Isolating Transformers, The Earth wire connection conundrum

[chala]

Originally Posted by transmitterdan

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.

I accept the view that any electrical shock will cause an injury.
Most of the shock received from electrical refrigerators in humid condition comes from the light switch. Condensation inside the refrigerator causes the switch plastic button to become live. If a hand touches the switch the electrical path across the body is normally from hand to hand or from hand to feet. In the past ELCB protection of electrical refrigerators was not required now it is mandatory. In a boat there is a good case for one isolation transformer or one inverter per refrigerator or simply the disconnection of the switch.

[Oceanride007]

I too have been zapped by a fridge, my nose actually, remember it well, unfortunately I condemned the refrigerator without investigating further. Chala good contribution pleased you coming around, slowly thats OK so am I. Au is Australia. We are not obliged to follow your ABYS regs here, but its interesting all the same.

[goboatingnow]

Originally Posted by DotDun

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.

Maybe so, I regularly see boats with 8-10KVA 230VAC generators and 3KVA shore power setups. makes no sense at all to rate ITs to other isolated power sources. Nor does it make sense electrically to switch shore and generator systems into the IT, since it complicates the ground wire issue. ( and requires it to be switched as well) .

dave

[colemj]

Originally Posted by DotDun

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

While true that some of them have 50A240V services available, almost all of the marinas we have been in stick boats like ours on 30A120V services. It only takes a small genset to exceed that. I have yet to see ubiquitous 50A240V only, but our marina experiences are extremely limited.

Mark

[DotDun]

Originally Posted by goboatingnow

Maybe so, I regularly see boats with 8-10KVA 230VAC generators and 3KVA shore power setups. makes no sense at all to rate ITs to other isolated power sources. Nor does it make sense electrically to switch shore and generator systems into the IT, since it complicates the ground wire issue. ( and requires it to be switched as well) .

dave

Yes, the 'complication' does require a 3-pole transfer switch.

[DotDun]

Originally Posted by colemj

While true that some of them have 50A240V services available, almost all of the marinas we have been in stick boats like ours on 30A120V services. It only takes a small genset to exceed that. I have yet to see ubiquitous 50A240V only, but our marina experiences are extremely limited.

Mark

Boaters in S. Florida would revolt if they had to schedule running their appliances due to small shore power. Full tilt, my boat will pull 6kva, my IT will handle this with some headroom.

Any marina wired in the last 5 years only offers 30A via adapters, it's not even on the pedestals anymore. I don't carry, nor had a need for 30A to 50A adapter. Most boats I'm familiar with are at least twin 30A.

[transmitterdan]

Originally Posted by DotDun

Any marina wired in the last 5 years only offers 30A via adapters, it's not even on the pedestals anymore.

A tiny fraction of all marinas were wired in the last 5 years. It will be a while before 30A outlets are not the majority around the country.

I don't see a compelling advantage to forcing gensets through the IT. The generator is an IT itself. So is an inverter.

Several disadvantages have been identified here and there are others not yet mentioned. On balance it seems a lot of work and problems for not enough advantage.

[Scott Berg]

The reason most 120/240 boats feed a genset into an IT is to avoid an unbalanced leg when feeding split 120 v subsystems, not a problem with pure 230v boats of course.

Secondly,

In my opinion, the biggest reason for an IT in US marinas, besides corrosion protection, is to prevent a dropped neutral fault in a 50a 120/240 dockside supply where it is commonly used to feed two 120 v with a common neutral. If that neutral drops I've seen fires caused as one side fell to 40v and the other jumped to 200 (isn't ohms law wonderful)

both cases are avoided with an IT with a properly wired secondary

[chala]

Originally Posted by Oceanride007

We are not obliged to follow your ABYS regs here, but its interesting all the same.

Yes but you have to comply with Australian Standard AS-3004 if still is: ELECTRICAL INSTALLATIONS- MARINAS AND PLEASURE CRAFT AT MAINS VOLTAGE and also to local regulations and States and Territory Laws.
Cheers.

[DotDun]

Originally Posted by transmitterdan

A tiny fraction of all marinas were wired in the last 5 years. It will be a while before 30A outlets are not the majority around the country.

I don't see a compelling advantage to forcing gensets through the IT. The generator is an IT itself. So is an inverter.

Several disadvantages have been identified here and there are others not yet mentioned. On balance it seems a lot of work and problems for not enough advantage.

