Marine AC - Why Tie Neutral and Earth?

[s/v Jedi]

Originally Posted by Thumbs Up

I am not arguing against the merits of a floating AC supply, It is just that you have your outlet grounds tied to the wrong place.

Only in case of the 50A shore power, right? So explain me why it isn’t dangerous to have shore ground potential on your appliances metal surfaces on other boats but it is on mine? I mean explain how one gets electrocuted, shocked, or a fire hazard is created?

[boatpoker]

Originally Posted by Thumbs Up

He is also grounded to the neighbor boat's DC. Although the galvanic isolator is there, is the isolation transformer passing it's earth connection? (depends on how it is configured)

A few thousand surveys and I've yet to see a boat with a GI and an isolation transformer.

[Thumbs Up]

Originally Posted by s/v Jedi

Let’s take a different approach: you show me a “dangerous” situation with ungrounded neutral and I’ll explain why it isn’t dangerous. Nobody ever came up with something except for the two simultaneous appliance isolation failures that are touched simultaneously... which highly unlikely situation is protected by gfci outlets.

You have your grandpa's drill plugged into one socket of a GFCI outlet. You have your grandpa's saw plugged into the other outlet of the same GFCI. The drill has a white wire fault to the case, the saw has it's black wire touching the case. You touch the drill with one hand and the saw with the other.....

[Thumbs Up]

Originally Posted by s/v Jedi

So there is one exception and that is when the 50A shore power is active. In that case a breaker trips. This is one of the reasons why a true floating power distribution is better: you need at least two isolation faults before a breaker trips. If you install a detector, you can most likely prevent that by installing an isolation fault monitor which warns when the first fault develops so it can be fixed before another fault develops. This is why they use floating power distribution in demanding high availability situations like operating rooms etc.

What breaker is going to trip? I like a floating power distribution (IT) system. What I am saying is that you have the outlet grounds connected wrong.

[Thumbs Up]

Originally Posted by s/v Jedi

Only in case of the 50A shore power, right? So explain me why it isn’t dangerous to have shore ground potential on your appliances metal surfaces on other boats but it is on mine? I mean explain how one gets electrocuted, shocked, or a fire hazard is created?

Explain what the advantage is. If there is a fault from isolated power to shore ground, nothing trips. And it is dangerous, (on other boats), that is part of why we want an isolated system in the first place. Your system is negating some of the advantages.

[Thumbs Up]

You have three isolation transformers but still have not eliminated shore ground.

[boatpoker]

Originally Posted by s/v Jedi

When you create a short between a hot and boat ground, nothing happens.

This appears to discount American built boats with AC ground and DC Negative bonded. Are you not in the USA ?

[Thumbs Up]

I am pasting this from the 2013 thread:

What I want to address here is the floating /non floating system as a result of installation of an isolating transformer

In ABYC solutions, where a local seawater earth is effectively established, the major safety factor of an isolating transformer, ie that earth is not in the fault path is removed. BUT, there is a robust debate, that isolating power transformers should not have floating secondaries. for example the common 110V tool transformer on UK building sites, is a centre tapped 110V secondary , with the centre tap connected to primary earth wire. Hence earth is in the fault return path but the max voltage that can be experienced in such a fault is 55V.

So we have a debate unpon installing an isolating transformer

1. A true floating AC output , no neutral, both are HOT, but earth is not in the return path, no danger to boat occupants if they touch one or the other HOT wires.

2. The inadvertent establishment of seawater(earth) referenced secondary output, in effect producing boat L & N . This could occur due to dampness, teh protective earth coming in contact with underwater or engine/prop shaft

3. The deliberate attempt to establish not only a local boat 'Ground", but to attempt to earth that ground, via underwater metals or even a hull plate.


Its clear that (3) is the preferred ( ABYC) US approach, yet such an approach adds earth as a fault path. ( note not a fault path TO earth). with this system if the secondary HOT ( L) touches anything on the boat and the user touches that, then a fault path to the neutral side of the transformer exists via seawater which causes a shock. ( this assumes the 'thing' is not connected to the protective wire).

What I dont understand is , by deliberately defeating the isolation it reintroduces the swimmer in freshwater risk, as fault currents can now flow through seawater , ie out of one underwater fiting and back through another, especially if the protective wire is poor made or connected.

ABYC, does not like (1) above and to be fair , several other code bodies do not like floating 1:1 POWER transformers. Why?

