Neutral and CPI Diagram: Hybrid Inverter Safety Guide

Neutral wiring and insulation control: electrical safety in standalone mode

Do you own a hybrid inverter like the SOFAR ESI with backup/EPS function? Excellent choice to guarantee your energy independence! But have you considered the earthing system configuration and continuous insulation monitoring in standby mode?

These technical aspects are crucial for compliance with NF C 15-100 standards , validation by Consuel (the French electrical safety certification body), and above all, the safety of personnel during backup operation. In this comprehensive guide, we will demystify these concepts and explain how to correctly configure your photovoltaic system with storage for safe and compliant operation.

"A photovoltaic installation with a backup mode that is incorrectly configured in terms of the neutral system can present serious electrical risks and be rejected by the Consuel (French electrical safety certification body). Understanding IT, TT, and TN-S systems is essential for any hybrid installation."

Part 1: Standalone mode and grounding schemes (SLT)

What is off-grid mode?

Autonomous mode , also called backup mode or EPS (Emergency Power Supply), is a function that allows your hybrid inverter to continue to power certain electrical circuits even during a power outage.

Hybrid inverters like the SOFAR ESI 3K-S1 to 6K-S1 incorporate this backup function which ensures:

• A near-instantaneous switchover (switching time ~10 ms)
• A 230 V~ single-phase power supply (L+N+PE) at 50/60 Hz
• A nominal power output of up to 6 kW depending on the model
• A continuous power supply via solar panels and/or the battery

During a network outage, your inverter immediately detects the absence of voltage and automatically switches to its backup output to power your priority equipment (refrigerator, lighting, alarm system, internet box...).

Earthing systems: IT, TT, TN-S

In stand-alone mode, your electrical installation is no longer connected to the public grid but forms an isolated electrical network . It is therefore crucial to define the earthing system (ES) applicable to your emergency exit. Three main systems exist:

Default configuration: IT scheme (risky)

Hybrid inverters like the SOFAR ESI are transformerless (without a galvanic isolation transformer). Therefore, in backup mode, the AC output initially operates in a network isolated from earth – which corresponds to an IT scheme .

In this system:

• No live conductor (phase or neutral) is directly connected to earth
• The conductors remain at a floating potential relative to earth
• A first insulation fault does not cause automatic disconnection.
• ⚠️ DANGER : Without continuous insulation monitoring (CPI), this first fault goes unnoticed

This IT configuration without CPI is not compliant with the requirements of NF C 15-100 and will be rejected by Consuel.

Recommended configuration: TT/TN-S (secure) system

To ensure safety and compliance, it is strongly recommended to connect the neutral of the emergency exit to local earth. This configuration transforms the IT system into a TT or local TN-S system .

How do we create this neutral-to-earth connection?

Two methods are possible:

1. Method 1 - Activation via the inverter : SOFAR ESI inverters include a Neutral Point Grounding (NPG) parameter in the advanced menu. By activating this function, the inverter automatically creates an internal connection between the neutral and the PE in EPS mode.
2. Method 2 - External contactor : Installation of a neutral connection device at the electrical panel which automatically connects the neutral to the PE when switching to autonomous mode.

Advantages of the TT/TN-S system in backup mode:

• ✅ Protection against indirect contact ensured by residual current devices (RCDs)
• ✅ Compliant with the requirements of NF C 15-100
• ✅ Validation guaranteed by Consuel
• ✅ No external CPI required on the AC part
• ✅ Functions identically to the public network (familiar to users)

"According to French regulations, if the installation is normally in a TT system with the public grid, switching to standalone mode must guarantee a consistent and safe SLT. Grounding the AC side neutral is therefore essential for transformerless inverters."

Part 2: Continuous Isolation Monitoring (CPI)

What is a CPI and what is its purpose?

Permanent Insulation Monitoring (CPI) is an electrical monitoring device that continuously measures the insulation resistance between live conductors (phase and neutral) and earth in an electrical network.

Its main role is to detect the first insulation fault in an IT network scheme, before a second fault occurs and creates a danger to people.

