Technical Note: TN0057 - Rev 05


Please note that for Powerchain systems, the installation note titled "Installation of a Powerchain System" should be followed for wiring an external contactor and associated relay control wiring:

This technical note here is still a useful reference for component specifications and operation.

The following information applies to single SP PRO systems and older advanced multiphase systems which contain green communications cards and have charge stage link wiring connected between each SP PRO.

All SP PRO products have the ability to control and accept AC Source supplies larger than the internal wiring and contactor allows for. This is achieved by adding an external contactor and current transformer (CT) which works instead of the internal contactor and CT.

With this arrangement, AC Source supplies of up to 1000A (250A for firmware revision 8.0 or less) can be integrated into an SP PRO system.

Simplified Schematics

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Figure 1: External contactor in single phase system

NOTE: Circuit protection devices not shown.

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Figure 2: External contactor in three phase system

NOTE: Circuit protection devices not shown.

Additional Requirements

The external AC Source contactor and CT components need to be supplied. These need to be rated to suit the incoming supply. Their general specifications are listed below.

  • Contactor  Rated to suit AC source:
    • 240 VAC Control coil
  • Current Transformer:
    • A suitable CT is available from Selectronic rated at 250A/5A  5VA, stock code ELCT250/5. Alternatively a suitably rated CT may be supplied by the installer. Any current transformer rated higher than the expected maximum load current and with a 5A secondary with a capacity of 5VA or greater is suitable. See Table 1 below.
  • CT wiring:
    • See Table 1 for suitable combinations of cable size, CT VA ratings and cable length. The table is calculated for cable temperatures up to 40o C.
    • Run a separate circuit for the CT back to SP PRO terminals.
    • Use twin active or similar wire to keep cables close together.
  • Control Relay
    • 240 VAC rated contacts
    • Control coil not exceeding 60 VDC.
    • NOTE: If a control relay with a 12V coil is used then the internal SP PRO 12V supply may be used as the “Relay supply”.

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Table 1: Maximum cable length for External CT wiring for different combinations of cable size and CT VA ratings


  • Cable lengths shown in table 1 are the maximum allowed. Any length less than or equal to those specified for a given combination of cable size and CT VA rating may be used.
  • Table assumes maximum cable temperature of 40o C. For cable temperatures up to 65o C, reduce maximum cable length values by 10%.


The SP PRO provides AC supply through the AC Load terminals as normal. The SP PRO relay output is held in the active state feeding supply to the external control relay which is held open, preventing any incoming AC Source supply from feeding the contactor coil.

Upon detection and synchronisation of incoming AC Source, the SP PRO relay output is de- activated, which via the control relay allows the AC Source supply to feed the contactor coil. The SP PRO internal AC Source contactor is held open.


The SP PRO can be installed in the normal manner. The external contactor and CT must be wired within reasonable proximity to the SP PRO, minimising the length of control and CT wiring.

No connection is required to the AC Source neutral connection point.

The power supply for the control relay cannot be supplied by the AC Source. This supply needs to come from either the battery supply or a small auxiliary power supply from the AC Load. If a control relay with a 12V coil is used then the internal SP PRO 12V supply may be used. This ensures the external contactor can be held open when SP PRO is running in standalone operation.

The voltage and CT sense wiring to the AC Source Active and External CT terminals must be appropriately fused to suit wire size used. In fault circumstances whereby the AC Source is supplied to system when the SP PRO has no battery supply, the internal contactor may close in parallel with the external contactor.

SP PRO Configuration

Three additional configuration settings are needed to enable the external AC Contactor/CT system. Do this for each inverter when configuring a three phase system.

AC Source – AC Input
The Extern. Contactor/CT control must be Enabled.

The External CT size must also be set. This can be set in 5A increments from 50A to 1000A.

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NOTE: The Alternate AC Source Power setting is not part of the External Contactor control settings.


The SP PRO Relay Output which operates the external control relay must be set.

Set External AC Contactor Control Output to the Relay Output - in this example Relay 3.

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System Test
When installed and configured, test the operation of the system. o Verify correct standalone operation.
  • Return SP PRO to Idle mode
Always ensure DC supply to SP PRO units before feeding any AC Source supply.

  • Switch on AC Source
    • AC Source should feed through to loads. Verify AC Source voltage and current measurements are correct within SP LINK  Data View Now. Do this for each inverter when configuring a three phase system.
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NOTE: AC Source Power MUST be a positive number when supplying load. If value is negative, shut system down, swap CT sense wiring and retest. If AC Source current is in error, verify current transformer ratio has been correctly set.

  • Set SP PRO to On mode and verify unit will synchronise to AC supply.

Additional Information
Visit the Selectronic: Knowledge Base or contact Selectronic via Customer Portal