MEN's and power assist - RCD

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Russell Brown asked May 2, '24

MEN's and power assist - RCD

I think I found a case where I NEED to turn OFF power assist when using a generator (shore power) to ensure our vans RCD's connected to the inverters AC out, continue to function.

1) Obviously when on inverter the Multiplus II turns ON its MEN's relay so our vans RCD safety switches function as normal.

2) Shore Power (GRID) aka caravan park.

Even with power assist ON and working, The Multiplus sees the shore connection and correctly assumes its using the shore power (grid) Earth and so MEN's relay OFF and RCD safety switches function as normal

THE PROBLEM

When we plug our generator into our vans shore power connection the inverter will assume its grid shore power and so turn OFF the MEN's relay thinking there is a shore power earth.

This is actually fine as I have a 16A RCD safety switch on our shore power connection BEFORE the inverter feeding shore power to AC IN and it's a special product that does trip the RCD even when on generator. It's made by a company called PowerSafe and called an RCD DUO.

The issue is if I turn ON power assist while using the generator.

I assume the Multiplus thinks it has a shore power earth, and so if power assist kicks in all the vans RCD's coming off the Multiplus AC output will NOT have an inverter MEN's, rather the multiples assumes a shore power MEN's earth and there is not any earth as its the generator plugged into the shore power connection.

So in this case if a fault occurs the 16A shore power "PowerSafe RCD DUO" should trip which will disconnect the generator (shore power), but the inverter will continue to operate and the vans RCD's will not trip. Now as shore power was just disconnected, the multiples I assume will now turn ON the MEN's relay and now the vans RCD's will trip.

BUT not too sure all that delay helps the person being shocked.

Generator powerassist men rcd tripping

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jays-1 commented · Jun 02 at 07:35 AM

I think you are overly concerned here. While best protection by an RCD occurs in a properly grounded (Earthed) system, an RCD will work fine in an ungrounded system.

RCDs do not detect current leakage to ground, they detect an unbalanced current between the line and neutral. While typically the unbalanced current difference appears as ground current, the ground current is not used by the RCD for proper operation and protection against electrical shock. In older residences without a ground wire in the US, the NEC allows use of RCDs with no grounds.

Example 1: The system is properly grounded (Neutral is connected to ground at the power origin - generator or inverter) and a person touches the line and ground. In this case, since the neutral is connected to ground at the origin, ground current can flow, bypassing the neutral conductor, and shock will occur. The RCD will trip as it sees an unbalanced current between the line and neutral and the person is only momentarily shocked - in some cases they will not even feel it.

Example 2: The system is ungrounded (both line an neutral are isolated from ground) and a person touches the line and ground. In this case, since the line and neutral are isolated from ground, no current can flow and no shock will occur. Also, the RCD will not trip, but the person is not shocked. Although no shock occurs, this is an undesirable situation in that the person has energized both themselves and the ground with line voltage and this should be avoided.

Note that in both example 1 & 2, the person is not shocked. However, example 1 is better because a fault has occurred (a person is touching the line conductor) and the circuit RCD disconnects the circuit.

One more: Example 3 - You have two different circuits and a person touches the line of circuit 1 and the neutral of circuit 2. In this case no ground current occurs (and therefore it does not matter if this system is properly earthed or not), but an unbalanced current occurs in circuit 1 and the RCD will trip providing protection.

Example 3 also will occur in a single unground circuit if line can somehow find a path back to the source bypassing the neutral. For example, this can occur in an ungrounded system by dirt/oil in the source generator causing a resistance path between ground and the neutral.

To summarize: RCDs work in both grounded and ungrounded circuits, but provide additional protections with proper grounding.

Note also that Circuit Breakers and RCDs provide no protection whatsoever when someone contacts the line and neutral of the same circuit. This is why working on an energized system remains hazardous.

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Answer 1 · 2024-06-02T07:16:01Z

kevgermany answered · Jun 02 at 07:16 AM

I solved this on my boat by wiring as follows:

Generator/shore power both go to a selector box. This either connects shore power or generator to the Multiplus. But not both.

Generator has earth/neutral bonded, shore connection does not

Earth from shore power goes through galvanic isolator to a common earth point connected to the steel hull

All other earths including generator go to the common earth point.

So on solar (most of the time) earth connection provided by the multiplus in my EasySolar, on generator by its connection to the hull, on shore power via the isolator to the hull.

But get a qualified electrician to check for your installation.

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