How the CPI works:

• Continuous measurement of conductor insulation relative to earth
• An audible and/or visual alarm is triggered if the insulation falls below the regulatory threshold.
• Immediate signaling to allow for rapid intervention
• No automatic shutdown on the first fault (continuity of service)

Devices integrated into SOFAR ESI inverters

SOFAR ESI hybrid inverters internally integrate certain monitoring functions compliant with IEC 62109 standards:

• RCMU (Residual Current Monitoring Unit): Detection of leakage currents and earth faults
• PV insulation fault detection : Monitoring the insulation resistance between the photovoltaic array and earth
• IsoFault Alarm - Low insulation impedance in case of DC side fault

⚠️ IMPORTANT : These internal devices primarily monitor the continuous side (PV/Battery) . They do not replace a dedicated CPI on the AC side if the backup output operates in an IT configuration.

CPI required in IT schemes

The NF C 15-100 standard (sections 411.6 and 537) requires permanent insulation monitoring in accordance with the NF EN 61557-8 standard for any installation operating in an IT system.

CPI not mandatory in TT/TN-S scheme

If you follow the recommendations and connect the neutral wire of the emergency exit to earth (via the Neutral Point Grounding function or an external contactor), then:

• ✅ The neutral system becomes TT/TN-S (referenced to earth)
• ✅ Protection against indirect contact is ensured by residual current devices (RCDs)
• ✅ An external CPI is no longer mandatory for the AC section
• ✅ The installation complies with the requirements of NF C 15-100

Differential protection required:

In a TT/TN-S system, each circuit supplied by the emergency output must be protected by a residual current device (RCD) :

• Type B 30 mA (or type A if upstream Type B) for general circuits
• Sensitivity ≤ 30 mA for personal protection
• Type B DDR is recommended as the primary device for detecting residual DC components.

Part 3: Risks and Regulatory Compliance

Risks of an IT configuration without CPI

Leaving the emergency exit in an IT (floating neutral) configuration without an external fire alarm exposes the system to major hazards.

• First invisible fault : An insulation fault on a backup circuit (damaged cable, faulty device) does not cause any interruption and does not trigger any alarm
• Neutral point displacement : The floating potential is raised to a dangerous potential relative to earth.
• Risk of electric shock : Any contact with a metallic mass or the earth can cause an electric shock.
• Second fault = deadly danger : If a second fault occurs on another phase, a direct short circuit will occur.
• Consuel non-compliance : The installation will be rejected during the inspection due to inadequate personal protection.

Consuel requirements for installations with batteries

The Consuel certificate form for photovoltaic installations with batteries clearly states:

"If no galvanic isolation is present (transformerless inverter), a TT system should be implemented by grounding the neutral on the AC side."

This requirement aims to:

• Ensure the correct operation of differential devices in emergency mode
• Avoid operating in an unmonitored IT system (without CPI)
• Ensure SLT consistency between network mode and standalone mode
• Protecting users against the risks of electrocution

🔗 Guide to French legislation

Compliant solutions: two possible approaches

To ensure the compliance of your installation with a hybrid inverter in standalone mode, two approaches are validated by standards and Consuel:

Our Wattuneed recommendation:

For 95% of residential and commercial installations in France and Belgium, Solution 1 (neutral connected to earth) is the most suitable. It combines simplicity, compliance, and controlled cost while guaranteeing an optimal level of safety.

Part 4: Step-by-Step Setup Guide

Step 1: Checking the current configuration

Before making any changes, check the current configuration of your SOFAR ESI inverter:

1. Access the inverter's advanced menu (LCD screen or SofarSolar app)
2. Navigate to System Settings > Advanced Settings
3. Locate the Neutral Point Grounding (NPG) parameter
4. Check its status: Enabled or Disabled

Step 2: Activation of the neutral-earth connection

Recommended method - NPG activation in the inverter:

1. In the Advanced Settings menu, select Neutral Point Grounding
2. Set the parameter to Enabled .
3. Confirm the change
4. Wait for the Success confirmation message
5. Restart the inverter to apply the changes

⚠️ WARNING : This operation must be carried out by a qualified electrician . Modifying the neutral wiring affects the electrical safety of the installation.