I understand we're discussing mouse nuts in comparison with the theme of this thread, but the only disadvantage I've heard is sizing of the IT. Whether you put the transfer switch between shore power and IT or between IT and distribution box isn't really an issue. (yes, it requires a 3 pole xfer switch)

IMO, just because some marinas only offer small shore power service isn't a good reason to install an IT that can't handle the load of the whole boat.

Again, connecting the generator thru the IT (1) cleans up 95+% of the harmonic distortion that gensets are famous for plus (2) balances the genset when in a 240v/120v system.

Admittedly, generator produced harmonic distortion is not a problem until it's a problem. But wiring it as I suggest, it's guaranteed not to be a problem.

If this isn't of value to you, then by all means, install an IT that matches the shore power cord vs. one the can handle whole boat power requirements. Just because it isn't a value to you doesn't make it wrong.

[Oceanride007]

I sometimes wonder if this discussion is going the wrong way. ie reinventing the wheel. I know most US commercial ships have isolated Neutral which to me (other than comparing 3ph to 1 ph) is pretty much the same as IT supply, single phase requirement is done by Ph to Ph.
I do not know the standard that applies to commercial vessels they must exist of course, do they also cover 1ph vessels?. The one vessel I am familiar with that connects to shore regular are tugs. The IT is before the transfer switch. And if I recollect Harmonic distortion is not minimised by a Transformer they pass thru unless the Transformer is connected Star/Delta, Not sure about filters. Superyachts use a magic box, they may be worth a look at as well.
I see no reason why you need to bring the earth connection out from a generator or inverter, they don't need to be connected, they could be isolated, am I wrong here.

[s/v Jedi]

I just got severly shocked from an isolation transformer thread! Bzzzz, pok! I'm out.

p.s. The blonde SailorChic posted the codes that say no ground on secondary side of IT. It's done, over, the techs were right

[Oceanride007]

Well lets also not talk about if refrigerators are the most troublesome of AC devices, (I agree with Chala). But to you Chala or anybody does that mean they should or should not be protected by RCD, or you want a separate IT/inverter for it.

We are undecided about which side of the IT to connect the Transfer switch. But it looks to me consensus is for the load side. Its up to the owner isn't it how big to make the IT?.

I really don't think a IT will help out with Harmonic distortion issues.

Regarding earth/neutral point of the Generator or Inverter, it is not ship practice to connect them to anything they remain isolated (usually). Sailor Chic advised its in code somewhere Personnally I don’t know the difference between a NEC and a ABYC is that it, end of story. Come on keep it going if nothing else this thread has been thought provoking and educational.

[sailorchic34]

Lets see, the NEC, that is national electric code,also called NFPA 70 as that's the official code, is the USA national electric code. It applies coast to coast exactly the same way. Electrical engineers have it so easy.

BTW, NFPA is the National Fire Protection Association and has a 24 ish, 3" thick each, binder set of codes defining everything from fire codes to natural gas and propane codes, fueling docks, Building life safety and hospital design guides and electrical codes are but a small part of it.

In the past, I've read probably half of the NFPA codes cover to cover, which is very dry reading. Luckily, I still have most of my sanity left and only a slight tic to show for it.

Unlike the plumbing codes, where there are at least 6 standards used in different parts of the country, plus numerous local plumbing codes and requirements. The mechanical codes also vary some what coast to coast.

ABYC is a standard, but is only "code" for commercial vessels as included by referenced in 46 CFR. Recreational vessels in the US, should comply with 33 CFR, which ok, is rather incomplete, as even GFI's are not listed as a requirement. But it's thousands of pages long, so It will take a while to catch up with reality.

So in the USA, the NEC is the bible for electrical design. Though officially that stops at the dock box.

Interesting that ABYC does show a IT ground that also connects to engine negative terminal. But it does NOT show shore power ground connected to IT ground or engine ground. So in effect for a IT, the ABYC boat system is still isolated from shore.

http://www.marinewiring.com/wp-conte...-Diagram-6.jpg

So in effect, both NEC and ABYC agree that the IT system is completely separate from shore ground as the shore ground/earth does not connect to the boat, and boat side is decoupled via the IT. So no ground loops exist to shore or from shore or adjacent boats (more likely).

[pbiJim]

Originally Posted by sailorchic34

NEC, that is national electric code,also called NFPA 70 as that's the official code, is the USA national electric code. It applies coast to coast exactly the same way. Electrical engineers have it so easy.

I think that there may also be some local codes that need to be adhered to in some places, in addition to NEC. NYC comes to mind.