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dbx120 avatar image
dbx120 asked

AC Input cable burned. No AC connected.

I am trying to get to the bottom of an issue that has now happened twice to a caravan manufacturer we sell Victron Inverters to.

The AC input cable has burned up, even though the system was not connected to site power, so there should have been no power on the AC input.


The AC input is wired from the inlet plug, through a 16A RCD to AC input 1 on a 12v 3000VA Multiplus. There is an oven and a fridge wired in parallel with the inverter, directly from the RCD.

The report from the recent customer was this occurred when the aircon was being used. The inverter was being run from the AC output of the inverter, from the batteries. Normally the Air con uses about 700W, but was drawing 1400W this time.


A local electrician was hired to change the AC input cable, and the problem persisted afterwards. I think this rules out the likelihood that a staple could have been put through the active and neutral on the input cable.


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MultiPlus Quattro Inverter Charger
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3 Answers
Alexandra avatar image
Alexandra answered ·

@DBX120

An appliance has a fault. The fault is returning on the neutral/earth line and since the earth is common (ground relay) whether is is on in or out is common so can burn anywhere there is insulation damage.

The damage could have happend before install, pulling through made the insulation thinner or shaved it a bit? It may have been caught on or pulled through a pinch point. Or, an this is a common one, it is too small to take the fault current. A combination of issues.

Why does the device rcd or other protection not trip?

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dbx120 avatar image dbx120 commented ·
It doesn't make sense that the AC breakers haven't tripped. As wkirby pointed out, it would take a lot of AC current to melt the 2.5mm2 cable...
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wkirby avatar image
wkirby answered ·

There is more going on here than your diagram shows.
In order for that cable to burn, current must be flowing through it. Without Site Power, it is impossible for any current to flow through that cable, so something is wired incorrectly.

Do you know which conductor in that cable is burning? You have an arrow pointing at the CPC, so if it is the CPC that's burning, then you have fault current in this conductor.
You did not mention the CSA of the cable used there, but I assume it is at least 2.5mm² for 230V? The MultiPlus cannot generate enough AC current to burn that cable, so then I begin to think that DC battery current is returning down that conductor somehow with some sort of shared earthing arrangement. Are all the battery negative connections sound and properly connected?

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mufassa avatar image mufassa commented ·

Depending on the country, if the CPC is carrying current in some countries the CPC for a 2.5mm T+E is only going to be 1.5mm (i.e. in the UK), around 14-16amps max for that conductor subject to how it is installed

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dbx120 avatar image dbx120 commented ·

Spoke to the caravan manufacturer again, and they have confirmed that there is an earth connection to the caravan chassis from the AC inlet. This cable was also burned. The battery negative is also connected to the chassis, at a different point.


I think this confirms wkirby's suggestion that it is DC current running through the earthing. This would also explain why the AC breakers have not tripped.

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They have not earthed the chassis of the inverter, which may have helped prevent this issue. As the inverter already connects the AC earth to the chassis (as long as the battery negative is connected to the chassis) I think that the extra AC earth connection can only cause problems? In a house there would not be two earth connections, only one from the main switchboard.

The Victron schematics show only a single chassis connection made from the negative bus bar:

https://www.victronenergy.com/upload/documents/MultiPlus-system-example-5KW-24V-230V-AC.pdf




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Kevin Windrem avatar image Kevin Windrem dbx120 commented ·
Unless there is a fault inside the Multi, the DC circuitry is isolated from the chassis and the AC PE connections. Connecting the Multi chassis to the battery negative with equal sized conductors is the recommended way to control DC faults.
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dbx120 avatar image dbx120 Kevin Windrem commented ·
I just tested some Multi's and a Quattro, the Multiplus and Multiplus-ii both had isolation from the earth to the battery negative, the Quattro did not. I don't have any other reason to think the Quattro is faulty, could it be that the Quattros are different in this respect?
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dbx120 avatar image
dbx120 answered ·

I don't know which of the three conductors has burned, hopefully, I will get a chance to inspect it in person or get some photos.

I think it's probably 2.5mm2 for the AC. I think the battery current is a likely cause. I assume that the battery negative is connected to the chassis, so if the battery negative is compromised, there could be a return passage through the vehicle chassis and then the AC earth if there is no earth from the inverter chassis to the battery or vehicle chassis.

There would need to be a ground connection from inlet to the chassis for this to be the cause, which sounds odd but isn't impossible.



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