral wire from the working neutral wire using two wires.

（2） The role of N line and PE line

N line (neutral line): In a three-phase four wire power supply system, the N line is a wire that is led out from the neutral point of the transformer. Its main function is to provide a current circuit for the load and ensure the balance of the three-phase voltage. In single-phase loads, the N line also plays a role in returning current to the power supply.

PE wire (protective grounding wire): The main function of PE wire is to connect the exposed conductive parts of electrical equipment to the ground, in order to prevent electric shock accidents caused by the equipment shell being charged. In electrical systems, the role of PE lines is to provide a low impedance path for fault currents to safely flow to the ground, triggering protective devices (such as residual current devices) to act, cutting off power, and protecting personal and equipment safety.

The following diagram illustrates the connection methods of N and PE wires in various grounding systems.

Figure 1: TN-C system

In the TN-C system, the N line and PE line are integrated throughout the entire system

Figure 2: TN-S system

In the TN-S system, the N line and PE line are separated throughout the entire system.

Figure 3: TN-C-S system

In the TN-C-S system, the N line and PE line are usually only integrated before the power supply entry point of the low-voltage electrical device throughout the system, and then divided into two lines after the power supply entry point.

Figure 4: TT system

The TT system is a system where the neutral point of the power supply is directly grounded, and the exposed conductive parts of the electrical equipment are also directly grounded. The grounding of the neutral point of the power supply is usually called working grounding, while the grounding of the exposed conductive parts of the equipment is called protective grounding. In the TT system, these two grounds must be independent of each other. Equipment grounding can be achieved by each device having its own independent grounding device, or by multiple devices sharing the same grounding device.

Figure 5: IT System

The IT system is a system where the neutral point of the power supply is not grounded, and the exposed conductive parts of the electrical equipment are directly grounded. IT systems can have neutral wires, but IEC strongly recommends not setting neutral wires. Because if a neutral line is set, if a ground fault occurs at any point of the N line in the IT system, the system will no longer be an IT system.

2. Reasons for Reliable Connection of N-PE in Off grid State of Hybrid Inverter

（1） Ensure grounding continuity and safety

In off grid applications, hybrid inverters typically rely on their own batteries and solar panels for power supply, without a grounding reference from the public grid. Connecting the N and PE wires together can ensure the continuity and integrity of the grounding system, effectively preventing the equipment casing from being charged and reducing the risk of electric shock. In addition, when a fault occurs in the system (such as equipment leakage), the N-PE wires connected together can guide the fault current to the ground, trigger the action of the protection device, cut off the power supply, and thus protect personal and equipment safety.

（2） Provide stable voltage reference

In off grid mode, hybrid inverters require a stable voltage reference point to control the output voltage and frequency of the inverter. Connecting the N and PE lines together can provide a stable reference potential for the inverter, ensuring the stable operation of the system. This connection method can also prevent system voltage drift, especially when there are large load changes or inductive loads in the system, which helps maintain stable voltage output.

The control circuit and power circuit inside the hybrid inverter usually require a common grounding reference point. Connecting the N and PE wires together can meet the design requirements of the internal circuit of the inverter and ensure its normal operation. In addition, this connection method can simplify the grounding design of the inverter, reduce additional grounding devices, and thus reduce the complexity and cost of the system.

（3） Complies with relevant standards and specifications

The electrical installation regulations in many countries and regions require that the N and PE lines must be connected together in off grid systems to ensure the safety and reliability of the system. For example, in the TN-C-S system, a PEN wire is used between the power supply and the user input point, and after the user input point, the PEN wire is divided into N wire and PE wire. This design not only complies with relevant standards, but also balances the safety and economy of the system.

3. Regulatory requirements of various countries

（1） China

In China, according to standards such as the General Code for Electrical and Intelligent Construction of Buildings (GB 55024-2022) and the Safety Requirements for Off grid Wind Solar Complementary Power Generation Systems (GB/T 29544-2013), the N and PE lines in off grid systems must be reliably connected. These standards explicitly require that in off grid mode, the N and PE lines need to be reliably connected to ensure the continuity and safety of the grounding system.

（2） The European Union

In the European Union, the design and installation of electrical systems typically follow IEC (International Electrotechnical Commission) standards. According to the IEC 60364 standard, in the TN-C-S system, after the PEN line is divided into N and PE lines at a certain point, it should not be merged or in contact with each other. But in off grid mode, in order to ensure the continuity and safety of the grounding system, it is usually required that the N line and PE line be reliably connected at the power inlet.

（3） United States

In the United States, the design and installation of electrical systems typically follow NEC (National Electrical Code). Although NEC mainly targets grid connected systems, in off grid systems, in order to ensure the continuity and safety of the grounding system, it is usually required that the N line and PE line be reliably connected at the power inlet.

（4） Australia and New Zealand

In Australia and New Zealand, the design and installation of electrical systems typically follow AS/NZS (Australian/New Zealand Standards). According to AS/NZS 3000, in off grid systems, it is required that the N line and PE line be reliably connected at the power inlet to ensure the continuity and safety of the grounding system.

（5） Japan

In Japan, the design and installation of electrical systems typically follow JIS (Japanese Industrial Standards). According to JIS C 8450, in off grid systems, it is required that the N line and PE line be reliably connected at the power inlet to ensure the continuity and safety of the grounding system.

4. Design of N-PE Relay in Inventronics Hybrid Inverter Products

Inventronics' hybrid inverter products are designed with full consideration of regulatory requirements for N-PE connections in off grid conditions. Its product is equipped with dual relays, which can reliably connect the N line and PE line when working off grid. The specific design is shown in the following figure. This design not only complies with the legal and regulatory requirements of countries and regions such as China, the European Union, the United States, Australia, New Zealand, and Japan, but also ensures the safety and stability of the system in off grid mode.

Figure 6: The built-in N-PE relay of the Inventronics hybrid inverter

5. Summary

The N-line and PE line of the hybrid inverter must be reliably connected in an off grid state, in order to ensure grounding continuity and safety, provide stable voltage reference, meet the internal design requirements of the inverter, and comply with relevant standards and specifications. Inventronics' hybrid inverter products are designed with built-in relays to reliably connect the N and PE lines during off grid operation, fully complying with the legal and regulatory requirements of various countries, providing users with a safe and reliable off grid power solution.