installation with bypass in switzerland

Standard AC/DC installation for villa
Use of the next3 and bypass schemes


This document is an aid for system installers. It is intended to explain the various bypass circuits that are possible when the inverter needs to be shut down or serviced, the overvoltage protection for the PV and the special parameters that must be programmed during commissioning and could be required by the DSO. 


Basic concept

The next3 is an all-in-one hybrid solar inverter (with battery) with backup functionality. This device is connected between the grid and the house, directly after the introduction and the main meter. The grid is connected to the AC-source terminal, the loads to the AC-Loads and AC-Flex terminals. An internal relay in the unit next3 opens in case of mains failure and the terminals AC-Loads and AC-Flex can be supplied from the batteries in case of main failure.

The AC Flex output is a second output of the inverter that can be controlled (internal relay in the unit) or simply used to measure consumption separately (next3 monitors/records all current flows). 

A bypass should be installed to provide redundancy in case of UPS failure. A double bypass can be installed if AC-Flex is used.

Exampleof adding to an installation with existing electrical cabinet:

  • an added bypass box/table

  • of surge arresters and DC protection devices for solar installations

In the (Swiss) regulations, DC and AC must be separated.Precautions must also be taken so that the unit can be stopped easily and clearly in case of problems.

Technical principle diagram of the unit 
schematic representation of the next3

Note: The relay that disconnects the feeder complies with the grid directives. The functionalities concerning the grid interaction have been certified (anti-islanding , NA protection, frequency behaviour, ...). The grid standard must be selected by the installer during commissioning according to the requirements of the DSO. For Switzerland, the network operators usually require the German standard VDE-ARN4105.

Electrical schematics ac


The next3 has connections and a transfer capacity (max. current flowing through the unit between AC source and AC loads) of 80Aac. This is usually more than the ampere rating of a standard house connection. So the next3 is protected by the input fuse. However, it is good to install a circuit breaker and bypass to isolate the unit in case of maintenance. Thus, the AC source input can be isolated by this circuit breaker and the AC load output can be isolated by the bypass.

The bypass comes in different types: 

  • automatic or manual

  • single or double (included with AC-flex)

Note that a bypass must always pass through a fully off position and implies a switching delay and thus a micro-interruption.

The neutral conductor can be left continuous (solid neutral) or interrupted, but this must be communicated to the unit during programming, as it must manage a correct neutral-earth connection. This connection neutral-earth must comply with the installation standards of the respective country. See below for comments and diagrams.

A simple, full automatic bypass box is available from Studer-Innotec or can be made by an electrician or installed in the main distribution box of the house if space is available.

Simple manual bypass

In this case, the source switch must have a zero position that guarantees a separated position.

Examples of components that can be used for this manual source selector switch :

  • HIM404 (Hager) 4P 40A

  • HIM304 (Hager) 3P 40A (with continuous neutral) 

  • HIM406 (Hager) 4P 63A

  • OT40F4C (ABB) 4P 40A

  • OT40F3C (ABB) 3P 40A (with continuous neutral)

Manual double bypass

The same type of manual double bypass can be realised with two switches if the AC-FLEX output is used:

Simple automatic bypass

In this case, an automatic bypass is realised by relays controlled by the presence or absence of voltages. 

As with any source switch, a mechanical interlock is mandatory to ensure that no short circuit is possible. An electrical interlock of the commands guarantees a defined status.

Independent automatic bypass

This is the variant preferred by Studer-Innotec.

In this case, the priority command is given to next3 via a K3 relay. When the next3 supplies a voltage, it is activated and switches off the mains.

Delays caused by K4 when switching on K2 (inverter) and K5 when switching on K1 (mains) provide real interruptions where loads can be demagnetised and phase jumps can be avoided. This applies to cases where the inverter was in operation but not synchronised with the mains. The delay of 100ms is sufficient.

Manual forcing on the mains is possible with a circuit breaker that interrupts the coils of K2 and K3:

An automatic bypass box with 40A can be ordered as an accessory directly from Studer-Innotec.

Bypass with a minimum of components

A control contact (AUX) provided by the inverter is used to control the bypass and in next3 gives priority to the inverter when it is fully functional. 

The normally closed position of AUX1 is used to have the contact at rest or when the unit is completely switched off. 

This saves two priority relays, but has the disadvantage that an AUX is used and is not optimal when changing units. This is because if the next3 unit is completely removed, the C-NC bridge must be established for the transfer to the mains, and the line is live. It is therefore necessary to disconnect the power supply via the mains and thus in the house.

In addition, the lack of time periods can lead to multiple transients between positions.

Below is the variant for a continuous neutral conductor (see chapter Connection neutral conductor-earth). You must ensure that this variant without interruption of the neutral conductor is permissible according to local regulations:

The settings for the AUX1 relay, which provides the contact for normal operation of the unit, are made in special menus accessible with the EXPERT/FAXMANN user level.

Change the user level in the menu of the remote control with the code 187382.

Changes to the settings should be made during commissioning with the unit switched off to avoid unintentional manipulation of the bypass.

