Inline RCD tripping

[blackrose]

I have a Honda EU30i generator in a shed on the bank with huge opening at the back for exhaust fumes, and open door at the front. The generator is fitted with an earth rod and the cable is N-E bonded at the C-form 16A plug that goes into the generator. I also fitted one of these inline RCDs in the cable near the generator for extra safety.  

 

I live off grid so when I start the generator the boat is still running from the batteries/inverter and then I switch over to generator on my AC selector switch. This sort of thing.

 

 

Sometimes the inline RCD trips as soon as I switch over and sometimes it runs fine until I switch on the battery chargers. After a couple of attempts the RCD stays on and everything is fine, but today took about 5 resets until the RCD stayed on. The boat's RCBO never trips. 

 

Is the inline RCD just too sensitive and can I get rid of it without compromising safety too much?

[MtB]

An oblique question springs to mind immediately. What measures are in place to ensure phase-syncing at the point you switch over using that AC selector switch? 

 

Or maybe it crudely disconnects-before-connecting. 

 

I doubt this is related to the in-line RCD tripping though. What current rating is it and what current is the boat drawing when it keeps tripping? 

[jonesthenuke]

Does your boat electrical system earth the neutral to the hull?

[Jen-in-Wellies]

43 minutes ago, MtB said:

An oblique question springs to mind immediately. What measures are in place to ensure phase-syncing at the point you switch over using that AC selector switch? 

 

Or maybe it crudely disconnects-before-connecting. 

 

I doubt this is related to the in-line RCD tripping though. What current rating is it and what current is the boat drawing when it keeps tripping? 

The question here is the switch a Break before Make switch? If it is, then that is good. If it is Make before Break, then that is bad. Allows out of sync phases to connect momentarily. 

 

12 minutes ago, jonesthenuke said:

Does your boat electrical system earth the neutral to the hull?

And is there either an isolation transformer, or a galvanic isolator on the input line from the generator?

[MtB]

And...  is that RCD thing connected the right way around?

 

 

 

(It's probably directional.)

 

[jonathanA]

i have a very similar set up except my generator isn't some puny petrol thing but a 4 cylinder yanmar diesel.   I don't have an inline RCD but a small consumer unit next to the generator with RCD and MCBS. I often switch between generator/inverter and back so 'er indoors can keep watching call the midlife or strictly nose diving.  

 

Can't say i've ever had the RCD at the generator trip (or the one on the boat) when switching. your intermittent trip and then the multiple resets before setting suggests an earth leakage fault somewhere. I'd start at the boat shore inlet plug/socket.  Also check that you haven't mixed up the neutral/earth somewhere, but i think that would trip everytime. 

Could be a sensitive inline RCD, pretty sure those are the normal 30mA trip, but worth checking its not 10mA or something.

Do you need it? i would say if your generator is close to the boat and your using decent extension leads not cheap orange or white 'domestic flex then probably not, but if your generator is a fair distance away and/or you plug other things into it then worth having. I run power tools and stuff off mine as well as the boat so hence i fitted a small consumer unit and MCBs feeding several 16A sockets. As i also run long extension leads with multiple connectors i like the protection of the RCD and i've definitely had a tingle once or twice in the rain when unplugging things....  (yes i know i should have isolated the supply first but i'm lazy) 

 

3 hours ago, MtB said:

An oblique question springs to mind immediately. What measures are in place to ensure phase-syncing at the point you switch over using that AC selector switch? 

 

Or maybe it crudely disconnects-before-connecting. 

assuming its a proper break before make selector switch as usually fitted to boats then there is no phase syncing to do probably not a good idea to do it if running a big reactive load (e.g big motors).  

 

 

 

[Jen-in-Wellies]

2 minutes ago, jonathanA said:

i have a very similar set up except my generator isn't some puny petrol thing but a 4 cylinder yanmar diesel.   I don't have an inline RCD but a small consumer unit next to the generator with RCD and MCBS. I often switch between generator/inverter and back so 'er indoors can keep watching call the midlife or strictly nose diving.  

