Cruisers Forum
 


Join CruisersForum Today

Closed Thread
  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
 
Thread Tools Search this Thread Rate Thread Display Modes
Old 28-06-2018, 09:16   #1
Registered User

Join Date: Oct 2007
Posts: 114
Onboard Isolation Transformer vs Safety

Hello all,
In a recent post about galvanic isolators, a fellow sailor recommended an isolation transformer as a better alternative to the galvanic isolator in both galvanic isolation as well as safety. For this discussion the word "safety" means protection from electrocution. The person to be protected may be a nearby swimmer or a diver while he/she is cleaning the bottom of a vessel, while that vessel is plugged into shore power.

First question :
Is it safe, to remove an existing galvanic isolator and disconnect the shore power grounding conductor from the underwater metals of your boat by installing an isolation transformer?

Second question :
If the shore power "hot conductor" brought on board your vessel shorts to the underwater metals of your boat, is it possible for the diver to get electrocuted due to this single point failure?

Thank you in advance.

Ahmet





__________________

Ahmet Erkan is offline  
Old 28-06-2018, 09:49   #2
Registered User

Join Date: Jun 2013
Location: canada
Posts: 2,191
Re: Onboard Isolation Transformer vs Safety

ISO transformers are superior. But they are 10x bigger, 10x heavier, and 5x more $. That's why not as common.

Generally mounted near boat outlet. Cable only a few feet long.

Output ground of transformer is still to boat ground.


If you really concerned. Put an elci breaker as the wire enters boat.
__________________

smac999 is offline  
Old 30-06-2018, 07:47   #3
Registered User

Join Date: Oct 2007
Posts: 114
Re: Onboard Isolation Transformer vs Safety

Quote:
Originally Posted by smac999 View Post
ISO transformers are superior. But they are 10x bigger, 10x heavier, and 5x more $. That's why not as common.

Generally mounted near boat outlet. Cable only a few feet long.

Output ground of transformer is still to boat ground.


If you really concerned. Put an elci breaker as the wire enters boat.

Yes I am concerned, I do not want to kill anybody because of a common failure on my boat.
About the ELCI breaker. Have you ever experienced 30mA electric current through your body? 5mA?

OK do not try this at home.

As the current through your body increases, long before it gets to 30mA, your joints will start twitching, muscles spasming, your teeth chattering, and even if a weak medical condition does not kill the person,he or she will not be able to swim away let alone breathe through scuba gear.

It is my belief that placing the isolation transformer at the marina pedestal is the only way to make the system as safe as using a galvanic isolator on board.
I originated this post to urge our marine power system experts (you may be one smac999) to stop making the recommendation to non-experts that it is OK to disconnect the galvanic isolator on board and leave the shore power grounding (green) conductor disconnected from the boat's underwater metals because you have an isolation transformer.

More later if there is interest.
Cheers
Ahmet
Ahmet Erkan is offline  
Old 30-06-2018, 10:07   #4
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 22,243
Onboard Isolation Transformer vs Safety

You do know that underwater electrocution is extremely unlikely in salt water?
It’s fresh water that it’s a killer, and I believe that most often it’s docks electrical wiring that is the killer, not the boats so much.
I am not saying don’t be unsafe, just responding to what is apparently the most danger is all.
I am not a professional electrician, so I won’t comment further.

However it may be prudent to hire a real licensed professional if noting else to inspect an installation. Sometimes its better to hire a Pro.
a64pilot is offline  
Old 30-06-2018, 20:23   #5
Moderator
 
Adelie's Avatar

Join Date: Feb 2010
Location: La Ciudad de la Misión Didacus de Alcalá en Alta California, Virreinato de Nueva España
Boat: Cal 20
Posts: 6,981
Re: Onboard Isolation Transformer vs Safety

If you have an isolation transformer the electricity on the shore does not transmit to the boat, only the energy in the electricity. At the transformer the energy in the shoreside electricity is transformed into magenetism in the iron core. This energy is then converted back to electricity on the boat side of the core.
If the shoreside shorts the electricity wants to go back to earth. If there is a grounding wire from the boat side bonded to the transformer the short may go thru the boat and from there into the water to get back to earth.

As A64 indicates, if there is a shoreside short that gets onto the boat and tries to go to earth thru the water it is only a problem in fresh water. Fresh water not as conductive as saltwater and and the human body is substantially saltwater. As such it provides a more conductive path for shorts. In salt water the body is less conductive than the water so electrocution is unlikely.
__________________
A house is but a boat so poorly built and so firmly run aground no one would think to try and refloat it.
SailboatData
Adelie is offline  
Old 30-06-2018, 20:50   #6
Moderator
 
Paul Elliott's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 4,505
Images: 4
Re: Onboard Isolation Transformer vs Safety

[I am not a credentialed marine electrician.]

What Ahmet is concerned about is the failure case where a shorepower connection hot wire shorts to the case of the isolation transformer (this case is typically connected to the boat's ground). Should this happen, there can indeed be a path for the fault current that will electrocute a swimmer (much more likely in fresh water).

Ahmet, did I get this right?

