Marine AC - Why Tie Neutral and Earth?

[Baronkrak]

Originally Posted by s/v Jedi

Edit: in the diagram you can see that I simply eliminate the neutral by just not connecting it from the genset nor the 120/240 50A shore power. I simply only take L1 and L2. Where this isn’t possible is with a 120V 30A shore power connection. This is tackled by using L1 and N to power the primary windings of an isolation transformer, then jumpering its output for 1:2 240V and not grounding either output conductor.

I can see a problem with your wiring. Let say L1 or L2 leaks in any of your Victron"s or any other appliances that share the same earth wire. As you said nothing would happen because you have a floating earth if it is that everything connected to that earth wire would become active until the opposite active as well start to leaks. If you are lucky a circuit breaker may open before the earth wiring catch fire.

[Wotname]

Originally Posted by Thumbs Up

There is a convention for naming the earthing arrangement on these different system types we are referring to denoted by letter abbreviations such as TT, IT, TNS, TNCS, and TNC. Most residential power supplies are TNCS which in North America is also called a "Multigrounded Neutral" system (Australia and New Zealand call it Multiple Earthed Neutral ). An isolated power supply is designated IT, has no connection to earth, and is what is used in hospital operating rooms, some ships etc. What Wottie is describing ( fully floating Active and Neutral AC output) is an IT system (I think).

Yes, it is!



A floating ground, I found out, is just the term we use when the safety ground of a three prong outlet is missing (such as an extention cord with the broken ground prong) on a TNCS system. My question now is: If we add the earthing conductor to the IT system (tying it to the so called nuetral conductor on the output of the isolation transformer) what are the implications?

My question is somewhat similar but one important difference - what are the implications if we add the earthing conductor to the IT system without tying it to the so called neutral conductor and without tying it to the actual physical earth / ground.

Is still an IT system (I think it is but not sure), does it allow us to make better use of GFCI protection? GFCI circuit breakers? I can't quite wrap my head around it (so to speak). Here are some good study material links:
https://www.emobilitysimplified.com/...-tn-tt-it.html
https://www.studiecd.dk/cahiers_tech...evolutions.pdf
https://en.wikipedia.org/wiki/Earthing_system
https://www.ecs.csun.edu/~bruno/Mult...nal_4-17-7.pdf
https://www.isolatedpowerspecialist....stions-answers
https://engineering.electrical-equip...ation%20fault.
https://stevenengineering.com/Tech_S...DFs/45HIPS.pdf
Here is a previous conversation about this conundrum:
https://www.cruisersforum.com/forums...um-109344.html

The above blue and red are my responses.

The earlier CF thread is a great find, it appears to cover most (all?) of the aspects of this thread although I haven't had time to read it fully ATM. It is now on my reading list!

[Thumbs Up]

My question is somewhat similar but one important difference - what are the implications if we add the earthing conductor to the IT system without tying it to the so called neutral conductor and without tying it to the actual physical earth / ground.

Not sure where that leaves it to tie in. If it were to tie in between the output of the transformer (into the nuetral or hot leg, they are basically the same right?) but before a double RCD breaker it then seems like any fault current would bypass one of those RCDs and trip that main breaker. If this works, there should be an example of it somewhere, maybe I am missing something.

[Thumbs Up]

Learning about line isolation monitors.
https://www.amazon.com/Schneider-Ele.../dp/B076HB6KJV

Types of LIMs
Several types of line isolation monitors are available. Reviewing them not only helps determine
requirements for a system, but helps identify the equipment currently used in the hospital.
Ground Detector. The first unit is not actually a LIM, but rather the original “ground detector,” which is
essentially a balanced bridge device. Ground detectors were standard equipment until about 1970, so
many of these units are still in use. Inexpensive to build and reliable because of its simplicity, the
ground detector is unaffected by and does not create any radio frequency (RF) interference. However,
it only recognizes unbalanced resistive or capacitive faults; it cannot recognize a partially grounded
system. This inability to sound an alarm (to recognized balanced fault systems) is the main reason
codes and standards no longer allow its use.
Systems in the field have been observed to allow as much as 30 mA (30,000 μA) to flow from line to
ground without sounding an alarm. This very hazardous condition can cause an electrical hazard to the
patient or medical staff.
Ground detectors may still be used if they were installed before 1971. Even though not required by
code, hospitals should consider revising these systems to match current standards.
Dynamic Ground Detector. The first dynamic ground dete................https://stevenengineering.com/Tech_S...DFs/45HIPS.pdf

These systems are supposed to warn of a fault without turning the power off. But for a boat is less of a convenience to lose power than an operating table.

