AC & DC ground

[rogerthebodger]

Is this still connected in a non-standard way Roger ?

Two possible fault scenarios:

1) A positive DC positive connection is made to the hull, risking greatly accelerated stray current erosion.

2) An AC positive connection is made to a point in the DC negative circuit, making everything in that circuit live.

No 1 is the main reason for the lack of DC connection to the hull

As I am the only one working on the electrics. A connection between AC positive and DC negative is very unlikely as both AC and DC circuits on in separate plastic conduit/ trucking so chafe and short between AC cables and DC cables very unlikely

 

[JOHNPEET]

With no connection between the Hull and DC will have no interconnection between the AC and DC. But with a short between the AC and the earth will cause the earth leakage to trip thus shutting of the mains AC supply

As I said the engine and prop shaft have no interconnection between DC at all, the only anodes I have is anodes welded to the steel hull so no DC connection

My propeller and prop shaft are all stainless steel, so no shaft anodes are needed unlike a bronze propeller and stainless steel.

The coupling to the engine is an isolated (rubber) coupling and no metal connection across the coupling

The earth leakage short to hull has been tested several times and also have mains reverse indicator that is checked every time commotion to shore power

“With no connection between the Hull and DC will have no interconnection between the AC and DC. But with a short between the AC and the earth will cause the earth leakage to trip thus shutting of the mains AC supply“

This doesn’t answer my question which was in essence, the same as the second point that PaulRainbow made in his comment. So will take your response to that.

 

[JOHNPEET]

No 1 is the main reason for the lack of DC connection to the hull

As I am the only one working on the electrics. A connection between AC positive and DC negative is very unlikely as both AC and DC circuits on in separate plastic conduit/ trucking so chafe and short between AC cables and DC cables very unlikely

So without an intentional low impedance bond between the DC neg bus and hull, should you get a DC insulation fault resulting in 12v or 24v pos connected to the hull, your DC circuit fuse won’t blow and the hull will remain at 12 or 24v pos until you somehow otherwise discover the fault!

Agree with your risk assessment that a fault between AC and DC maybe unlikely, but obviously not impossible! There are places where both systems come closely together - battery charger, inverter. Also need to consider direct contact, whether accidental or not.

Are you using a galvanic isolator or a residual current device for your earth leakage protection? (You mentioned a GI in an earlier response) Not too familiar with a GI being used to prevent electric shock from direct contact - does that provide the required tripping characteristics as an RCD would?

 

[rogerthebodger]

I have both an RCD that will shut off mains if there is a short between the hull and the mains line of neutral

The GI is to prevent and Galvanic leakage current eroding my hull anodes

The trip point of the RCS is such that the trip point is set to the requires for the reverse phase leakage

s you say the only possible is a leakage between the ac and DC positive, but this was very careful installed to prevent this happening

 

[PaulRainbow]

So without an intentional low impedance bond between the DC neg bus and hull, should you get a DC insulation fault resulting in 12v or 24v pos connected to the hull, your DC circuit fuse won’t blow and the hull will remain at 12 or 24v pos until you somehow otherwise discover the fault!

Agree with your risk assessment that a fault between AC and DC maybe unlikely, but obviously not impossible! There are places where both systems come closely together - battery charger, inverter. Also need to consider direct contact, whether accidental or not.

Those are why the connection is usually made. As you say, several appliances onboard with AC and DC inside. But according to Roger you don't even need to Earth the inverter case, or any other cases with dedicated Earth connections.

Are you using a galvanic isolator or a residual current device for your earth leakage protection? (You mentioned a GI in an earlier response) Not too familiar with a GI being used to prevent electric shock from direct contact - does that provide the required tripping characteristics as an RCD would?

A GI offers no electric shock protection at all. I'm pretty sure Roger has an RCD or equivalent.

When he spoke of his GI in another thread, a while back, it was incorrectly installed though, not sure if he changed it, hence my question in post #19

 

[PaulRainbow]

I have both an RCD that will shut off mains if there is a short between the hull and the mains line of neutral

The GI is to prevent and Galvanic leakage current eroding my hull anodes

The trip point of the RCS is such that the trip point is set to the requires for the reverse phase leakage

s you say the only possible is a leakage between the ac and DC positive, but this was very careful installed to prevent this happening

How did you isolated the internal components of all of your appliances that have AC and DC parts inside ? Things like chargers, inverters, generator, fridge etc ?

What do you perceive the problem with connecting DC negative to the hull would be ?

 

[Nomic]

Hi all,

You guys seem to actually know what you are speaking about, so I thought I'd ask my questions here.

Based on what I understood, there seem to be 4 ways to ground (or not) AC systems on a metal hull. Here is a quick scheme to illustrate them:


In practice, there can of course be several sources and loads. All sources are assumed to be on board. For example, AC from shore goes to an isolation transformer which is considered the source.
Casings are all isolated from the hull to avoid ground currents through the hull.

