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spyros-damigos asked

Galvanic isolator size

I have a 8 meter boat with a Multiplus 2000 and one AC outlet that will operate either a coffee machine or small microwave. On board AC panel has an earth leakage safety and a 16A fuse. What size isolator do I need?

In case I have a 16A galvanic isolator and I visit another port with a 32A shore power plug and a 16A adapter will I be OK?

If I understand correctly a 32A galvanic isolator will work with both a 16A and 32A shore power whereas a 16A isolator will work only with a 16A shore power supply?

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Trevor Bird answered ·

@Spyros Damigos Imagine you are fed with an earth leakage breaker ( RCD like already in your boat) that has a rating of 30mA. That rating is the maximum it takes to trip. It can by specification be 50% less than that. So minimum trip can take place at 15mA and maximum at 30mA. Typically they actually trip at just over 20mA.

Earth leakage will create an earth current in your earth lead where the galvanic isolator is installed and if you have an RCD supplying your vessel a maximum of 30mA can flow in the circuit before the RCD trips.

However let’s assume the RCD has failed. Now the maximum that can flow in the earth lead is whatever the overcurrent trip rating of the breaker is. If you are fed with a 15amp breaker that will be 15amps. If 32 amp breaker that will be 32 amps.

The galvanic isolator is simply some diodes that need about 1.2 volts forward bias before they conduct. That isolates you vessel from other vessels in a marina if you are all plugged into supply pedestals from galvanic voltages.

The truth is a galvanic isolator in your case could be rated at a lowly 1amp and be fine provided your RCD is doing it’s job.

It would be customary however to have a galvanic isolator rated at the supply current rating feeding your boat.

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Trevor Bird avatar image
Trevor Bird answered ·

I’m fact @Spyros Damigos you may check if your 8 m boat has the AC ground connected DC ground. I have a 12 m French sailing boat where the AC ground is isolated to the DC ground. If I fitted a galvanic isolator it would be a waste of money because DC ground ( prop shaft, prop and “P” bracket ) are not connected to AC ground so they are already isolated from all other boats in a marina.

If the AC ground and the DC are isolated don’t fit a galvanic isolator as it will do nothing at all.

If you do have them bonded at only one bonding point, install the galvanic isolator in that line and just keep your AC ground wire perfectly intact.

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spyros-damigos avatar image spyros-damigos commented ·
@Trevor Bird it will be a new installation and if I need to use an inverter when on the go I must either bond the ground to the DC or give it a separate dedicated ground I believe to be safe, correct?
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Trevor Bird avatar image Trevor Bird spyros-damigos commented ·

@Spyros Damigos if you have a marine style inverter it will have a neutral to ground relay so that when in inverter mode the neutral is tied to a ground. That ground can be DC ground and still the mains AC ground and the boat DC ground can be isolated. In that way if you are in a marina, plugging into the marina pedestal will not tie your underwater metal to other boats, but when inverting while away from the marina you have the neutral and ground ( boat ground) connected.

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