[Wotname]

Originally Posted by Thumbs Up

Not sure where that leaves it to tie in. If it were to tie in between the output of the transformer (into the nuetral or hot leg, they are basically the same right?) but before a double RCD breaker it then seems like any fault current would bypass one of those RCDs and trip that main breaker. If this works, there should be an example of it somewhere, maybe I am missing something.

For the sake of simplicity, I will call the third conductor 'the third conductor' rather than 'earth' or 'ground' conductor. It's only purpose is to connect (tie) any metal cases of any devices which are plugged into any of the outlets.

Unless double insulated, most pluggable devices with a metal chassis or cover have the metal parts tied to the earth/ground prong of the plug. The third conductor connects all such metal together across whatever is plugged in at the time.

This should (?) provide protection should there be leakage from either of the source conductors to the case as described upthread.

What I haven't thought through is the implications (good or bad) of tying the third conductor the physical ground - maybe it is covered somewhere in the 23 pages of the previous thread you linked.

[s/v Jedi]

So for those familiar with electrical system but wondering about Yankee power, here is the simplest way to understand:

The transformer that delivers power to a house... they hang it up high in the air on a post... with high voltage wiring 440V or so running across bare cables that span such poles... crazy! Anyway, the secondary winding with L1 and L2 conductors connected to it is 240V but it has a center tap which they ground using a grounding rod driven into the earth/ground and they connect Neutral to that center tap. So between L1 and neutral you get 120V which is shifted 180 degrees in phase from the 120V that exists between L2 and neutral.

These feeds are 100A (25kW) or 200A (50kW) for most houses.

So the ground wire (green/yellow, earth, bare copper) on the outlets all go to the ground busbar as shown in my diagram. This is where the ground wires from transformers, inverter/chargers, boat-side of galvanic isolator and isolation transformer etc. go. It is not connected to DC negative and not to AC Neutral. It’s only job is to connect all external metal surfaces of appliances together.

If the housing of my first Multiplus shorts to L1 and then the coffeemaker housing shorts to L2, the ground wiring carries that short circuit current which triggers the breaker(s).

Someone mentioned that this is dangerous in my diagram as it can cause fire: no sir, it is not. This is exactly what the ground wiring is supposed to do: trigger the breakers. The wiring can safely carry that current, it is designed to do so.

GFCI outlets also trigger... except when these are directly wired appliances like Multiplus, auto transformer etc. Whenever possible wire something using a gfci outlet. Example: my water heater 120V element is wired to a dedicated gfci outlet.

Some have mentioned detectors that show a ground fault... these can be installed here as well, warning as soon as either L1 or L2 shorts to the ground wiring so that it can be fixed and we don’t get triggered breakers.

[Jammer]

Originally Posted by s/v Jedi

So for those familiar with electrical system but wondering about Yankee power, here is the simplest way to understand:

The transformer that delivers power to a house... they hang it up high in the air on a post... with high voltage wiring 440V or so running across bare cables that span such poles...

Actually, the most widely used distribution voltage in the USA is 7200 volts to ground (12470 volts phase to phase). The second most widely used distribution voltage in the USA is 14400 volts to ground.


Early 20th century distribution systems used 2300 volts phase to phase and nearly all have been converted ("wyed up") to 2300 volts phase to ground, which is 4160 volts phase to phase (In a 3 phase system the relationship between the wye and delta voltages is sqrt(3)). A few of these systems are still in operation and reflect the lowest voltage utility distribution systems in widespread use.


440/460 volt outdoor distribution systems are rarely used by utilities. 460 volts is a common voltage for moderately large industrial loads, and is sometimes used for electrical distribution within a larger building and then dropped to 120/208v as needed.

[s/v Jedi]

Originally Posted by Jammer

Actually, the most widely used distribution voltage in the USA is 7200 volts to ground (12470 volts phase to phase). The second most widely used distribution voltage in the USA is 14400 volts to ground.


