In-line RCD - PRCD

[NigelD]

Tried looking through some of the other topics but could not find an answer easily so thought I post his.

 

1. Am I right in thinking a PRCD or RCD works when there is an in-balance between the neutral and line currents and does not reference to earth, its function is independent of where the current is "leaking off" to?

 

2. A 2 pole PCRD or RCD will break both line and neutral circuits?

 

3. PCRD is better from a "human" protection point of view as it triggers at 10ma not 30ma ?

 

I am asking as I am thinking of using one between my genny and the shore line socket on my NB.

 

Thanks

 

 

 

[WotEver]

1. Yes and no. Yes because it measures L&N only and checks for an imbalance. No because where else would it go other than Earth?

 

2. Yes.

 

3. Only if it's a 10mA RCD.

Edited November 6, 2016 by WotEver

[NigelD]

Thanks for the feedback. Agree about the earth but I just wanted to check my understanding of the functioning of the RCD PRCD.

 

 

If the genny neutral is bonded to the genny chassis and the chassis (earth) is bonded to the hull then the return path is closed. If I now fit a PRCD as near as possible to the genny then it will protect the feed line from the genny into the boat and up to the distribution box and onboard RCD. So in this setup, the genny effectively looks like a "normal" marina connection with RCD but with a 10ma trigger (is perhaps a bit "safer"). This is of course not absolutely necessary but will cover the issue of a fault between the genny and the onboard RCD.

 

I would like to run the genny on the front deck where I have lots of room, it will be in its own enclosure with exhaust system etc. I am aware of the CO and fuel issues but am just trying to get my head around the electrical issues before I proceed.

 

Hope this is right?

[Chalky]

We had 10ma in the electronics labs at Lucas York Road. Whilst they were safer than the 30ma devices they were a pain in the proverbial since they kept false tripping. It's due to the false tripping that 30ma is widely used.

[NigelD]

I had suspected this as I had an older oven at home that was causing a 30mA RCD to trigger - turned out the oven was a bit leaky and had to be replaced. On the boat, I am mainly gas and it would be interesting to see how the 10 mA version behaves not very expensive probably worth a try. My main concern here is that I connect up the genny safely.

[WotEver]

Personally I don't see the need for an RCD at the genny.

[Geo]

I had suspected this as I had an older oven at home that was causing a 30mA RCD to trigger - turned out the oven was a bit leaky and had to be replaced. On the boat, I am mainly gas and it would be interesting to see how the 10 mA version behaves not very expensive probably worth a try. My main concern here is that I connect up the genny safely.

 

I think on a boat because of the level of moisture around that you are likely to get false trips. From personal experience of tripping 230V protected by a 30mA trip you will not even get a tingle let alone a shock.

[Geo]

Personally I don't see the need for an RCD at the genny.

 

Without it there could be several metres of live generator cable between the generator and the consumer unit. A leak before the RCD does not always trip it. I know theoretically it should but the main underground 100A AC feed into a house that I own had an earth leak and it would only occasionally trip the house RCD during very heavy rain. It took 4 years to finally identify the fault and where it was.

[WotEver]

 

Without it there could be several metres of live generator cable between the generator and the consumer unit

What pray is a 'live generator cable'?

[Chalky]

From personal experience of tripping 230V protected by a 30mA trip you will not even get a tingle let alone a shock.

 

Less than 4ma will kill you if you make contact in the wrong place.

[WotEver]

Less than 4ma will kill you if you make contact in the wrong place.

I'm not trying the crocodile clips on the genitals again, it really tingles.

[Iain_S]

 

Without it there could be several metres of live generator cable between the generator and the consumer unit. A leak before the RCD does not always trip it. I know theoretically it should but the main underground 100A AC feed into a house that I own had an earth leak and it would only occasionally trip the house RCD during very heavy rain. It took 4 years to finally identify the fault and where it was.

 

I'd have thought that theoretically an earth leak before the RCD wouldn't trip it.

[NigelD]

The scenario I was thinking about is: if the genny is happily running and there is a "leak" somewhere, then the main RCD would trip but the genny would still be buzzing away and the line back to the genny is still at 230V, it could be that this is where the danger is. Not very likely but on my trad, mid engined, the distribution box and RCD are a long way from the inlet socket - appreciate I could change things round etc to reduce the distance but that's the scenario at the moment.

[WotEver]

The scenario I was thinking about is: if the genny is happily running and there is a "leak" somewhere, then the main RCD would trip but the genny would still be buzzing away and the line back to the genny is still at 230V, it could be that this is where the danger is. Not very likely but on my trad, mid engined, the distribution box and RCD are a long way from the inlet socket - appreciate I could change things round etc to reduce the distance but that's the scenario at the moment.

If you were on shore power and there was a leak somewhere the RCD would trip and the shore power would still be supplying 230V. What's the difference?

 

Serious question, not trying to create an argument, just don't see the relevance.

[Keith M]

I would always design a system where by the RCD is within a maximum of 1 meter from the power source

 

Keith

[WotEver]

I would always design a system where by the RCD is within a maximum of 1 meter from the power source

 

Keith

I can't be bothered to look right now but I believe the BSS suggests this as best practice.

[NigelD]

If you were on shore power and there was a leak somewhere the RCD would trip and the shore power would still be supplying 230V. What's the difference?

 

I think there is normally an RCD in the shore line distribution box which will trip, so you would be protected if there is damage on the "feed side".

[Arthur Brown]

30mA 50 mS RCDs are common and are probably the best compromise between fast protection and nuisance tripping.

 

If the bollard has an RCD then having one on the boat would simply confuse locating any fault,

[BruceinSanity]

I can't be bothered to look right now but I believe the BSS suggests this as best practice.

It's a Recreational Craft Directive requirement, not more than one meter between inlet and RCD. We have bow and stern inlets and had to have an RCD at both ends.

[WotEver]

It's a Recreational Craft Directive requirement, not more than one meter between inlet and RCD. We have bow and stern inlets and had to have an RCD at both ends.

Thanks I'm now edumacated.

 

Glad I didn't waste my time looking in the BSS guide...

[Robbo]

I would be surprised if the generator didn't have a RCD built in ?

[NigelD]

30mA 50 mS RCDs are common and are probably the best compromise between fast protection and nuisance tripping.

 

If the bollard has an RCD then having one on the boat would simply confuse locating any fault,

 

Surely the shore line side distribution bollard/connection point, must have a RCD in?

[Robbo]

If the bollard has an RCD then having one on the boat would simply confuse locating any fault,

 

Not sure how it would confuse locating a fault?

[WotEver]

Not sure how it would confuse locating a fault?

I guess he means two RCDs tripping. A fault between bollard and boat could trip both.

[Robbo]

I guess he means two RCDs tripping. A fault between bollard and boat could trip both.

Not likely, as they only do downstream. If a issue happened on the boat it will trip the most sensitive one which could be the one on the boat or shore, but still not really an issue to troubleshooting what's wrong.