So this is a single fault that can be dangerous. But just how likely is that particular fault? And compare this to other faults that can happen without an isolation transformer (say I dropped my toaster in the bilge, and the shorepower connection has a poor ground contact).

Given the sorry state of some shorepower pedestals, I suspect that a properly installed isolation transformer is probably safer than not. But to be fair, I have seen conflicting advice on this topic.
__________________
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
www.sailvalis.com
Paul Elliott is offline  
Old 01-07-2018, 09:53   #7
Registered User

Join Date: Oct 2007
Posts: 114
Re: Onboard Isolation Transformer vs Safety

Quote:
Originally Posted by Paul Elliott View Post
[I am not a credentialed marine electrician.]

What Ahmet is concerned about is the failure case where a shorepower connection hot wire shorts to the case of the isolation transformer (this case is typically connected to the boat's ground). Should this happen, there can indeed be a path for the fault current that will electrocute a swimmer (much more likely in fresh water).

Ahmet, did I get this right?

So this is a single fault that can be dangerous. But just how likely is that particular fault? And compare this to other faults that can happen without an isolation transformer (say I dropped my toaster in the bilge, and the shorepower connection has a poor ground contact).

Given the sorry state of some shorepower pedestals, I suspect that a properly installed isolation transformer is probably safer than not. But to be fair, I have seen conflicting advice on this topic.

You got it right on the target Paul.
The other folks are right too, fresh water or brackish water does make the electrocution hazard much more probable.
I am astonished that the core/case of the onboard isolation transformer is not bonded to the shore power grounding conductor in compliance with NEC and all similar safety standards. As you say "the sorry state of some shore power pedestals" is known. However, most pedestals do have at least a magnetically tripped (regular household) circuit breaker. It is not possible to be 100% safe but why not wire the transformer right to make things as safe as possible.

Why try to steal the galvanic isolator's purpose and misrepresent its performance? People in the electrical power industry know that diodes always short, they open only if the current is not limited. (ie: No circuit breaker) In our case we have two CB's in series, one on board and one hopefully on the pedestal, so the safety capability of the galvanic isolator is unquestionable. We need to make the galvanic isolator a better galvanic isolator instead of replacing it with a 60Lbs of very expensive weight providing questionable safety.
Best regards.
Ahmet
Ahmet Erkan is offline  
Old 01-07-2018, 10:48   #8
Registered User

Join Date: Jun 2007
Location: SW Florida
Boat: FP Belize, 43' - Dot Dun
Posts: 3,834
Re: Onboard Isolation Transformer vs Safety

I have serious doubts that you can find even one failure where a shorepower hot conductor touched the case of a properly installed isolation transformer. I can't even imagine what kind of fault would cause this to happen. If this did happen, it would/should clear the breaker on the dock. FWIW, the case on my isolation transformer is non-metallic.
DotDun is offline  
Old 01-07-2018, 12:23   #9
Registered User

Join Date: Oct 2007
Posts: 114
Re: Onboard Isolation Transformer vs Safety

Quote:
Originally Posted by DotDun View Post
I have serious doubts that you can find even one failure where a shorepower hot conductor touched the case of a properly installed isolation transformer. I can't even imagine what kind of fault would cause this to happen. If this did happen, it would/should clear the breaker on the dock. FWIW, the case on my isolation transformer is non-metallic.

Here is a failure scenario :
The screws on the terminal block that terminate the hot conductor to the transformer primary get loose. (ie: Not torqued at installation, no lock washer, transformer too close to engine vibration etc etc could cause this)
The heat generated at the loose terminal is transferred through the conductor to the strain relief at the shore cable entry and causes the insulation on the hot conductor to get soft and the conductor contacts the chassis which is connected to the underwater metals.

"If this did happen, it would/should clear the breaker on the dock." DotDun

ANSWER : Agreed but the isolation transformer's manufacturer is instructing the user to connect the boat ground to the case of the transformer and not to connect the shore power ground to the case. Therefore the circuit breaker at the pedestal will not trip.
Since your specific brand isolation transformer has a non conductive chassis it appears like the manufacturer knows what they are doing. Hopefully the shore power ground is bonded to the core of the transformer inside the non metallic transformer case.

In your boat the risk is minimized but it is still a single point failure which will electrocute the diver.
If you use a galvanic isolator, the single point failure will be eliminated. If you use a properly designed galvanic isolator the galvanic isolation will become just as good as what the isolation transformer can provide.
Furthermore if you use the isolation transformer to do what it is meant to do, then you need a much smaller isolation transformer that performs what they do best, which is to provide common mode noise attenuation for your sensitive electronics. A 300W isolation transformer at about 10Lbs and maybe for $100 to $200 is the best protection for your TV, computer, phone charger etc.
By the way, if I hear a valid explanation that an isolation transformer can provide better galvanic isolation compared to a galvanic isolator, I will admit I am wrong.

