Marine AC - Why Tie Neutral and Earth?

[Wotname]

Can someone explain the logic of using a tied neutral/earth AC system on a sailing boat?

Is it simply a NEC code requirement or is there some inherent safety benefit in not having a fully floating Active and Neutral AC output?

I understand why shore power uses a Neutral / Earth connection but why do this with an onboard AC supply?

Regarding using shore power on board, surely a safer system is to isolate the shore side from the boat side with an isolating transformer thus creating a fully floating A&N output onboard.

[Dsanduril]

I'll weigh in on this at the risk of being flamed. The use of an earth ground and a neutral tied to that are really quite antiquated, and codes and regulations have not caught up.

The codes are based on fault devices (circuit breakers and fuses) that only trip when the full current they can support is exceeded. As a result, the protective earth ground was born, trying to create two independent return paths so that if one return path was breached or failed the other could carry sufficient current, easily, to trip the overcurrent protection.

With the advent of RCDs (residual current devices) this thinking is a bit outdated for a properly designed system. You no longer need to provide a full return path for protection, if you instead provide a device that can check the balance between out-and-back (down to a few mA if that's your desire) there isn't really a need for a PE connection of any kind. If for some reason, through some connection of which you are unaware or was created accidentally (such as your body) electricity starts to flow where it shouldn't (and in a completely floating system even that is difficult) the RCD can open the circuit with very little errant flow. This is a big change from the rather crude overcurrent devices that used to be all that was available. And if the RCD is also an overcurrent protection device it can still perform its function by tripping when the current exceeds the circuit's design capabilities.

The newer NEC codes even recognize this, in wiring a premises that has receptacles without a PE ground it is permissible to replace an ungrounded receptacle with a GFCI (what we Yanks call a specific RCD) and to then install three-prong "grounded" receptacles downstream of that GFCI even though there is no actual earth ground. This allows the user to plug in items that have three-prong grounded plugs without actually having to install a ground.

2017NEC 406.4(D)(2) Non–Grounding-Type Receptacles. Where attachment to an equipment grounding conductor does not exist in the receptacle enclosure, the installation shall comply with (D)(2)(a), (D)(2)(b), or (D)(2)(c).

(a) A non–grounding-type receptacle(s) shall be permitted to be replaced with another non–grounding-type receptacle(s).

(b) A non–grounding-type receptacle(s) shall be permitted to be replaced with a ground-fault circuit interrupter-type of receptacle(s). These receptacles or their cover plates shall be marked “No Equipment Ground.” An equipment grounding conductor shall not be connected from the ground-fault circuit-interrupter-type receptacle to any outlet supplied from the ground-fault circuit-interrupter receptacle.

(c) A non–grounding-type receptacle(s) shall be permitted to be replaced with a grounding-type receptacle(s) where supplied through a ground-fault circuit interrupter. Where grounding-type receptacles are supplied through the ground-fault circuit interrupter, grounding-type receptacles or their cover plates shall be marked “GFCI Protected” and “No Equipment Ground,” visible after installation. An equipment grounding conductor shall not be connected between the grounding-type receptacles.

[smac999]

Every single house is too. . It’s standard electrical.

The earth is useless unless it’s tied to something to compleate the fault circuit.

Without the tie you have a 2 wire system h + n.

[Wotname]

Originally Posted by smac999

Every single house is too. . It’s standard electrical.

The earth is useless unless it’s tied to something to compleate the fault circuit.

Without the tie you have a 2 wire system h + n.

It sort of makes sense for a house. For several reasons (AFAIK), the power generating station has historically tied the neutral to the physical earth. The house is attached to the physical earth and so on.

It doesn't make sense on a boat, the inverter will work with a floating A&N. There is no need for a PE as far as I can see so why is it done. Joining to N to the E at the source (inverter) doesn't make it safer, as far as I can see, it makes it unsafer - but I could be wrong but I can't see where I am wrong.

[Jammer]

Originally Posted by Wotname

Can someone explain the logic of using a tied neutral/earth AC system on a sailing boat?

There are several reasons.


Classically, the reason usually given is that bonding the neutral leads to rapid, predictable tripping of overcurrent devices in the event of a short between hot and ground. Without a bond, an inadvertent short between hot and ground causes the whole boat to be hot which poses safety risks should a person touch a grounded surface and a neutral surface. These problems can be difficult for lay people to recognize and resolve as you get a situation where you get a bad shock when touching two metal objects, usually only some of the time under specific circumstances.