The connect/disconnect depending on the presence of the network must be configured. Click on the middle part of the synoptic.

It is possible to have a version without using the AUX contact by realising the priority with a contact :

And on the configuration icon:

Change the operating mode for relay AUX1 to AUTO:

And the type of condition to "Ext bypass":

Automatic double bypass

For this case, the terminal of AC-Flex is used and must be switched automatically together with AC-Loads. A second source switcher can be added that has a similar control logic as the first. 

The neutral conductor system

The neutral conductor system can be :

  • continuous (solid-neutral=a bridge is formed between the neutrals and they are never interrupted) 

  • be interrupted through the unit and a bridge established between earth and neutral conductor in the unit when it is in isolated operation.

In all operating cases, the T-N connection must be correct so that the residual current devices can function.

The standards for photovoltaic inverters require that all poles are disconnected when islanding is detected and this is therefore the configuration recommended by Studer-Innotec. Local installation regulations should be followed as a priority.

An example is given for the manual double bypass:

An internal relay in next3 ensures the connection between neutral and earth during island operation. This earth relay is open when connected to AC-Source, since in this case the neutral position is established by the connection made during feed-in. In island mode, the internal T-N relay switches over and establishes the neutral potential so that the downstream residual current devices can function properly.

A check of the connection is constantly carried out by the unit through measurements to ensure safe operation.

The default operation of the unit is for a neutral control managed by the unit. If a change is desired, this is done in the system menus :

And on the configuration icon:

Select "self-managed" or "solid neutral" for the earthing management.

Disabled mode disconnects the neutral, never bypasses the earth and no insulation test is performed. You should only switch on this mode if you have taken all necessary precautions to ensure that the neutral conductor is correct and that the safety of persons is guaranteed.

Commissioning: isolation test and short-circuit current

Isolation tests

When commissioning an electrical system, insulation tests are carried out on the wiring (in some countries according to local regulations). They are carried out by applying high voltages to the cables and measuring the leakage current. This procedure must be carried out without the next3 in the loop. The overvoltage protection included in the next3 invalidates the tests, as the next unit is protected against mains overvoltages by electronic voltage limiting elements (VDR).

The insulation between the circuits and earth is tested in the factory for each next3 device manufactured in accordance with the IEC/EN 62109 and IEC/EN 62477 safety standards before final assembly of the surge protection devices. The manufacturer Studer-Innotec therefore guarantees the good insulation of the device to earth in this case. The installer must check everything concerning the wiring without the presence of the next3.

Short circuits current and overload behavior

In stand-alone operation (offgrid), when the inverter is not connected to the grid but is operated with batteries, the next3 cannot pass the standard tests for short-circuit currents that are carried out when commissioning an electrical system due to its output impedance. 

Nevertheless, in the event of a fault (short circuit, overload), the current is electronically limited by the unit. If a short circuit occurs, it is detected and the unit switches off automatically. Safety is guaranteed in this case. In the island situation, the electronic current limitation is 45A in the event of a short circuit phase-neutral in the cold state and 65A in the event of a short circuit phase-phase.

A next3 detects a short circuit when the voltage drops to less than 50 % of the nominal voltage with maximum current limitation, then the unit switches off after 0.5 seconds. In case of a limited overload, if the voltage drops to less than 80 % of the nominal voltage with current limitation, the unit switches off after 3 seconds. By default, the unit will attempt to restart up to 3 times after 1 second. The number of attempts is reset to zero after 30 seconds of normal operation. 

All these parameters can be set in "Expert" mode. Studer-Innotec recommends leaving them at the default values.

schematic dc

PV dimensioning

The installation must be done according to the local standards. In Switzerland, it is now required that AC and DC are separated. The DC wiring therefore has its own table and its own channels.

The next3 has two independent 8kW PV inputs that can be used to operate two strings of modules in series. Each MPPT can convert up to 22Adc of photovoltaic (PV) current during operation. For this reason, each MPPT has 2 inputs and up to 4 strings can be connected to the unit. The two upper inputs (MPPT1) are internally connected, as are the two lower inputs (MPPT2). The parallel connection of two PV strings for one MPPT can also be done outside the next3, e.g. with a Y on the roof. The short-circuit current of the panel max. is 27A.

The DC voltage of a string must be less than 900V. For starting, the voltage must be greater than 350V.

There is a switch on the unit with which the solar modules can be switched off.

PV protective housing

Outside the unit, only surge arresters corresponding to the keraunic level need to be added.

For the fire brigade, a visible external switch in a small DC box (as DC is separated from AC).

Professional solar installers are familiar with the requirements and there are no DC boxes supplied by Studer-Innotec.

Battery protection

A fuse should protect the battery cables. 

With a lithium battery, the protection devices are internal to the battery, which also protects itself with a contact in the event of simple overcurrent or incorrect use of the battery (undervoltage, overvoltage). An additional fuse is usually not required. Please observe the battery manufacturer's instructions. 

For lead-acid batteries, a fuse is required at the positive terminal closest to the battery.