Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 16-08-2013, 00:02   #16
Registered User
 
CarinaPDX's Avatar

Join Date: Jan 2010
Location: Portland, Oregon, USA
Boat: 31' Cape George Cutter
Posts: 1,682
Re: Isolating Transformers, The Earth wire connection conundrum

Raymond-

In the US the neutral and safety ground are "earthed" at the transformer near your house or apartment (or marina) and should not be earthed again as you suggest. The theory is that contact with neutral (a current-carrying part of the circuit) will be close enough to the potential of the surroundings that electrocution is not possible. The safety ground is connected to metal enclosures of electrical devices so that if there is a problem with leakage from the hot lead it will be shunted back to the transformer through the safety ground. Thus only leakage from the hot lead that is not intercepted by the safety ground is a risk. The ABYC basically calls for replicating this arrangement on the secondary side of an isolation transformer. As Dave notes, this is not unambiguously good, but at least things are well defined and many failure modes will trip breakers or an ELCI. By not earthing the secondary both leads are floating relative to the earth (seawater) potential so that would not normally be a return path (a good thing). But it complicates the failure modes. Your choice. Edit: Dan posted while I was writing this, and he gives a good example of the odd failure modes that can happen.

Dave's question was whether to earth the secondary or not; my question assumes that the secondary is earthed, and asks where is the best place for the ELCI. The advantage of having the ELCI after the transformer is that it can also be used to protect with a generator or inverter source.

There is no perfect answer to these questions, just tradeoffs. And standards aren't always the best choice.

As for the drownings, they are called ESDs (electro-shock drownings). The ABYC rep I referred to earlier lost his son in fresh water to ESD. Needless to say, he is very insistent on following the standards. I have always resisted bonding my thru-hulls - they are quite electrically isolated - but understand that wiring according to ABYC would greatly reduce any chance that an ESD might happen. Choices...

Greg
__________________

__________________
CarinaPDX is offline   Reply With Quote
Old 16-08-2013, 01:46   #17
Registered User

Join Date: Apr 2007
Location: Australia
Boat: Island Packet 40
Posts: 1,376
Images: 7
Re: Isolating Transformers, The Earth wire connection conundrum

Thank you very much gentlemen.

My system is 240v and consequently deserves a little more paranoia than a 110v system.

The original system was put in with only shore power in mind and when I decided to add an inverter I installed a receptacle on the shore power connection and a plug on the line to the boats distribution system as I preferred to have a very visual indication of where the connection between the two sources were.

When I installed the genset I maintained this system with another receptacle beside the other two.

I noticed that the green earth lead on the genset is connected to the alternator housing. I don't know whether the earth terminal on the inverter is connected to anything inside, three pin, polarized plugs are standard here and whilst many plugs now come with just live and neutral I have never seen a receptacle without the third, ground or earth wire connection.

I am guessing that the situation is that when I am connected to shore power there is a ground connection to all the receptacles and when on either inverter or genset it is floating.

On the DC side of things I have come to the conclusion that to keep life simple the cranking and house systems should be completely isolated from each other and from the hull. I am working towards that with each having it's own engine alternator and the solar regulator for the cranking batteries from the solar/wind busses with two relays which disconnect it when the ignition switch is turned on.

For safety reasons I tried to keep the boat all 12v even to having an engine driven compressor and air operated power tools but finally gave up on it and installed the inverter and genset, the availability of the safe stuff was just to limited and it was all pretty expensive.
__________________

__________________
RaymondR is offline   Reply With Quote
Old 16-08-2013, 02:22   #18
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
The issue of floating the output terminals to high potentials is sometimes raised. But in reality I don't believe its an issue. ( and really Nick in SV Jedi might comment here as I believe he floats his AC. )

I don't believe its an issue

(A) stray impedances , and capacitances mean its unlikely the output common mode voltage will shift much away from earth potential.

(B) you will only get " tingling" if there is an high impedance circuit to earth , if there is the secondary common mode voltage will also be held lower because of that.

(c) every day of the week I work with isolated floating DC power supplies, I've never seen common mode voltage issues.

(d) many boats away from shore power have inverters and gennys , unless you establish a proper seawater ground ( which ABYC doesn't require ) yiu have in effect floating AC , why is it different with transformers


ABYCs circuit requirements make it very clear , that isolating transformers , are only there to eliminate impressed current corrosion. A situation in itself brought around by the existence on US boats of bonded underwater fittings and a AC DC interconnection. So NO additional safety is imparted by using such a transformer. All the AC fault scenarios are carried over into the isolating circuit.

In the EU, these safety issues are addressed by the use of RCDs. You can argue as to their reliability , but in the EU they are code in houses and boats for decades now.