The second reason is that it constrains the maximum voltage between the electrical system (both hot and neutral) and ground. A floating ground system will typically sit at half the line voltage -- about 60 volts if the supply is 120 vac. But there's no guarantee it will stay there, because faults or inductive coupling from high voltage sources can drive the voltage difference much higher. So for example if a microwave over has a little leakage between its high voltage power supply and ground you could end up with 1000 volts between ground and neutral, and that will tend to cause arcing and damage somewhere unrelated. The fact that the ground is isolated allows this since all it takes is a few mA to get the voltage that high.


Finally it helps reduce electrical noise, at least usually.

[sailcrazy]

Following. I'm assuming the AC and DC system grounds are kept separate and not connected, and the bonding system is kept isolated from both of them?

[ttex]

Following also.

[Stu Jackson]

Books.


Nigel Calder's Boatowners Manual comes to mind.


Or else someone will have to go get their copy and type a whole lotta words.

[Lake-Effect]

Originally Posted by Wotname

Can someone explain the logic of using a tied neutral/earth AC system on a sailing boat?

Is it simply a NEC code requirement or is there some inherent safety benefit in not having a fully floating Active and Neutral AC output?

I understand why shore power uses a Neutral / Earth connection but why do this with an onboard AC supply?

Jammer gives some great technical reasons.

I'm going to stress the most practical reason: because that's how AC wiring is currently done. Same voltage, same breakers, same plugs and sockets, same loads and devices as ashore... then it should behave like ashore. Many of us have sufficient familiarity with adding an outlet, installing a ceiling fixture, etc, so doing the same on a boat poses no real challenge. If you then make it fully floating, you've changed expectations - especially the most basic one that if the hot ever touches a metal chassis or ground, a breaker or fuse will blow.

If you're an advanced user, and decide to take that risk on knowingly, go for it. But it's a terrible thing to be telling the less-technical user that floating AC is better, and they don't understand the safety tradeoff that fully floating might cause.

[Dsanduril]

Here are a couple of interesting articles on the subject:

https://www.benderinc.com/know-how/t...loating-system

https://www.lsp-international.com/po...grid%20system.

https://www.eaton.com/content/dam/ea...ap083005en.pdf

Note that there are places in the current code world where floating systems are preferred (moist/wet environments being one of them), and there is reduced risk of things like fire in a properly configured floating system because the fault currents are not nearly so high.

I don't think anyone here is advocating that it is safe to just go out and float your boat's AC system, but it is a topic worthy of discussion and maybe a chance to get standards to catch up with the modern world.

[ttex]

Just want to make it clear I did not intend to get all experimental with my boat I just find these discussions fascinating.

[John De Bree]

Following with interest

[boatpoker]

It is contrary to ABYC, NFPA, ABS standards and Transport Canada Regulations to tie the AC ground to AC neutral on a vessel.

Since the DC negative and AC ground are bonded (or should be), tieing the AC ground to AC neutral will introduce AC into your DC system. This is poytentially fatal. a fire hazard and the cause of ESD (electric shock drowning).

There are exceptions for inverters and generators but these should be automatically switched and that conversation gets a little beyond forum chat.

[Jammer]

Originally Posted by boatpoker

It is contrary to ABYC, NFPA, ABS standards and Transport Canada Regulations to tie the AC ground to AC neutral on a vessel. [...] There are exceptions for inverters and generators

::shrug:: I think the point of the OP is that the neutral is bonded somewhere. For shore power, it's bonded on the shore side. For inverters and generators, the bond is as you point out aboard.


Isolation transformers also involve a neutral-to-ground bond aboard the boat.

Since the DC negative and AC ground are bonded (or should be), tieing the AC ground to AC neutral will introduce AC into your DC system. This is poytentially fatal. a fire hazard and the cause of ESD (electric shock drowning).

Well, that is at best a oversimplification, and I suspect you know that. But it's off topic. The OP's point is that somewhere in the AC system, the neutral is bonded to the ground, and the OP asks the question why.

[boatpoker]

Originally Posted by Jammer

::shrug:: I think the point of the OP is that the neutral is bonded somewhere. For shore power, it's bonded on the shore side. For inverters and generators, the bond is as you point out aboard.


Isolation transformers also involve a neutral-to-ground bond aboard the boat.






Well, that is at best a oversimplification, and I suspect you know that. But it's off topic. The OP's point is that somewhere in the AC system, the neutral is bonded to the ground, and the OP asks the question why.

The OP said ... "Can someone explain the logic of using a tied neutral/earth AC system on a sailing boat?"
This is wrong (in most cases) and thats why I answered the way I did.

I thought he asked why they should be bonded on the boat and thats what I tried to answer. Yes I should have included isolation transformers in my exceptions, however given the OP's question it's clear that that part of the coversation is beyond his capacity at the moment. If someone does not understand grounding/bonding/AC/DC and how they interplay, they should just hire someone who does know.