Why EPS Neutrals and RCDs Confuse So Many Electricians (And What’s Really Going On)
If you’ve worked with hybrid solar inverters and backup power, you’ve probably run into this question:
Should the EPS neutral be connected to the main neutral bar in the switchboard, or should it be separate?
Many electricians notice something confusing. The Australian standard AS/NZS 4777.1 Clause 8.3.3.4.2 suggests that the EPS neutral should be separated in the switchboard in certain situations. Yet when you open the installation manual for a hybrid inverter like the Sungrow SH5-25T from Sungrow Power Supply Co., Ltd., the diagram often shows the EPS neutral connected directly to the main neutral bar.
Similar images from the Goodwee ESA model, Solax X3 Model, and Sig Energy are shown below.
This image shows a permanent connection of neutral at the inverter in the older versions of Solax X1 Hybrid inverters
So which one is right?
The short answer: both can be correct, depending on how the inverter is designed internally. Let’s walk through why this happens and how RCDs still operate properly in these systems.
The MEN System We’re All Familiar With
Most electricians working in Australia are used to the MEN (Multiple Earthed Neutral) system.
In a typical switchboard, all circuit neutrals terminate on the neutral bar. The neutral bar connects to earth through the MEN link. This keeps the neutral close to earth potential. Because of this, the common rule most electricians follow is simple:
All neutrals in the switchboard are normally connected together, and most of the time, that rule works perfectly, but hybrid inverter backup systems introduce something new. When an Inverter becomes a power source when the grid fails, a hybrid inverter switches into backup mode (EPS) and starts supplying power to essential circuits. At that point, the inverter is essentially behaving like a small generator. Just like any generator supply, the system must still maintain a neutral-earth reference so that protective devices like RCDs can detect earth faults.
Manufacturers achieve this in two different ways. Two Different Ways Inverters Handle Neutral
Neutral-Switching Inverters. Some hybrid inverters disconnect both the active and neutral conductors from the grid when a blackout occurs. When this happens, the inverter establishes its own neutral-earth bond internally (or through an external relay).
In these systems, backup circuits usually connect to a separate EPS neutral bar. That neutral bar returns directly to the inverter. The inverter creates the MEN reference while operating in backup mode. This is the scenario referred to in AS/NZS 4777.1 Clause 8.3.3.3.4.2, where separating the neutral prevents:
- Parallel neutral paths
- Multiple MEN points
- Incorrect RCD operation
Shared-Neutral Inverters
Other inverter designs take a different approach. Instead of switching the neutral conductor, they leave the grid neutral connected and only isolate the active conductors. In this architecture, the existing switchboard MEN remains the neutral reference for the whole system.
This is why the diagrams in the Sungrow manuals show the EPS neutral connected to the main neutral bar. The inverter does not create a new MEN point, so the system still only has one neutral-earth bond.
The Question Everyone Asks: How Does the RCD Still Work?
One of the most common questions electricians ask is this: If the EPS neutral is separate from the main neutral bar, how does the RCD detect an earth fault? The answer comes down to the basic operating principle of an RCD. An RCD simply compares the current flowing out on the active conductor with the current returning on the neutral conductor. This principle follows Kirchhoff’s Current Law, which states that the current entering and leaving a circuit must balance.
RCD Normal Operation under normal conditions:
Current flows from the inverter through the RCD. It powers the load. The same current returns through the neutral
Example:
Active current = 10 A
Neutral current = 10 A
Difference = 0
So the RCD stays on.
What happens during Earth Fault Condition?
Now imagine an appliance develops a fault and the active conductor touches its metal frame. Some of the current now flows through the earth path instead of returning through the neutral.
The current path becomes:
Active → appliance frame → earth conductor → earth bar → MEN reference → neutral.
The RCD now sees a difference between the active and neutral currents.
Example:
Active current = 10 A
Neutral current = 9.97 A
Leakage current = 30 mA
That imbalance causes the RCD to trip immediately.
The critical requirement is that there must always be a neutral-earth reference somewhere in the system, either at the switchboard MEN or inside the inverter during backup operation.
The Wiring Mistake That Causes Most RCD Problems
There is one mistake that shows up regularly in hybrid inverter installations. The active conductor of a circuit passes through the RCD, but the neutral returns directly to the main neutral bar instead of through the RCD.
When this happens, the RCD see active current leaving through the device but neutral current bypassing the device. The RCD immediately detects an imbalance and trips. A good rule to remember is: Every conductor of an RCD-protected circuit must pass through the same RCD.
The Takeaway
Hybrid inverter backup systems can look confusing at first, but the fundamentals remain the same. The system must always maintain one neutral-earth reference. RCDs operate by detecting current imbalance. Whether the EPS neutral is separate or shared depends on the internal inverter design. Once you understand how the inverter handles the neutral conductor, the requirements in AS/NZS 4777.1 and the manufacturer’s diagrams start to make a lot more sense, and like many things in electrical work, once you see the logic behind it, the mystery disappears.
Publish Date: 15/04/2026