(a) a concern for excessive common mode voltages, but in practice the secondary does not float excessively above earth potential due to leakage resistance and capacitance .

(b) A rather strange concern ( as Calder as described) of the floating HOT touching an appliance, and raising the potential of that appliance unbeknownst to teh occupant, then a subsequent fault on the other HOT causes the occupant to be inserted into the circuit.

Now this is rather a bizarre claim, and seems to completely ignore two things
(a) A RCD on the secondary side covers this
(b) double pole breakers and the presence of the protective fault wire, will mean a trip will occur in most cases.

I have seen some fault sceanarios debated where tingling can be experienced on flating secondaries , because of stray capacitive connections causing low fault currents to return to the secondary 'neutral'

So an RCD and double pole CB protected secondary does deal with the secondary HOT to appliance ( or hull) issue, in that if the rather bizarre double fault occurs, then the RCD will sense the fault current returning via the protective wire. Thats assuming the CB hasn't tripped anyway.

SO while I can understand the theoretical ABYC concern, it does seem to effectively predate RCDs and proper protective (earth) wiring.

ABYC in effects , defeats a claimed safety feature of isolating transformers, to simply deal with the galvanic corrosion issue.

S/V Jedi on the other hand , went to a lot of bother to ensure that the safety aspect, ie no earth return path was maintained in his AC system.

But that was your old system, correct?

[Thumbs Up]

Originally Posted by s/v Jedi
When you create a short between a hot and boat ground, nothing happens.

from Boatpoker
This appears to discount American built boats with AC ground and DC Negative bonded. Are you not in the USA ?

Nothing happens because there is no potential between isolated AC hot and anything (except the other hot wire (or wires if you are center tapped) coming from the transformer.

[s/v Jedi]

Originally Posted by Thumbs Up

You have your grandpa's drill plugged into one socket of a GFCI outlet. You have your grandpa's saw plugged into the other outlet of the same GFCI. The drill has a white wire fault to the case, the saw has it's black wire touching the case. You touch the drill with one hand and the saw with the other.....

That’s too easy! The breaker for the outlet trips. Current path: L1 of the saw to it’s metal housing to it’s ground wire to the outlet to the drill ground wire to it’s metal housing to it’s L2 short

[s/v Jedi]

Originally Posted by Thumbs Up

Explain what the advantage is. If there is a fault from isolated power to shore ground, nothing trips. And it is dangerous, (on other boats), that is part of why we want an isolated system in the first place. Your system is negating some of the advantages.

No... the breaker trips because L1 and L2 and ground come from shore power, so as soon as L1 shorts to ground or L2 shorts to ground, the breaker trips both as it is a mechanically linked double pole breaker.

Isolated power is from the 30A shore power, the genset and the inverters. When the 50A shore power is connected it only galvanically isolates ground and it doesn’t utilize shore neutral.

[s/v Jedi]

Originally Posted by Thumbs Up

You have three isolation transformers but still have not eliminated shore ground.

I have no idea what you talk about... there is only one isolation transformer. The 50A has an EMS (energy management system) with 50A contactor that disconnects shore power when something is wrong with it. That thing also has surge suppression.

Our boat only uses the 50A shore power for a couple of days a year because it’s fully solar powered.

[s/v Jedi]

Originally Posted by Thumbs Up

What breaker is going to trip? I like a floating power distribution (IT) system. What I am saying is that you have the outlet grounds connected wrong.

No I don’t have that wrong. How are the outlet grounds connected in your car or motorhome? Open the outlet, I bet it’s wired. Read up on chassis ground.

[s/v Jedi]

Originally Posted by boatpoker

This appears to discount American built boats with AC ground and DC Negative bonded. Are you not in the USA ?

This whole thread is about floating power distribution... which don’t have that, not even in the USA

[Thumbs Up]

Originally Posted by s/v Jedi

That’s too easy! The breaker for the outlet trips. Current path: L1 of the saw to it’s metal housing to it’s ground wire to the outlet to the drill ground wire to it’s metal housing to it’s L2 short

It would still trip in this case without the ground wire connected to anything. Introducing earth ground to this connection does nothing except create the possibility of an additional safety hazard in the water. If the the outlet grounds were instead connected to the auto transformer neutrals a fault to an appliance case would trip the RCD breaker. The way you have it a fault to an appliance case does nothing except connect one hot wire to the water.