Neutral Ground Connection

Hello Kevin,

My question is regarding marine wiring.

On my marine electrical, I’m using an isolation transformer in order to transform 220V to 110V.

Recently, the boat’s reverse polarity indication light is on. I’ve opened the circuit to see how this indication is measured and saw that there is a diode and led connected between the neutral and the ground.This is a really bad idea.

I’ve realized that in the current situation the output of the transformer is basically floating, therefore the neutral line can have any voltage. One of the solution that I’ve seen online is connecting the neutral line at the output of the transformer to the boat’s ground (that is not connected to the shore’s ground). So I have several questions about it:

  1. Is there advantage in connecting the neutral to the boat’s ground over working with floating voltage?
  2. Is it ok to make this connection?
  3. For about a year, the indication was off, what can be the cause for it to be on now?

Thanks in advance,


Hi Ilia,

At the source of power, the neutral must be connected to the ground. With an isolation transformer, this is on the output (boat size). Please connect the output neutral and ground together immediately. It is an extreme shock hazzard as is.

The reverse polarity indicator is connected to the input side of your transformer. With an isolator transformer, the light is more of a nuisance than a concern. It is telling you that the polarity is reversed (Neutral is hot) on the input. Since you have an isolation transformer, you really don’t care as long as the neutral and ground are connected together on the output side.

Hope this helps,