How Do I Connect External Current Transformer to MultiPlus II?

DIY
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1

I was trying to figure out how to connect my own external 100A/50mV CT to a MultiPlus inverter. It’s the newer unit with the 3-pin terminal plug instead of the 3.5mm stereo jack. I suspect my drawing shows how the external CT replaces the internal CT by removing the jumper and putting the external CT in it’s place. Used VE Config 3 to connect to the inverter via a Victron VE Bus to USB interface module, and select External CT on the General tab.

Thanks for everyone’s help with this.

Victron CT Ckt
Victron CT Ckt1920×1148 155 KB

2

Did you try consult the manual?

Screenshot 2025-02-07 at 06.41.41
Screenshot 2025-02-07 at 06.41.411332×264 44.5 KB

4

Did you enable an external CT in the inverter config?

5

itI did not have it enabled, though it doesn’t seem to make a difference. I enabled it since doing so may apply a different algorithm for the external CT.

Thanks, everything seems to be working for now.

6

Do you need to change anything in inverter config, for using Victron’s external CT? I thought is only a hw change (bypass), using same spec CT (but external, to be used in grid parallel topology).
btw, I have two questions:

  • I have the “stereo jack” CT, but MPII with terminals. Do you know the wiring of the 3.5mm jack?
  • do you know what is the terminating resistor inside MPII, so as to use the CT as a measuring device (not connected to MPII, but to a true-RMS meter, or a oscilloscope)?
7

See: Current sensor port on MP2 - VictronEnergy

8

You should be able to read the value of the internal “burden” resistor, as they are known in the world of CT’s, by turning the inverter off and measuring the resistance between the Com and Ext pins.

9

… after removing the jumper, of course. Thanx, but I want to avoid powering off the inverter. Maybe during the next (low bat) shutdown…

10

My MultiPlus II measured 26Ω across the Ext to Com terminals (jumper removed).

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11

:red_question_mark:Question: Should the internal current transformer (CT) be shunted when using an external CT?

I’ve carefully reviewed the instructions for connecting an external current sensor to the MultiPlus-II GX 48/3000/35-32. According to the manual:

  • To connect the external current sensor, we are instructed to remove the jumper between terminals INT and COM, then:
    • Connect the red wire of the external sensor to EXT.
    • Connect the white wire to COM.

In the wiring diagram, the internal current transformer (CT) remains physically in the circuit, but its secondary winding is left open (INT–COM disconnected). Here is where my concern arises:

:stop_sign: The issue:

If the internal CT is still carrying current (which it likely is), but its secondary winding is left open, this could lead to high voltage induction, due to the lack of a shunt or burden resistor.

This situation can cause:

  • Overheating, insulation breakdown, or even fire hazard.
  • Permanent damage to the internal CT over time.

:light_bulb: Hypothesis:

To protect the internal CT, I assume that it may be necessary to shunt it — possibly by placing a jumper between INT and EXT to avoid leaving the secondary winding floating.

However, this is not mentioned anywhere in the documentation.

:folded_hands: Questions:

  1. Does the MultiPlus-II internally disable or safely shunt the internal CT when the jumper between INT and COM is removed?
  2. Should we manually shunt the internal CT, e.g. by connecting INT to EXT when using an external CT?
  3. What is the safe and recommended way to wire the external CT to avoid damage to the internal transformer?
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12

If done correctly, the internal CT will be shunted by a Transient Voltage Suppressor, or TVS. This is a common practice for CT’s for the reason you mentioned.