Early 20th century distribution systems used 2300 volts phase to phase and nearly all have been converted ("wyed up") to 2300 volts phase to ground, which is 4160 volts phase to phase (In a 3 phase system the relationship between the wye and delta voltages is sqrt(3)). A few of these systems are still in operation and reflect the lowest voltage utility distribution systems in widespread use.


440/460 volt outdoor distribution systems are rarely used by utilities. 460 volts is a common voltage for moderately large industrial loads, and is sometimes used for electrical distribution within a larger building and then dropped to 120/208v as needed.

Yes, 7.2kV...30:1 windings. I mixed it up with marina wiring that I’m familiar with...

[Thumbs Up]

I skimmed through this 2013 thread https://www.cruisersforum.com/forums...um-109344.html. Jedi was part of that.
It was the exact same topic but a better thread. I find it ironic that the IT system designation identified earlier has the same anagram as IT (isolation transformer). I think that Jedi's position is that IT systems are so safe that you barely need RCDs. (correct me if I am wrong) Both threads have confirmed that dedicated outlet GFCIs as a way to protect from the (highly unlikely) opposite twin live fault scenario. What both these threads boil down to (I think) is what options we have for the "green" wire (ground prong connection on each outlet) or should it be used at all. One option was to connect it to the center tap on the transformer (if you have a center tap). A center tap can also be used to connect ground lamps for monitoring. You can't connect it to shore power ground or it would not be an isolated system. Connecting it to one of the outputs of the transformer creates a pseudo neutral but doesn't accomplish anything. (or does it?) If I understand Jedi's diagram he is connecting it to the transformer case (shore power green would connect to the shield so no interconnect) but I don't know what this accomplishes. (set me straight on this please Jedi). The only reference I could find to what I am suggesting was posted by the OP (Dave goboatingnow) in one post that I included below in red which is connecting the green wire to one of the outputs (pseudo neutral) but before a double pole RCD breaker (or breakers). You could then still have dedicated (5ma) outlet GFCIs and have the extra full circuit protection (which would actually trip from a small >30ma current on the ground wire. It would give an extra measure of safety. I think that Dave described it too in this post:

Several posters simply don't understand IT basics. There are several ways to configure IT. The one were are talking about is a fully floating system. This is similar to units used in hospitals.

In a full IT setup. It would be advisable to still fit an RCD and the usual breakers on the secondaries . No earth wire is installed

Lets examine the potential fault scenarios

(1) Single live fault to appliance case , ( ie battery charger etc ) this does not generate any shock hazard , however no fault is triggered either

(2) both live wires shorted to an appliance , depending on the potential ground or other paths. A short may generate and the breaker trips

(3) one live faulted to appliance A and the other live faulted to appliance B. a person touching both appliances will be inserted into the circuit and receive a shock. No protection will be extended. However this is a very unlikely fault situation.

The solutions are

( a) re establish ground referenced output and earth wire and rely on breakers and RCDs. Hence the IT provides no security as most shock hazards are fault through human to ground

(B) establish a fault protective wire , connecting one live of the IT to appliance case with an RCD then ahead of the common protective wire and IT secondary.

This wire must not lead to inadvertent ground connection , but it does protect a live fault to appliance as in scenario (1)

I called it a conundrum for a reason.

It depends on what you regard as the safety issues on board a boat. I personally agree with Nick , in that most shock faults are exposed live or appliance faults to ground via the human. A floating IT solution completely eradicates that issue, even though the existence of such a fault is not easily detectable.

Every system has fault scenarios that cause problems , its a balance of options

dave

[Thumbs Up]

Originally Posted by s/v Jedi

So for those familiar with electrical system but wondering about Yankee power, here is the simplest way to understand:

The transformer that delivers power to a house... they hang it up high in the air on a post... with high voltage wiring 440V or so running across bare cables that span such poles... crazy! Anyway, the secondary winding with L1 and L2 conductors connected to it is 240V but it has a center tap which they ground using a grounding rod driven into the earth/ground and they connect Neutral to that center tap. So between L1 and neutral you get 120V which is shifted 180 degrees in phase from the 120V that exists between L2 and neutral.

These feeds are 100A (25kW) or 200A (50kW) for most houses.

Fair enough. How is the euro power grid different? You guys just use two prongs? Australia three? No bare poles or hanging transformer? How's it work? Just curious.