Personally I think full floating DC isolated from everything and floating isolated AC but with a protective wire run to cause breaker and secondary side RCD trips offers the best solution to impressed corrosion and the safety of an isolated non ground return path.

Dave
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 16-08-2013, 02:27   #19
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Quote:
Originally Posted by RaymondR View Post
Thank you very much gentlemen.

My system is 240v and consequently deserves a little more paranoia than a 110v system.

The original system was put in with only shore power in mind and when I decided to add an inverter I installed a receptacle on the shore power connection and a plug on the line to the boats distribution system as I preferred to have a very visual indication of where the connection between the two sources were.

When I installed the genset I maintained this system with another receptacle beside the other two.

I noticed that the green earth lead on the genset is connected to the alternator housing. I don't know whether the earth terminal on the inverter is connected to anything inside, three pin, polarized plugs are standard here and whilst many plugs now come with just live and neutral I have never seen a receptacle without the third, ground or earth wire connection.

I am guessing that the situation is that when I am connected to shore power there is a ground connection to all the receptacles and when on either inverter or genset it is floating.

On the DC side of things I have come to the conclusion that to keep life simple the cranking and house systems should be completely isolated from each other and from the hull. I am working towards that with each having it's own engine alternator and the solar regulator for the cranking batteries from the solar/wind busses with two relays which disconnect it when the ignition switch is turned on.

For safety reasons I tried to keep the boat all 12v even to having an engine driven compressor and air operated power tools but finally gave up on it and installed the inverter and genset, the availability of the safe stuff was just to limited and it was all pretty expensive.
Yes so you have floating AC on board , ( invertor , genny , and if you had it, isolating traffo )

Separating cranking and domestic DC is very difficult in practice. , you need isolated alternators , isolated engines etc, beware of unexpected common DC grounds. I would always tie starter and domestic DC negatives together. There is no advantages in attempting to isolate their grounds.

Dave
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 16-08-2013, 02:47   #20
Registered User
 
CarinaPDX's Avatar

Join Date: Jan 2010
Location: Portland, Oregon, USA
Boat: 31' Cape George Cutter
Posts: 1,682
Re: Isolating Transformers, The Earth wire connection conundrum

Dave: I still haven't seen you comment on the location of the ELCI (RCD) - any thoughts?

When you talk about floating the 12VDC system, are you then isolating the engine from the prop shaft and not connecting the common ground point to an underwater "terminal" or "interface"?

As far as isolating the AC system that might be difficult to accomplish for all cases; IIRC in the US the Marine UL inverters are required to have relays to interconnect DC- to neutral/ground when operating. I think the battery charger isolates them but would need to check to be certain.

ABYC has taken us down a path which I feel is problematic, but at least attempts some consistency. If nothing else it would seem to be a boost for the zinc anode manufacturers. I will give the idea of floating everything some thought before starting the system rebuild.

Greg
__________________
CarinaPDX is offline   Reply With Quote
Old 16-08-2013, 03:04   #21
Registered User
 
CarinaPDX's Avatar

Join Date: Jan 2010
Location: Portland, Oregon, USA
Boat: 31' Cape George Cutter
Posts: 1,682
Re: Isolating Transformers, The Earth wire connection conundrum

Quote:
Originally Posted by goboatingnow View Post
ABYCs circuit requirements make it very clear , that isolating transformers , are only there to eliminate impressed current corrosion. A situation in itself brought around by the existence on US boats of bonded underwater fittings and a AC DC interconnection. So NO additional safety is imparted by using such a transformer. All the AC fault scenarios are carried over into the isolating circuit.
I'm not certain I buy that entirely. In my current marina the resistance in the safety ground is too high for it to do much good (it over-ranges the tester). So I don't think one is going to be well protected from shorts to metal enclosures, for instance. By installing the iso-xfmr the utility of the safety ground is restored (because neutral and ground are connected on the secondary side). Also, I have been in a marina in Europe that measured 28VAC between neutral and ground - not enough potential to drive lethal current but possibly trouble for shielded circuits. Again, an iso-xfmr eliminates that difference. I don't disagree with your main point, which is to say that the ABYC recommendations tend to undo some of the good things that an iso-xfmr does; and that leads to further dubious requirements like bonding to undo them in turn.