Which option would you consider safe for the hull and its inhabitant? Rodgerthebodger, you went for option 1, right?

An experienced marine electrician mentioned he goes for option 2, which creates a safety issue in my opinion as it seems to make RCDs useless. Or maybe the hull has enough capacitance to make RCDs trip in case of a live fault? What's your take on this?

Do you believe the galvanic isolator to be of any use in options 1 and 2, or is it an overkill?

Any 5th option maybe?

Thanks for you inputs

 

[Nomic]

How did you isolated the internal components of all of your appliances that have AC and DC parts inside ? Things like chargers, inverters, generator, fridge etc ?

What do you perceive the problem with connecting DC negative to the hull would be ?

To follow up on PaulRainbow’s point about DC negative grounding:

If there’s a DC negative leak, stray current will flow to DC negative through the hull to the grounding point instead of through the cable, since the hull conducts better than the cable. Detecting this requires to regularly disconnect the DC negative bond. Easy to do if a switch if fitted, but it still prevents continuous monitoring.

With a floating DC system, a positive leak would not trip any breaker, but hull isolation is easier to monitor continuously with an alarm system. The downside is indeed that an AC fault could make the DC system live, like you said, but a properly set up AC ground with an RCD should trip the second someone touches the DC system.

I'm not saying any is better than the other. Both approaches seem to have pros and cons to me.

 

[PaulRainbow]

Hi all,

You guys seem to actually know what you are speaking about, so I thought I'd ask my questions here.

Based on what I understood, there seem to be 4 ways to ground (or not) AC systems on a metal hull. Here is a quick scheme to illustrate them:
View attachment 208351

In practice, there can of course be several sources and loads. All sources are assumed to be on board. For example, AC from shore goes to an isolation transformer which is considered the source.
Casings are all isolated from the hull to avoid ground currents through the hull.

Which option would you consider safe for the hull and its inhabitant? Rodgerthebodger, you went for option 1, right?

An experienced marine electrician mentioned he goes for option 2, which creates a safety issue in my opinion as it seems to make RCDs useless. Or maybe the hull has enough capacitance to make RCDs trip in case of a live fault? What's your take on this?

Do you believe the galvanic isolator to be of any use in options 1 and 2, or is it an overkill?

Any 5th option maybe?

Thanks for you inputs

If shore power is vis an isolation transformer there is no point fitting a GI.

There must only be a single ground connection to the hull.

All Earths are connected together, typically using a busbar. Then there should be a single Earth connection to the hull.

That's it. But, note comments above relating to DC Earth.

So:

1 looks correct if you omit the GI
2, 3 and 4 are all incorrect.

 

[Nomic]

If shore power is vis an isolation transformer there is no point fitting a GI.

There must only be a single ground connection to the hull.

All Earths are connected together, typically using a busbar. Then there should be a single Earth connection to the hull.

That's it. But, note comments above relating to DC Earth.

So:

1 looks correct if you omit the GI
2, 3 and 4 are all incorrect.

Ok, thanks a lot PaulRainbow!

I know GIs are usually used between shore ground and hull in setups that do not have an isolation transformer, but it seamed to me that rodgerthebodger installed one between his AC ground and the hull.
Maybe he was referring to AC shore ground? If not, maybe the GI is here to prevent residual DC currents from flowing from the AC system to the hull? Not sure why there would be any DC component on the AC side, though. Rodgerthebodger, any info on this?

 

[Alex_Blackwood]

Ok, thanks a lot PaulRainbow!

I know GIs are usually used between shore ground and hull in setups that do not have an isolation transformer, but it seamed to me that rodgerthebodger installed one between his AC ground and the hull.
Maybe he was referring to AC shore ground? If not, maybe the GI is here to prevent residual DC currents from flowing from the AC system to the hull? Not sure why there would be any DC component on the AC side, though. Rodgerthebodger, any info on this?

Won't make a lot of difference if it installed on the incoming a.c. supply where it comes on board or on the earth between the ground and the hull. The purpose of the GI is to isolate stray (earth) currents in the shore supply from leavingthe boat via its earth(water) so is normally fitted at the incoming end. The design of the GI should not impinge on the operation of the vessels earth protection system (RCD) . It could be argued, if you are so inclined, that fitting at the hull end will prevent stray currents produced by the vessels equipment.

 

[PaulRainbow]

The GI is there (in systems without a IT) to stop your onboard Earthing from being connected to everyone elses Earth circuits (including the marina Earth) when you are plugged into shore power. It connects between the incoming shore power connection and the boats Earth circuits. It serves no purpose in systems with an IT.

 

[samfieldhouse]

Should the 240v AC and 12v DC systems share a negative busbar and connection to a grounding plate?

To me this seems to connect high power and lower power systems unnecessarily?

What are the advantages and risks of a shared negative busbar?

On a GRP boat my understanding, and my set up, is no, there is no need to connect them.
If one is running a 240v shore power and 12v domestic systems that don't interact, there is no need to connect them.