When a single-phase UPS with double pole back-feed protection is operating in battery mode, a voltage between neutral and earth may be observed. This phenomenon is typically due to the absence of a neutral-to-earth bond during battery operation, this creates an IT Earthing Network, resulting in a floating neutral.
This is normal as there is double pole isolation in these units. When the unit is online it is obviously having an earth from the supply. When it goes on battery or should the supply be disconnected, both poles will be disconnected from the supply thus creating something similar to an IT Earthing Network..
The supply has then no reference to the earth, so the first fault is not a safety hazard.
Schneider Electric single phase UPS units comply to CE standards so this shouldn’t be an issue but if for some reason the customer requires a system with earth reference, then they can use a transformer, such as SURT002.
The reason for double pole isolation, is that it is sold in countries where the plug can be easily reversed, e.g. the German Shuko Socket.
For the 3 phase and larger UPS systems, the neutral is not broken so this type will always maintain a reference to earth is a TN-CS network.
Here's a detailed explanation:
Why Voltage Appears Between Neutral and Earth in Battery Mode
Floating Neutral Condition
In battery mode, the UPS inverter generates the output voltage. When the inverter output loses the neutral-to-earth bond, the neutral is floating, it has no defined reference to earth.
This causes the neutral to drift electrically, and a voltage can appear between neutral and earth, often around half the line voltage. For a 230V system this can vary, usually it is a value between 50V and 115V depending on various factors, such as load, type of common-mode filters or EMI suppression components, capacitive coupling, cable layout, etc.
This occurs for one or more of the following reasons:
1. Double Pole Back-feed Protection
Double pole protection means both line and neutral are disconnected from the grid when the UPS switches to battery. This is required by standards like IEC 62040, to prevent back-feed into the grid and ensure safety. However, disconnecting the grid also removes the neutral-earth bond that was established at the main panel or transformer.
2. Capacitive Coupling and Common Mode Filters
UPS systems often include EMI filters and capacitive components between line, neutral, and earth.
These can create a capacitive voltage divider, causing measurable voltages between neutral and earth even when no current is flowing.
3. Separately Derived System
A UPS without an internal isolation transformer can still be considered a separately derived system.
In such cases, a neutral-earth bond must be switched in during battery mode to maintain reference. While this is common practice in the US the international units do not have such a bonding relay
And this causes the neutral to float, leading to voltage readings.
4. Are there any safety issues? No, the IEC 62040 has established that there in no danger associated with this. In fact, it becomes an IT earthing network, and this is a network used in critical installations such as hospitals, due to its enhanced safety characteristics.
5. How to resolve or prevent this? In most cases this is not an issue but where it causes problems, an isolation transformer can be installed on the UPS output to create a separately derived system with a stable neutral-earth bond.
In the US a bonding relay is used, this switches in a neutral-to-earth connection when the UPS enters battery mode. However, in Europe, many local regulations do not allow this configuration.
Ensure compliance with local regulations which requires a new earthing arrangement in island mode.
The above drawing shows the behaviour of a Line Interactive UPS. Online UPS units behave in a similar manner as far as the floating neutral is concerned.
To sumarise, Schneider Electric single phase UPS units comply to CE standards so this shouldn’t be any concerns, safety or otherwise
Released for: Schneider Electric Belgium