Greg
__________________
CarinaPDX is offline   Reply With Quote
Old 16-08-2013, 03:17   #22
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Quote:
Originally Posted by CarinaPDX View Post

I'm not certain I buy that entirely. In my current marina the resistance in the safety ground is too high for it to do much good (it over-ranges the tester). So I don't think one is going to be well protected from shorts to metal enclosures, for instance. By installing the iso-xfmr the utility of the safety ground is restored (because neutral and ground are connected on the secondary side). Also, I have been in a marina in Europe that measured 28VAC between neutral and ground - not enough potential to drive lethal current but possibly trouble for shielded circuits. Again, an iso-xfmr eliminates that difference. I don't disagree with your main point, which is to say that the ABYC recommendations tend to undo some of the good things that an iso-xfmr does; and that leads to further dubious requirements like bonding to undo them in turn.

Greg
But that resistance only matters if you can established a better local seawater ground. Otherwise you have a floating AC, where earth return to secondary neutral may or may not be in effect

As to Europe , note that contrary to the US , by design in many countries the neutral can be floating ( in single phase) . Hence in 240 land , it always is better to assume non polarised mains. With proper wiring practice , there is simply no issue with having two hot wires , the US does this with 240V all the time. Reverse polarity is only an issue where an assumption is made that a " neutral" exists.

Dave
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 16-08-2013, 06:12   #23
Registered User

Join Date: Jun 2007
Location: SW Florida
Boat: FP Belize, 43' - Dot Dun
Posts: 3,432
Re: Isolating Transformers, The Earth wire connection conundrum

Quote:
Originally Posted by CarinaPDX View Post
Dave: I still haven't seen you comment on the location of the ELCI (RCD) - any thoughts?

When you talk about floating the 12VDC system, are you then isolating the engine from the prop shaft and not connecting the common ground point to an underwater "terminal" or "interface"?

As far as isolating the AC system that might be difficult to accomplish for all cases; IIRC in the US the Marine UL inverters are required to have relays to interconnect DC- to neutral/ground when operating. I think the battery charger isolates them but would need to check to be certain.

ABYC has taken us down a path which I feel is problematic, but at least attempts some consistency. If nothing else it would seem to be a boost for the zinc anode manufacturers. I will give the idea of floating everything some thought before starting the system rebuild.

Greg
FWIW, I have an ELCI/RCD at the IT primary input AND the main breaker in the distribution box (it's only $$). My paranoia also led me to install MOVs and gas tubes on the primary side (be sure to spare some of each). My possibly flawed thinking was to try and protect the primary winding from transient spikes coming in on the shore power. But, admittedly, if a spike is such to get past the MOVs and gas tubes and cause an arc from the primary winding to screen tripping the ELCI, the damage is probably already done.
__________________
DotDun is offline   Reply With Quote
Old 16-08-2013, 10:16   #24
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Isolating Transformers, The Earth wire connection conundrum

Quote:
Originally Posted by CarinaPDX View Post
Dave: I still haven't seen you comment on the location of the ELCI (RCD) - any thoughts?

When you talk about floating the 12VDC system, are you then isolating the engine from the prop shaft and not connecting the common ground point to an underwater "terminal" or "interface"?

As far as isolating the AC system that might be difficult to accomplish for all cases; IIRC in the US the Marine UL inverters are required to have relays to interconnect DC- to neutral/ground when operating. I think the battery charger isolates them but would need to check to be certain.

ABYC has taken us down a path which I feel is problematic, but at least attempts some consistency. If nothing else it would seem to be a boost for the zinc anode manufacturers. I will give the idea of floating everything some thought before starting the system rebuild.

Greg
In an isolating traffo, I would put one RCBO, /ELCI , in a box beside the sure power inlet. I would then put a second one on the output of the secondary mounted on the AC panel. This would be after any internal power sources, i.e. traffo, genny or inverter.

enforced DC AC connections are a bad idea, Are you sure your right, are you mixing up neutral AC ground . i.e. earth switching relays

Dave
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 16-08-2013, 12:16   #25
Registered User
 
transmitterdan's Avatar

Join Date: Oct 2011
Boat: Valiant 42
Posts: 4,030
ABYC requires inverters with integral changeover switching to connect safety, neutral and DC minus when no AC input is present. It requires these three to be isolated at inverter when external AC is present. Thus some inverter manufacturers include internal relay that performs this function. To comply with other jurisdictions the DC minus connection is usually defeatable by a switch setting on the unit. Mastervolt for example has the capability.
__________________
transmitterdan is offline   Reply With Quote
Old 16-08-2013, 12:28   #26
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Isolating Transformers, The Earth wire connection conundrum

Quote:
Originally Posted by transmitterdan View Post
ABYC requires inverters with integral changeover switching to connect safety, neutral and DC minus when no AC input is present. It requires these three to be isolated at inverter when external AC is present. Thus some inverter manufacturers include internal relay that performs this function. To comply with other jurisdictions the DC minus connection is usually defeatable by a switch setting on the unit. Mastervolt for example has the capability.

could you find the reference to that please
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 16-08-2013, 12:43   #27
Registered User
 
sy_gilana's Avatar

Join Date: Jan 2005
Location: On board
Boat: Van de Stadt 50'
Posts: 1,101
Send a message via Skype™ to sy_gilana
Re: Isolating Transformers, The Earth wire connection conundrum

Hi, My old CAD program wont run on Win7 so I will try to explain what I have, and possibly open other options to you.