Step 3: Checking the equipotential bond

Ensure that the protective earth (PE) conductors of the emergency exit and the main network are connected together at the earth bar of the electrical panel. This connection is essential to guarantee a common earth reference in off-grid mode.

Step 4: Installation of residual current devices

Install or check for the presence of suitable residual current devices on the circuits supplied by the backup output:

• At the top of the emergency lighting panel : Type B 30 mA RCD (general protection)
• Per circuit : Type A 30 mA RCD (if Type B RCD upstream)
• Verify proper operation via the Test button

Step 5: Functional testing and validation

Once the configuration is complete, perform the following tests:

1. EPS failover test : Simulate a network outage (main circuit breaker tripping)
2. Automatic failover verification : The backup output must activate in less than 10 ms
3. Testing the RCDs : Press the Test button on each residual current device.
4. Neutral-to-earth voltage measurement : Should be close to 0V in standby mode (neutral connected)
5. IsoFault alarm check : No insulation fault alarm displayed

Frequently Asked Questions (FAQ)

What is an IT scheme and why is it risky in backup mode?

An IT system is a neutral grounding system where no live conductor is connected to earth (floating neutral). In emergency mode, this configuration is risky because a first insulation fault does not trigger any protection and goes undetected without a fire alarm control panel (FACP). A second fault can then create a life-threatening hazard.

Is my SOFAR ESI inverter equipped with a CPI?

SOFAR ESI inverters incorporate an RCMU (Residual Current Monitoring Unit) to detect insulation faults on the DC side (PV/Battery), but not a complete CPI compliant with NF EN 61557-8 for the AC side. If you leave the neutral wire floating (IT system), an external CPI is mandatory.

How do I activate the Neutral Point Grounding function on my inverter?

Access the Advanced Settings menu via the LCD screen or the SofarSolar app, locate the "Neutral Point Grounding (NPG)" setting, and switch it to "Enabled". This operation must be performed by a qualified electrician.

What type of DDR should I install for the backup output?

Install a 30 mA Type B RCD at the head of the emergency distribution board. The Type B detects faults with a DC component, which is essential with photovoltaic inverters. You can then install 30 mA Type A RCDs per circuit if a Type B is present upstream.

Will Consuel reject my installation if the neutral wire is not connected?

Yes, for transformerless inverters (without galvanic isolation) like the SOFAR ESI, the Consuel (French electrical safety certification body) requires either neutral grounding in backup mode OR the presence of a compliant external CPI (presumably a specific type of protection device). Without one of these two solutions, the installation will be deemed non-compliant with NF C 15-100.

Can I install an external CPI myself?

No, installing a CPI (Controlled Protection Interface) and modifying the neutral wiring system are operations reserved for qualified electricians . Incorrect configuration can create serious electrical hazards and invalidate your home insurance.

Conclusion: Safety and compliance first

The configuration of the neutral system and the insulation control in standalone mode is not a secondary technical detail: it is a fundamental safety element that conditions the conformity of your photovoltaic installation with storage.

Key points to remember:

• ✅ Transformerless hybrid inverters operate by default in an IT (floating neutral) configuration
• ✅ This IT configuration without CPI is dangerous and non-compliant
• ✅ The recommended solution is to connect the neutral to earth (TT/TN-S) via the NPG function
• ✅ Type B 30 mA residual current devices (RCDs) must protect all backup circuits.
• ✅ If you maintain an IT system, a compliant external CPI is mandatory
• ✅ Hire a qualified electrician for any modifications

The Wattuneed team is available to assist you with the configuration and compliance of your installation. Please feel free to consult our technical guides or contact our support team with any questions.

About Wattuneed SPRL
Wattuneed, the Belgian leader in photovoltaic and battery storage solutions, has been supporting individuals and professionals in their energy transition for over 10 years. We offer technical expertise, responsive customer service, and a complete range of Sofar Solar inverters, BTS batteries, solar panels, and accessories for self-consumption and energy independence in Belgium and France.