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Effects of Residual Current on Multiplus transistor circuitry.

Hi guys and gals and @Guy Stewart (Victron Community Manager)


Does anyone know what the effects residual current has on the transistor section of a multiplus inverters?

In my short (6 year) career of installing/commissioning 500kVA? of Victron inverters I have had 3 x 5kVA inverters blow transistors and these were all on on tiny loads of between 40W and 850W.

The latest scenario involves L1 inverter loosing transistors twice in 6 months in a 3 phase 15 kVA site (3 x 5kVA Multipluss) with double pole 25A MCBs on their output. After an absolutely ridiculous amount of work searching for "a" problem (more than 180 circuits). I have concluded one of 2 things:
1. I am unlucky enough to get 2 inverters with transistors from the "low quality" bins of the bell curve. OR.

2. Victron (and transformer based inverters in general) has a subtle earth leak/earth wobble stability "problem" - (no doubt multiplus II solves any unknowns here. (no, im not sure what i meant by earth wobble ;) )

Going with option #2 for now.

I have not seen SMA fail in the same apparent circumstances- then again i only have like 72kVA of SMA installation(s).

I have to assume the following:

1. there is some sort of electrical path from neutral and/or ground to the driving circuit of the inverter. (apologies: i have not stripped one yet to study the transformer-inverter interface properly.) in this situation perhaps a earth mat issue (surge?) can destabilize the driver circuit causing a thermal runaway?

2. the multiplus does not sense neutral current and in theory can source (or sink) full load on negative going side of the stack while having very low load on the positive going side (now that i write this down it sounds silly - HOWEVER! why in multiple cases did only the "negative" side of the stack blow and not the other?). - please someone correct me I have no drawings or circuit of the inverter to go on - what i do know is only one side of the stack blows.

So my Question: What residual current level is "safe" for the multiplus 5kVA and 10kVA? I want to put in the maximum rated RCD which is "safe" for the inverter (300mA is maximum for fire, but i fear with the grid transformer wiring this will not be sufficient so i want to go to like 1000mA - if possible - just to stop inverters blowing).


History of this topic starts back at this post on neutral problems https://community.victronenergy.com/questions/25355/common-neutral-in-domestic-situation.html


Some pictures. - currently considering installing client-side 1:1 isolation transformer.... (oh yes and there is an old 1940s 50mm2 paper insulated armoured cable running about 850m to 120kVA? of loads off a 100kVA transformer replace last year because the old one caught fire - "i wonder why? :D".)

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