Isolating Transformers, The Earth wire connection conundrum

[Saltyhog]

Originally Posted by Lake-Effect

So whether or not there's an isolation transformer on the boat, it's still possible for the shorepower hot to get shorted to boat ground somehow (eg chafe at a metal pass-thru, or a screw through the AC lead-in, etc)

Generally much less likely as the cabling is short and permanently installed, *and* is protected by the whole boat RCD. Not saying it could not happen, but probability of it spontaneously occurring is low. This would be *way* down on my risk analysis probability table.

[transmitterdan]

Originally Posted by DotDun

If in #2 you wire the transfer switch on the primary side of the IT, #7 becomes a mute point.

Thank you for your comment. There are lots of installations where the power capacity of the shore side is less than the on-board generating capacity. This is especially true of boats with inverters. The IT need only be sized for the shore power capacity. Any load sharing can be done after the IT which is one reason to not recommend putting the transfer relay prior to the IT.

Another reason has to do with ensuring that the wiring from shore plug to the IT is short and sweet. This goes a long way toward eliminating the risk of ESD deaths and avoiding corrosion problems.

[transmitterdan]

Originally Posted by goboatingnow

Would agree with most , however the outlet GFCIs are a bit over kill.

Note that running a ground wire to L2 without establishing a local seawater ground is tely of very little use and floats the case of every appliance up to. The IT float voltage

AC to DC connection is virtually useless, since earth ground is not in the circuit

Really it boils down to ABYC where all on board power sources including ITs and invertor generators are earth referenced OR fully floating outputs where ground wiring is essentially superfluous

Dave

Dave,

I don't think many standards bodies will have the stomach to eliminate RCD protection of outlets on deck, in the galley, head, engine room, etc.

Connecting the third wire to L2 is just to make the RCD test function work. If someone wants to connect that to the engine frame which might establish a sea water reference no harm can come of that so far as I can see. An RCD with no ability to test it is worse than useless.

One thing that the "floating secondary" crowd has been ignoring is static charge. A truly floating secondary with absolutely no reference to anything else on a plastic boat will take on a static charge. Unless there is something to drain it off then the voltage could exceed the typical breakdown voltage of most appliances. For this reason I think it a good idea to reference the L2 to the rest of the boat even though it "is virtually useless". I cannot see how it does any harm and it costs almost nothing.

[transmitterdan]

Originally Posted by Saltyhog

Generally much less likely as the cabling is short and permanently installed, *and* is protected by the whole boat RCD. Not saying it could not happen, but probability of it spontaneously occurring is low. This would be *way* down on my risk analysis probability table.

Exactly right Salty. We want to keep the short power connection short and to the point to make it extremely unlikely that ESD will ever be a problem. Nothing is 100% foolproof but proper application of an IT can reduce the risk a lot.

[goboatingnow]

Originally Posted by transmitterdan

Dave,

I don't think many standards bodies will have the stomach to eliminate RCD protection of outlets on deck, in the galley, head, engine room, etc.

Connecting the third wire to L2 is just to make the RCD test function work. If someone wants to connect that to the engine frame which might establish a sea water reference no harm can come of that so far as I can see. An RCD with no ability to test it is worse than useless.

One thing that the "floating secondary" crowd has been ignoring is static charge. A truly floating secondary with absolutely no reference to anything else on a plastic boat will take on a static charge. Unless there is something to drain it off then the voltage could exceed the typical breakdown voltage of most appliances. For this reason I think it a good idea to reference the L2 to the rest of the boat even though it "is virtually useless". I cannot see how it does any harm and it costs almost nothing.

currently EU regulations , via ISO for small craft do not specify Outlet RCDs. ( for example) they do mandate whole boat 30 mA
RCDs. There is no requirement for such devices in galley, head or engine spaces

Static charge isn't really an issue , in a boat with all that wiring , there are capacitive and high resistive paths to earth to bleed static charge.

The problem with referencing L2 , is referencing it to what. , what you mean is establishing a local boat seawater earth and earth referencing L2. ( a better basis and one I never understand would be to centre tsp the secondary and reference the centre tap to seawater

Once you establish a seawater earth reference , you introduce earth fault paths back into the equation

Dave

[DotDun]

Originally Posted by transmitterdan

Thank you for your comment. There are lots of installations where the power capacity of the shore side is less than the on-board generating capacity. This is especially true of boats with inverters. The IT need only be sized for the shore power capacity. Any load sharing can be done after the IT which is one reason to not recommend putting the transfer relay prior to the IT.

Another reason has to do with ensuring that the wiring from shore plug to the IT is short and sweet. This goes a long way toward eliminating the risk of ESD deaths and avoiding corrosion problems.

Agree, capacity is a design consideration. My point is with the gen/inverter connected to the primary of the IT, all concerns about whether those sources are or are not grounded goes away. I don't see the cabling issue as a big concern, yes it has to be proper, but no less safe with a transfer switch in the path.

