SMA MULTICLUSTER BOX 12 - Manual (Page 12)

4 Product Description SMA Solar Technology AG
12 MC-BOX-12-3-20-BE-en-10 Operating Manual
Residual-current device
Figure 4: Overview of the residual-current device
4.2 Grounding in the Multicluster System
With TN-S, TN-C-S and TT systems, the neutral conductor must be grounded for protection against indirect contact with
live components. The following options for grounding the neutral conductor are available in the multicluster system:
• If the multicluster system is using a generator as grid-forming source, the multicluster system must be grounded outside
the Multicluster Box on the generator side.
• If the multicluster system is connected to the utility grid, it is grounded via the utility grid when in parallel grid
operation. However, in case of grid failure, the multicluster system must disconnect from the utility grid. For this
disconnection, either only the line conductors are disconnected, or in case of all-pole disconnection, the line
conductors and the neutral conductor.
With all-pole disconnection, the multicluster system with line conductors and neutral conductor are disconnected from the
utility grid in the event of grid failure. This disconnection does not ground the neutral conductor in the grid of the
multicluster system. Therefore, in multicluster systems with all-pole disconnection, the grounding contactor of the
Multicluster Box must ground the neutral conductor in the event of grid failure. The grounding contactor ensures the
necessary protection in case of indirect contact with live components. The grounding contactor is in fail-safe design.
If the neutral conductor of the multicluster system is connected to the utility grid, no further grounding is permitted in the
electricity grid of the multicluster system. Therefore, if the multicluster system is connected to the utility grid via NA Box or
Grid Connect Box the grounding contactor of the Multicluster Box deactivates the connection between neutral conductor
and ground potential.
If the Grid Connect Box is used, the all-pole disconnection can be deactivated (see operating manual of the
Grid Connect Box). Deactivation of all-pole disconnection is only possible if the utility grid is configured as a TN-C-S
system. The multicluster system may not be operated on a TT or TN-S grid.
Position Designation Explanation
A Test button The functionality of the residual-current device must be tested
regularly (see Section 10.1, page 36).
B Switch lever Top position: ON - residual-current device is switched on.
Bottom position: OFF - residual-current device has tripped or
is switched off.