Isolating Transformers, The Earth wire connection conundrum

[transmitterdan]

Originally Posted by goboatingnow

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

In the US they will never remove the RCD (GFCI) requirement IMHO. We need to design the system to work with them. I don't understand the aversion to RCDs or GFCIs. They cause no harm in my view and offer a certain level of comfort to some.

Originally Posted by goboatingnow

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.

For many the question will be where does that stray current go? Someone will get a harmless tingle from it and go bonkers. Also, regulators want to know exactly the maximum voltage that will appear on an insulated wire to ensure that the insulation is properly rated. The only way to do that is give all voltages a reference plane. Whether or not that reference plane includes the water is inconsequential.

Originally Posted by goboatingnow

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

I am not suggesting that L2 has to be referenced to sea water and definitely not attempting to reference it to earth (more on this below). It should be referenced to the most likely thing that people on board will be referenced to. The machinery and appliance cases are the most likely things.

One reason not to reference the third wire to the center tap is that it lowers the test current of the RCD test function by 2X thus foiling the proper functioning of the RCD test button.

Originally Posted by goboatingnow

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

The IT prevents this quite nicely. If the L1 should somehow become connected to the water (but by some magic not connected to the L2 reference plane) there would be no current leaving the boat and trying to get to shore. What current there is would try to get back to the reference plane on the boat from whence it came. Thus the electric field would be very localized at the point where the L1 is connected to the water. It would not spread out away from the boat because it has no affinity for the earth. I believe there would be no risk of ESD from this condition. This is easily proven by experimentation but I think we already have the answer. Generator powered boats do not have an earth fault problem and all generator and inverter L2 wires in US boats wired per ABYC are referenced to machinery and possibly the water. No one is concerned about internally powered boats causing shocks to swimmers.

To retain this feature earth ground should never connect to anything on the boat except the optional ELCI prior to the IT and the electrostatic shield of the IT. Earth ground must not connect to anything else on the boat period end of story.

Bottom line, an IT equipped boat will be no greater hazard to swimmers than a generator powered boat today. And it is a lot less hazard than a shore power fed boat without an IT.

[DotDun]

Originally Posted by transmitterdan

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.

The IT needs sized to match boat power requirements, not necessarily shore power nor on-board generating capacity. Protection on the primary to match IT capacity is required. On board generating capacity exceeding shore power capacity is not the norm in the US. US marinas offer 50A240V (12kva) shore power as the norm.

As I've stated before, I'm a fan of the genset connected to the primary side of the IT to clean up harmonic distortion. There is nothing wrong with doing this.

[Lake-Effect]

More points; not sure if they've been raised yet:

1) In North American-style AC single-phase supply, the 'neutral' conductor is always bonded to ground at the source. A generator, or an isolating transformer are both considered new 'sources', so the ABYC expects the neutral to be bonded to ground at the output of those devices. Remember the 'A' in ABYC stands for American so of course they are going to follow the US electrical practice.

2) I believe European standards have made fault-detecting breakers ( >30 mA leakage) mandatory on all marine shore-power hookups for many years now, so that's an added consideration for why Euro boats do X while American boats do Y.

3) I don't think galvanic corrosion is a good enough reason for not carrying shore ground through to the onboard AC sockets, iso transformer or not. There are already measures (eg galvanic isolators) to protect against creating galvanic corrosion paths

There are many good technical arguments in this thread in favour of a fully-floating transformer output. But the overriding argument against making this widespread is that of practice - it deviates from electrical practice in many parts of the world - hence you can hardly expect a standards body to bless or endorse it.

[DotDun]

Originally Posted by Lake-Effect

3) I don't think galvanic corrosion is a good enough reason for not carrying shore ground through to the onboard AC sockets, iso transformer or not. There are already measures (eg galvanic isolators) to protect against creating galvanic corrosion paths

What purpose would shore ground to onboard AC sockets serve when using an IT?

[Lake-Effect]

Originally Posted by DotDun

What purpose would shore ground to onboard AC sockets serve when using an IT?

Mainly because That's How It's Done (... standards). When you're on shore-power you are part of the shore AC system.

Look, I do get your technical arguments for the relative merits of what you propose, and in your collective hands, I'm sure you'd make a safe and effective system on your own boats. But unless you propose a separate electrical standard, with your own connectors, and certification process for equipment, you're stuck with the shore conventions and practices.

[sailorchic34]

Originally Posted by Lake-Effect

There are many good technical arguments in this thread in favour of a fully-floating transformer output. But the overriding argument against making this widespread is that of practice - it deviates from electrical practice in many parts of the world - hence you can hardly expect a standards body to bless or endorse it.

Actually the NEC happens to address this under VII. Isolated power. The section relates to hospital surgery suites, but its one of the few that address isolation transformers in the NEC. Oddly enough the systems have UL listings too. Bold added by me.

517.160 Isolated Power Systems.
(A) Installations.
(1) Isolated Power Circuits.
Each isolated power circuit shall be controlled by a switch or circuit breaker that has a disconnecting pole in each isolated circuit conductor to simultaneously disconnect all power. Such isolation shall be accomplished by means of one or more isolation transformers, by means of generator sets, or by means of electrically isolated batteries. Conductors of isolated power circuits shall not be installed in cables, raceways, or other enclosures containing conductors of another system.