[goboatingnow]

Originally Posted by DotDun

Agree, capacity is a design consideration. My point is with the gen/inverter connected to the primary of the IT, all concerns about whether those sources are or are not grounded goes away. I don't see the cabling issue as a big concern, yes it has to be proper, but no less safe with a transfer switch in the path.

In my view IT systems should never be in series with inboard power generation, only in switched parallel. It makes no sense to have to rate ITs to carry onboard power gen , when such generators are not part of the shore power infrastructure anyway.
Dave

[transmitterdan]

Originally Posted by goboatingnow

In my view IT systems should never be in series with inboard power generation, only in switched parallel. It makes no sense to have to rate ITs to carry onboard power gen , when such generators are not part of the shore power infrastructure anyway. Dave

Dave,

I agree. The IT has enough jobs without adding to its requirements.

[DotDun]

Originally Posted by transmitterdan

Dave,

I agree. The IT has enough jobs without adding to its requirements.

Medical ITs do it all the time.

[transmitterdan]

Originally Posted by DotDun

Medical ITs do it all the time.

Only because they don't have the luxury of referencing a gen set or inverter to something other than earth so they can't put them on the secondary side. If they referenced the inverter to the secondary of the IT then that would cause them lots of problems. But we don't have those problems.

Also, they have to size the IT for the maximum load. A boat does not have to do that. The IT can be smaller than on-board generating capacity. We should not force the IT to be bigger than minimum size required.

[chala]

Originally Posted by Saltyhog

normally non-conducting metal is connected to the safety green wire.

Can you just confirm the above “non-conducting metal”

Originally Posted by Saltyhog

I'd be sure to only permit double insulated appliances aboard.

I agree since long before 21.06.2009 and not only appliances but the complete installation shall be of the double insulation principle.

Originally Posted by chala

It could also be said that if, due to double insulation, earth wires are no longer required then also there will be no requirement for installing a Galvanic Isolator.

[goboatingnow]

Originally Posted by chala

Can you just confirm the above “non-conducting metal”

I agree since long before 21.06.2009 and not only appliances but the complete installation shall be of the double insulation principle.

Good luck with finding that double insulated fridge ......

Dave

[Saltyhog]

Originally Posted by goboatingnow

Good luck with finding that double insulated fridge ......

Dave

Exactly! So that's the real point of my question. I'm not arguing, I'm just not sure of your point. In the OP drawing, you showed an IT supplying on-board outlets where the the green safety wire is connected to L2. I'm not sure if you're proposing that, or if you're proposing removing the green safety wire from the outlet wiring. You say full floating. In my local terminology, that suggests no green safety wire, just a full differential mains voltage not referenced to any local ground. If the green safety wire is wired to outlets, I doubt the installation will stay "floating" for very long as some safety grounded chassis or enclosure will find a connection to local DC ground. Now it's no longer floating but referenced to the local gnd. Again, I'm not arguing, just trying to see where you're coming from.

[goboatingnow]

Originally Posted by Saltyhog

Exactly! So that's the real point of my question. I'm not arguing, I'm just not sure of your point. In the OP drawing, you showed an IT supplying on-board outlets where the the green safety wire is connected to L2. I'm not sure if you're proposing that, or if you're proposing removing the green safety wire from the outlet wiring. You say full floating. In my local terminology, that suggests no green safety wire, just a full differential mains voltage not referenced to any local ground. If the green safety wire is wired to outlets, I doubt the installation will stay "floating" for very long as some safety grounded chassis or enclosure will find a connection to local DC ground. Now it's no longer floating but referenced to the local gnd. Again, I'm not arguing, just trying to see where you're coming from.

I'm not arguing either case, merely a discussion about the merits and demerits each type. Yes in a fully float there is no earth wire. Equally I know of no device that ever connects either live or neutral to do ground , hence your supposition has little merit.

Note that even if the appliance faults to ground , once the IT has no ground connection , nothing happens. People really need to stop thinking about earth connections per say

Dave

[Saltyhog]

Originally Posted by goboatingnow

I'm not arguing either case, merely a discussion about the merits and demerits each type. Yes in a fully float there is no earth wire. Equally I know of no device that ever connects either live or neutral to do ground , hence your supposition has little merit.

Note that even if the appliance faults to ground , once the IT has no ground connection , nothing happens. People really need to stop thinking about earth connections per say

Dave

Your right of course. I never meant that the grounded appliance chassis would be intentionally connected to DC ground, only that there's good opportunity for that to occur. I'm not sure, but I believe my air-conditioning unit has its chassis connected to the green safety wire. When the pump is running, wouldn't that chassis be connected to seawater. I've never looked into it, so maybe there is some isolation between the seawater plumbing and the reset of the chassis.