[Baronkrak]

Originally Posted by Wotname

For the sake of simplicity, I will call the third conductor 'the third conductor' rather than 'earth' or 'ground' conductor.

Very wise and what colour would you choose for that conductor?

[Wotname]

Originally Posted by Thumbs Up

Fair enough. How is the euro power grid different? You guys just use two prongs? Australia three? No bare poles or hanging transformer? How's it work? Just curious.

Australia -

240V single phase delivered into the house on 2 wires (A & N).
Three conductor power wiring in house (A, N & E).
Neutral and Earth buses tied together in the house meter box and tied to a physical earth stake.
RCD breakers in the meter box on most power circuits.
House power outlets three prongs (A,N & E).
Very few downstream RCD outlets.

[Wotname]

Originally Posted by Baronkrak

Very wise and what colour would you choose for that conductor?

Dayglo yellow or rescue orange but not brown, blue, red or black

[Baronkrak]

Originally Posted by s/v Jedi

Someone mentioned that this is dangerous in my diagram as it can cause fire: no sir, it is not. This is exactly what the ground wiring is supposed to do: trigger the breakers. The wiring can safely carry that current, it is designed to do so.

GFCI outlets also trigger... except when these are directly wired appliances like Multiplus, auto transformer etc. Whenever possible wire something using a gfci outlet. Example: my water heater 120V element is wired to a dedicated gfci outlet.

Some have mentioned detectors that show a ground fault... these can be installed here as well, warning as soon as either L1 or L2 shorts to the ground wiring so that it can be fixed and we don’t get triggered breakers.

Your wiring diagram is for the least unconventional.
A better approach could easily make it acceptable.
The fact that your ‘ships ground" can become active is unacceptable.
The generator wiring is incorrect.
At least one "breaker" should be an ELCB, (Earth Leakage Circuit Breaker).
In conclusion floating ground is a myth.

[smac999]

Originally Posted by s/v Jedi

It’s good to see that people are actually starting to think it over and discuss specifics instead of parroting “ must be grounded”

So here are the reasons for having the ground wired to outlets as well as still using gfci outlets and double pole breakers.

If you do not wire the ground conductors to outlets then theoretically it is possible that two appliances with metal housings develop a short to the housing, with one shorting L1 and the other L2. This creates a lethal voltage on two exposed metal surfaces that can electrocute a person.
If the ground wiring is installed, this creates a hard short flipping the breaker.

Someone posted a diagram showing ground faults and a gfci outlet. So here we have a resistance instead of a hard short. The resistance limits current so that a breaker isn’t triggered, but it can potentially still hurt or even kill a person. The way the diagram is drawn, this will never trigger the gfci protection like mentioned in that comment. However, if you have two gfci outlets and ground faults develop in between them, then they trigger when the current exceeds the gfci trigger value. The outlets still protect even though the test buttons don’t work.

The above also suggests to make every outlet a gfci outlet and not use “downstream” normal outlets.

Double pole breakers are needed because both conductors are hot. For US installations, 220V circuits always have double pole breakers for this reason.

I guess everyone knows my wiring diagram by now but I’ll attach it again. This is for US wiring... for EU wiring you simply leave the auto transformers out.

Edit: in the diagram you can see that I simply eliminate the neutral by just not connecting it from the genset nor the 120/240 50A shore power. I simply only take L1 and L2. Where this isn’t possible is with a 120V 30A shore power connection. This is tackled by using L1 and N to power the primary windings of an isolation transformer, then jumpering its output for 1:2 240V and not grounding either output conductor.

are you using the n-g relay in the auto transformer? I'm guessing not?

so if you take the hot feed wire at the AC panel and short it to the green ground wire what happens?... under inverter, gen , 240v, and 120v shore?

I'm curious. but here are my guesses:

no breakers will trip except on 240v shore because current has nowhere to go. you'll just end up with a hot ground bus

it would clear on the 240v shore because of the n-g bond on shore.

if the n-g bond is made in the gen it will clear the fault. even though the N is not used. if the n-g bond is not made at gen it will not. since the n is not used the bond may have been overlooked and missing.

on inverter the auto transformer should have the n-g bond on controlled from the quatro and it will clear faults. if the relay is not used it will not.

on 120 shore that iso transformer will never clear a ground fault. it's not designed to create 240v usa power with a floating ground. it's deigned to create 230v euro power with a grounded neutral