 

Can't say i've ever had the RCD at the generator trip (or the one on the boat) when switching. your intermittent trip and then the multiple resets before setting suggests an earth leakage fault somewhere. I'd start at the boat shore inlet plug/socket.  Also check that you haven't mixed up the neutral/earth somewhere, but i think that would trip everytime. 

Could be a sensitive inline RCD, pretty sure those are the normal 30mA trip, but worth checking its not 10mA or something.

Do you need it? i would say if your generator is close to the boat and your using decent extension leads not cheap orange or white 'domestic flex then probably not, but if your generator is a fair distance away and/or you plug other things into it then worth having. I run power tools and stuff off mine as well as the boat so hence i fitted a small consumer unit and MCBs feeding several 16A sockets. As i also run long extension leads with multiple connectors i like the protection of the RCD and i've definitely had a tingle once or twice in the rain when unplugging things....  (yes i know i should have isolated the supply first but i'm lazy) 

 

assuming its a proper break before make selector switch as usually fitted to boats then there is no phase syncing to do probably not a good idea to do it if running a big reactive load (e.g big motors).  

 

 

 

Added to that, it is amazing how often shore leads and plug/sockets trip bollard RCDs. Water in plugs from rain/snow, cuts and nicks in wires.from getting trapped between boat and jetty, miswiring. Seen them all. Generators connected by shore line will suffer the same. 

[blackrose]

6 hours ago, MtB said:

An oblique question springs to mind immediately. What measures are in place to ensure phase-syncing at the point you switch over using that AC selector switch? 

 

Or maybe it crudely disconnects-before-connecting. 

 

I doubt this is related to the in-line RCD tripping though. What current rating is it and what current is the boat drawing when it keeps tripping? 

 

Well I asked that question about switching over a couple of years ago and I was told the switch breaks all connections before it makes them and was safe to do. 

 

If it trips as soon as the supply is switched it's very low current draw, almost nothing. If it trips once I switch the battery chargers on its probably about 1.3kW.

6 hours ago, jonesthenuke said:

Does your boat electrical system earth the neutral to the hull?

 

Yes. Edit * Sorry that was a mistake. AC Earth is bonded to the hull. The generator is also earthed with an earth rod.

5 hours ago, Jen-in-Wellies said:

 

And is there either an isolation transformer, or a galvanic isolator on the input line from the generator?

 

Galvanic isolator

5 hours ago, MtB said:

And...  is that RCD thing connected the right way around?

 

(It's probably directional.)

 

 

Yes it is directional and it is connected the right way around.

3 hours ago, jonathanA said:

Could be a sensitive inline RCD, pretty sure those are the normal 30mA trip, but worth checking its not 10mA or something.

Do you need it? i would say if your generator is close to the boat and your using decent extension leads not cheap orange or white 'domestic flex then probably not, but if your generator is a fair distance away and/or you plug other things into it then worth having. I run power tools and stuff off mine as well as the boat so hence i fitted a small consumer unit and MCBs feeding several 16A sockets. As i also run long extension leads with multiple connectors i like the protection of the RCD and i've definitely had a tingle once or twice in the rain when unplugging things....  (yes i know i should have isolated the supply first but i'm lazy) 

 

 

The generator is about 10m from the boat. Perhaps I'll try changing the RCD for a different model and see if that works any better. 

 

The fact that sometimes it trips and sometimes it doesn't with exactly the same switching and loads leads me to suspect it's too sensitive.

Edited January 10 by blackrose

[Tony Brooks]

15 minutes ago, blackrose said:

Yes. The generator is also earthed with an earth rod.

 

Doesn't that provide an alternative "earth" path for the neutral, this will bypass the RCD and thus give an imbalance between line and neutral?

 

Does the way it trips alter with soil dampness?