(2) Circuit Characteristics.
Circuits supplying primaries of isolating transformers shall operate at not more than 600 volts between conductors and shall be provided with proper overcurrent protection. The secondary voltage of such transformers shall not exceed 600 volts between conductors of each circuit. All circuits supplied from such secondaries shall be ungrounded and shall have an approved overcurrent device of proper ratings in each conductor. Circuits supplied directly from batteries or from motor generator sets shall be ungrounded and shall be protected against overcurrent in the same manner as transformer-fed secondary circuits. If an electrostatic shield is present, it shall be connected to the reference grounding point.

So from the above you can see that both the NEC (NFPA 70) and NFPA 99 and UL, require isolation transformer secondary sides to be ungrounded.

So yes, MAJOR standards bodies do endorse it, least wise in the US.

Wish I had checked the code first.

Edit: BTW 517.160 applies to any isolated transformer installation, not just surgery suites.

Are we done yet!!

[DotDun]

Originally Posted by Lake-Effect

Mainly because That's How It's Done (... standards). When you're on shore-power you are part of the shore AC system.

Please quote the standard that mandates shore ground extend past an IT.

Originally Posted by Lake-Effect

Look, I do get your technical arguments for the relative merits of what you propose, and in your collective hands, I'm sure you'd make a safe and effective system on your own boats. But unless you propose a separate electrical standard, with your own connectors, and certification process for equipment, you're stuck with the shore conventions and practices.

I think you may be confusing a 3rd wire safety "ground" on the secondary of an IT with shore ground. Extending shore ground past the IT would serve no purpose and I don't believe any standard advocates doing so.

[Lake-Effect]

[edited]

sc - does the NEC standard also mandate the presence of a 'local' AC ground, 3-prong outlets, and that the neutral of the IT or genny must be bonded to the local ground?

I would guess there's also a set of standards around the construction and certification of the equipment used in surgical suites... That possibly excludes microwave ovens from Wal-Mart?

[edit. wrong again, L-E]

In a perfect world, there would be a separate marine standard of course - everything double-insulated and salt-water corrosion-resistant. And no colour other than white.

[Lake-Effect]

Originally Posted by DotDun

Please quote the standard that mandates shore ground extend past an IT.

I think you may be confusing a 3rd wire safety "ground" on the secondary of an IT with shore ground. Extending shore ground past the IT would serve no purpose and I don't believe any standard advocates doing so.

Ack - I just checked ABYC - I thought it did mandate the ground connection continuing on past the IT. Nigel Calder has set me straight. Sorry.

[sailorchic34]

Originally Posted by Lake-Effect

[edited]

sc - does the standard also mandate the presence of a 'local' AC ground, 3-prong outlets, and that the neutral of the IT or genny must be bonded to the local ground?

I would guess there's also a set of standards around the construction and certification of the equipment used in surgical suites... That possibly excludes microwave ovens from Wal-Mart?

In other words I think there must be a compelling, recognised reason for having more than one ground in an electrical system. Perhaps you folks have a solid enough argument put together that the NEC or others would carve out such an exception for vessels on shore-power.

In a perfect world, there would be a separate marine standard of course - everything double-insulated and salt-water corrosion-resistant. And no colour other than white.

Actually the requirement for isolated power in the NEC, is its not connected to earth ground. I'll need to check more about a separate floating ground. But don't quite see the purpose of that. BTW that's for wet location installations, which surgery suites are considered. That does not cover boats as the NEC stops at the power pedestals as far as the code is concerned. ABYC does not address isolated power. 46 CFR had one paragraph in thousands of pages and 33 CFR does not even require GFI for recreational boats.

But as far as the NEC and UL listed isolated transformers serving wet locations, the secondaries are ungrounded. Good enough for me.

I expect that with the fast pace that ABYC moves, they will have a isolated power chapter sometime in the next 30-50 years. I still believe that Jedi's system meets the intent of the code as it relates to isolated power.

EDIT: As far as a separate ground wire goes, the general consensus is if one is used, it terminates at the first panel after the IT and does not connect to building ground or earth. Its not quite black and white. But the gist is if you add a ground wire to a IT secondary circuit it does not connect back through the primary side or to earth.

Oh here is a nice basic summation of the code:http://static.schneider-electric.us/...0104DB0701.pdf

This mainly relates to healthcare, but I think the wet locations could also apply to boats.

[chala]

Originally Posted by goboatingnow

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

Dave

Mine is triple insulated, it is an ice box. but let be serious 240 v AC electric refrigerator can be one of the most dangerous appliance on board. And how to tame it may take many posts.

[transmitterdan]

Originally Posted by chala

Mine is triple insulated, it is an ice box. but let be serious 240 v AC electric refrigerator can be one of the most dangerous appliance on board. And how to tame it may take many posts.

Can you site a case where a boater was injured by shock from their refrigerator?

[pbiJim]

Originally Posted by Lake-Effect

2) I believe European standards have made fault-detecting breakers ( >30 mA leakage) mandatory on all marine shore-power hookups for many years now, so that's an added consideration for why Euro boats do X while American boats do Y.

I think that NFPA 303 now requires similar current leakage sensing devices on US docks as well.

[chala]

Originally Posted by transmitterdan

Can you site a case where a boater was injured by shock from their refrigerator?

I know case where boater did get shocked.

[transmitterdan]

Originally Posted by chala

I know case where boater did get shocked.

I meant a case where the cause was determined so we could consider how risky refrigerators might be. I don't think refrigerators are the most risky device on a boat.