question

Type A and Type B RCDs at Multiplus/Quattro output. Rationale for use.

I understand that Victron use / recommend a Type A RCD, as it protects against AC and pulsing DC residual currents. While perhaps intuitive to use an RCD that detects DC, my understanding is that for the RCD to trip on the basis of DC, then there must be both a fault in the inverter (to produce pulsatile DC) and a ground fault in an appliance (to incur a residual current fault). If this is the case, there would have to be a double point failure for the DC element of the RCD to operate.

Am I missing something?

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AIUI the Multis/Quattros are based around toroidal transformers (there is a youtube video explanation of this) which would seem to meet the UK requirement for "simple separation between AC and DC circuits" in S 9.12 of the IET Code of Practice.

So (in the UK at least) you do not need more than a type A RCD provided it can handle bidirectional current flows.

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Usually is it dictated by local law / legal requirements and regulations. So it is best to check if there if the regulation is met.

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"Am I missing something?"

Possibly:

Perhaps a DC offset in the sinewave will cause the AC measuring element (CT) to saturate and therefore there will be no RCD protection when an earth fault occurs?

My understanding is that Transformerless inverters do not have electrical isolation between DC and AC circuits and therefore there could be a strong rationale for the use of these types of RCDs in that instance.

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I think it is the reverse way the DC component can cause maloperation of the RCD ? ABB has a goo guide here - https://library.e.abb.com/public/9f0e99de3bc740288bc41ab95667f72f/RCD%20Technical%20Guide%20EN.pdf

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Hello. If the QUATTRO/MULTIPLUS II has a transformer that separates AC and DC circuits, wouldn't a TYPE AC RCD be sufficient?

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Type AC RCDs were the most common type in use and until relatively recently, the prevalence of DC equipment both transformer isolated and otherwise has led to the type A coming down in price to the point where it is comparable to the AC but offers suitability in more circumstances.
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IMHO nobody should be using type AC RCDs for anything these days. I fully expect they will be dropped from the regs in the not too distant future.
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