Originally the Shore power connection had L/E/N connected directly to the IT. The earth wire was connected to the frame of the IT, not the N.

On the other side, the L and N went through a contactor (Gen-Shore) an Earth Leakage, and a thermal. The earth wire is connected to the IT frame.
There is a CT fro AC Amperes, and a freq reed meter.

The AC ground does not tie in with the DC ground.

While tied to a dock in Mallorca, I noticed that the anodes on one side of the boat were deteriorating fast. I started measuring various things, with a silver cell in the water etc, found nothing really definite. Then I put my MM between my boat and the Jongert next to me. 25v AC! and 3v DC!!!!, with ME as the anode!

I disconnected the ground wire in the shore power cable and the voltages went away.

Now the IT only gets L and N from the shore, a new bigger 2 core cable. All other "earths" "Grounds" are created inside the boat. The inverter ground, generator ground and IT ground are commoned on the AC master panel.

Hope this helps, and if you can see any problems with my installation, please let me know.

PS the Jongert was old, and had many strange electrical problems, like the nav lights would glow dimly when he ran his pressure washer or dive compressor in the engine room.
__________________
Tight sheets to ya.
http://gilana.org
sy_gilana is offline   Reply With Quote
Old 16-08-2013, 12:52   #28
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Isolating Transformers, The Earth wire connection conundrum

if your description is correct you actually connected the boat earth ( secondary) to the shore earth using the frame of the IT. This is a mistake and effectively removes the galvanic protection afforded by the IT. It reintroduces the problems of AC as if the IT wasn't there.

The shore power earth should only be connected to the electro static shield. it can also be connected to the IT case , but then the secondary side boat earth cannot be connected to the case. Only one earth to the case

ABYC requires secondary side case earthing, I would ( and others like smart gauge ) argue that the primary side shore earth be carried to the IT case but you must ensure that the IT case doesnt cause leakage currents to flow.

Now that you have disconnected the shore earth , you have a very dangerous condition, that is that everything on the primary side of the IT has no protective earth. A fault in that side will not cause the breakers to trip.

However as you have a Euro boat , its most likely the RCD on the shore pillar or the one immediately after the shore power enters the boat at least providing some safety cover

Its not a good idea though

dave
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 16-08-2013, 13:08   #29
Registered User
 
transmitterdan's Avatar

Join Date: Oct 2011
Boat: Valiant 42
Posts: 4,030
Quote:
Originally Posted by goboatingnow View Post

could you find the reference to that please
Dave,

I may be mistaken about the part where DC minus should be switched to safety ground. I think ABYC expects this to be connected always on board. So maybe only the neutral and safety ground have to be switched. I am not an ABYC member so don't have copies of their docs. Here is a link to Mastervolt inverter guide that describes optional neutral relay. It connects safety to neutral when AC is missing.

http://images.mastervolt.nl/files/Qu...bi080731EN.pdf
__________________
transmitterdan is offline   Reply With Quote
Old 16-08-2013, 13:14   #30
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Isolating Transformers, The Earth wire connection conundrum

Quote:
Originally Posted by transmitterdan View Post
Dave,

I may be mistaken about the part where DC minus should be switched to safety ground. I think ABYC expects this to be connected always on board. So maybe only the neutral and safety ground have to be switched. I am not an ABYC member so don't have copies of their docs. Here is a link to Mastervolt inverter guide that describes optional neutral relay. It connects safety to neutral when AC is missing.

http://images.mastervolt.nl/files/Qu...bi080731EN.pdf

The ABYC docs are widely available on line and I have a copy. Yes ABYC requires a DC negative to AC earth connection , but not necessarily at the inverter. ( i.e. there has to be one on board.) mind you I personally use RCDs and would never interconnect AC earth and DC neg.

Yes what you mean is Neutral Earth relay, which establishes a local safety circuit for AC , when the shore poower is removed and removes it ( as ABYC requires only ONE earth connection at the shore side and never on the boat), when on shore power is present and the inverter isn't running

Note Id love to a see ABYS address local grounds in the case of multiple synchronised AC sources, like power assist inverters etc. !!!! where do you connect the neutral and protective earth then. !

Technology ahead of standards bodies, thats an unusual one!!

dave
__________________

__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Reply

Tags
rum

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 20:36.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.