[jonesthenuke]

16 minutes ago, blackrose said:

  5 hours ago,  jonesthenuke said:

Does your boat electrical system earth the neutral to the hull?

Yes. The generator is also earthed with an earth rod.

 

So you have the neutral connected to earth on the boat and on the land. As a result some current may pass back to the generator via the hull/water/land rather than via the neutral wire in your shore line. This will cause tripping of the RCD.

[Tony Brooks]

1 minute ago, jonesthenuke said:

So you have the neutral connected to earth on the boat and on the land. As a result some current may pass back to the generator via the hull/water/land rather than via the neutral wire in your shore line. This will cause tripping of the RCD.

 

Snap. We must have been typing at the same time.

Edited January 10 by Tony Brooks

[blackrose]

18 minutes ago, jonesthenuke said:

So you have the neutral connected to earth on the boat and on the land. As a result some current may pass back to the generator via the hull/water/land rather than via the neutral wire in your shore line. This will cause tripping of the RCD.

 

No I don't think so? I have the neutral connected to earth at the generator.

 

* I made a mistake when I answered the question earlier. Neutral is not connected to the hull (why would it be?) Earth is connected to the hull.

 

 

Anyway I've just ordered a different inline Masterplug brand RCD which is arriving tomorrow from Amazon, so I'll fit it as soon as it arrives and see if it's any better. The one that's fitted is some unknown brand and looking at the reviews some people are saying their's were faulty.

Edited January 10 by blackrose

[Tony Brooks]

13 minutes ago, blackrose said:

Neutral is not connected to the hull (why would it be?

 

Not sure what is going on in any inverter, but neural and earth may be connected internally.

[Jen-in-Wellies]

Is the changeover switch switching both live and neutral?

[blackrose]

7 minutes ago, Tony Brooks said:

 

Not sure what is going on in any inverter, but neural and earth may be connected internally.

 

Yes neutral and earth are connected in my inverter, but Jonesthenuke asked if neutral was connected to the hull. I misread his question because I assumed he was talking about the AC earth bond to the hull. I'm not sure why anyone would connect neutral to the hull, although as you say, there may be an indirect connection through the inverter.

 

3 minutes ago, Jen-in-Wellies said:

Is the changeover switch switching both live and neutral?

 

That I don't know. How would I find out?

Edited January 10 by blackrose

[Jen-in-Wellies]

2 minutes ago, blackrose said:

That I don't know. How would I find out?

With everyrhing powered off, undo the four screws on the chsngeover switch box and take a look. If you see  three live and three neutral wires going through the switch, it is switching both. 

[blackrose]

6 minutes ago, Jen-in-Wellies said:

With everyrhing powered off, undo the four screws on the chsngeover switch box and take a look. If you see  three live and three neutral wires going through the switch, it is switching both. 

 

Ok thanks I'll have a look. Is switching both L and N what it should be?

Edited January 10 by blackrose

[MtB]

1 hour ago, blackrose said:

Neutral is not connected to the hull (why would it be?)

 

This is one helluva big question, about which there are probably hundreds of internet forum threads arguing about it!

[jonesthenuke]

1 hour ago, blackrose said:

 

No I don't think so? I have the neutral connected to earth at the generator.

 

* I made a mistake when I answered the question earlier. Neutral is not connected to the hull (why would it be?) Earth is connected to the hull.

 

 

Anyway I've just ordered a different inline Masterplug brand RCD which is arriving tomorrow from Amazon, so I'll fit it as soon as it arrives and see if it's any better. The one that's fitted is some unknown brand and looking at the reviews some people are saying their's were faulty.

Some inverters have an internal relay which connects the inverter output neutral to earth (on the boat this will be the hull). This function is optional and selected by a switch on the Mastervolt which we have. It is an appropriate function as otherwise the inverter output is floating and a live to earth fault will not trip MCBs or blow fuses.

 

I suggest that to determine if this is the cause on your boat is not a trivial issue which can be diagnosed by exchanges on this forum. You need either a set of as-built wiring diagrams or detailed inspection of the installed wiring, plus an understanding of how switches and the inverter are set up. 

 

One option is to remove the earth rod connected to the generator (or the N-E link), There are pros and cons to this and I suspect I will get shouted at for even mentioning it. As above the sensibility of doing this requires a detailed understanding of you particular setup.

 

PS. 

1. Does your galvanic isolator have indication that it is operating (e.g LEDs).  If so what is that showing? The current down the shoreline may cause enough volt drop to make the galvanic isolator conduct, shorting the shoreline earth to the hull by that route.

2. Buying new parts when the root cause is not known is seldom the best approach (apologies if this offends) 

Edited January 10 by jonesthenuke

[MtB]

On 09/01/2026 at 17:36, blackrose said:

Is the inline RCD just too sensitive and can I get rid of it without compromising safety too much?

 

Um.... how much compromising of safety would you be comfortable with? 

 

 

[GUMPY]

On 09/01/2026 at 17:36, blackrose said:

Is the inline RCD just too sensitive and can I get rid of it without compromising safety too much?

As yourself the question what is it protecting?

 

It's only protecting the cable between it and the next  RCBO on the boat.

 

Do you feel lucky?

[MtB]

3 minutes ago, GUMPY said:

It's only protecting the cable between it and the next  RCBO on the boat.

 

 

Is it?

 

I'm no expert but isn't it protecting people rather than the cable? 

[Tony Brooks]

10 minutes ago, MtB said:

 

Is it?

 

I'm no expert but isn't it protecting people rather than the cable? 

 

Yes, and possibly the hull from corrosion if there was a short to the hull.

[blackrose]

1 hour ago, MtB said:

 

This is one helluva big question, about which there are probably hundreds of internet forum threads arguing about it!

 

I've never seen any threads about direct connection of neutral to the hull. There may be indirect connection though the inverter but that's not what was asked in this thread. The question was "Is neutral connected to the hull" not "Are neutral and earth connected inside your inverter." 

 

The outcome might be the same but I read those as two different questions.

 

 

1 hour ago, jonesthenuke said:

Some inverters have an internal relay which connects the inverter output neutral to earth (on the boat this will be the hull). This function is optional and selected by a switch on the Mastervolt which we have. It is an appropriate function as otherwise the inverter output is floating and a live to earth fault will not trip MCBs or blow fuses.

 

I suggest that to determine if this is the cause on your boat is not a trivial issue which can be diagnosed by exchanges on this forum. You need either a set of as-built wiring diagrams or detailed inspection of the installed wiring, plus an understanding of how switches and the inverter are set up. 

 

One option is to remove the earth rod connected to the generator (or the N-E link), There are pros and cons to this and I suspect I will get shouted at for even mentioning it. As above the sensibility of doing this requires a detailed understanding of you particular setup.

 

PS. 

1. Does your galvanic isolator have indication that it is operating (e.g LEDs).  If so what is that showing? The current down the shoreline may cause enough volt drop to make the galvanic isolator conduct, shorting the shoreline earth to the hull by that route.

2. Buying new parts when the root cause is not known is seldom the best approach (apologies if this offends) 

 

As I've said, my inverter is N-E bonded. My GI has a gauge and it's operating normally.

58 minutes ago, MtB said:

 

Um.... how much compromising of safety would you be comfortable with? 

 

 

 

That's why I asked the question. There's really no point pushing the question back at me. If you read one of the previous posts you'll see that someone asked whether the inline RCD was really necessary and I see plenty of (most) people running generators without any RCD at the generator output so my question was valid.

Edited January 10 by blackrose

[GUMPY]

36 minutes ago, Tony Brooks said:

 

Yes, and possibly the hull from corrosion if there was a short to the hull.

How  can an RCBO protect from corrosion?