Cheers
Ahmet


Ahmet Erkan is offline  
Old 01-07-2018, 17:29   #10
Registered User

Join Date: Jun 2007
Location: SW Florida
Boat: FP Belize, 43' - Dot Dun
Posts: 3,834
Re: Onboard Isolation Transformer vs Safety

Quote:
Originally Posted by Ahmet Erkan View Post
Here is a failure scenario :
The screws on the terminal block that terminate the hot conductor to the transformer primary get loose. (ie: Not torqued at installation, no lock washer, transformer too close to engine vibration etc etc could cause this)
The heat generated at the loose terminal is transferred through the conductor to the strain relief at the shore cable entry and causes the insulation on the hot conductor to get soft and the conductor contacts the chassis which is connected to the underwater metals.

"If this did happen, it would/should clear the breaker on the dock." DotDun

ANSWER : Agreed but the isolation transformer's manufacturer is instructing the user to connect the boat ground to the case of the transformer and not to connect the shore power ground to the case. Therefore the circuit breaker at the pedestal will not trip.
Since your specific brand isolation transformer has a non conductive chassis it appears like the manufacturer knows what they are doing. Hopefully the shore power ground is bonded to the core of the transformer inside the non metallic transformer case.

In your boat the risk is minimized but it is still a single point failure which will electrocute the diver.
If you use a galvanic isolator, the single point failure will be eliminated. If you use a properly designed galvanic isolator the galvanic isolation will become just as good as what the isolation transformer can provide.
Furthermore if you use the isolation transformer to do what it is meant to do, then you need a much smaller isolation transformer that performs what they do best, which is to provide common mode noise attenuation for your sensitive electronics. A 300W isolation transformer at about 10Lbs and maybe for $100 to $200 is the best protection for your TV, computer, phone charger etc.
By the way, if I hear a valid explanation that an isolation transformer can provide better galvanic isolation compared to a galvanic isolator, I will admit I am wrong.

Cheers
Ahmet


1) My post stated 'a properly installed' isolation transformer.
2) Your failure scenario depends on an improperly installed isolation transformer.
3) You need to provide evidence that your failure scenario has ever actually happened, please provide links.
4) The same failure you describe can happen with a breaker/distribution panel. the hot conductor heats up, the insulation melts and touches ground.
5) An properly installed isolation transformer provides physical galvanic isolation, whereas a galvanic isolator depends on semiconductors, which have and do fail in a manner that no one knows it happened.
6) A shoreside pedestal with a ELCI breaker will trip if the shorepower hot touches the metallic case of a boat-grounded isolation transformer. My isolation transformer has an ELCI breaker ahead of the primary input.
7) An isolation transformer provides a means to step up/down voltage and/or create split phase power when none exist on the dock.
8) There is a reason expensive medical devices are installed behind an isolation transformer, they provide the best safety for the patient, workers, and the equipment.
DotDun is offline  
Old 01-07-2018, 20:56   #11
CF Adviser
 
Pelagic's Avatar

Cruisers Forum Supporter

Join Date: Oct 2007
Boat: Van Helleman Schooner 65ft StarGazer
Posts: 7,846
Re: Onboard Isolation Transformer vs Safety

I agree with DotDun.

You can nitpick failure scenarios due to improper connections, but I think the key question is....:

Where are the most usual and highest probability of failures?
....and does a (properly installed) onboard isolation transformer protect you and your investment from that most common failure?

There is no doubt in my mind that the biggest weakness and danger is from then power runs along a floating dock and corrosion/stress damage at the plug in pedestal, whose quick pin connectors are constantly being abused by weather and rough handling.

I have seen it so many times
Rats eating the partially submerged cables
Click image for larger version

Name:	20180621_103303.jpg
Views:	71
Size:	88.4 KB
ID:	172851Click image for larger version

Name:	20180621_103254.jpg
Views:	79
Size:	89.2 KB
ID:	172852

Just happened last month at our dock where they fried themselves when they finally ate thru the core. (Smell of more than a few dead rats lasted a week)

Same with accelerated corrosion, on the pedestal plug due to years of abuse. We have all repaired them.

To my understanding, an isolation transformer like my Victron specifically isolates the boat from large or small ground faults, from damaged marina cables, or exposed pedestals.

While it may not protect the rats or anyone swimming around electrified floating docks. It will protect you and your boat
Pelagic is offline  
Old 01-07-2018, 21:30   #12
Registered User

Join Date: Mar 2016
Boat: McCurdy & Rhodes Custom 46
Posts: 261
Re: Onboard Isolation Transformer vs Safety

As someone who has experienced very unpleasant electrical shocks (when I was young I learned slowly) I will say that I want no part of more experiences like those. I have also spent time designing, and getting fda and CE approval for, medical surgical equipment. Isolation transformers were common.

It is clear to me that an isolation transformer is a technically superior system in terms of protecting a boat. I also believe that the vast majority of issues are installation or inspection and maintenance problems as pointed out by Pelagic.
I’m rebuilding my shore power and will use an isolation transformer.
But a ground isolator is not a bad compromise either.
dfelsent is offline  
Old 02-07-2018, 04:51   #13
Registered User

Join Date: Oct 2007
Posts: 114
Re: Onboard Isolation Transformer vs Safety

Quote:
Originally Posted by DotDun View Post
1) My post stated 'a properly installed' isolation transformer.
2) Your failure scenario depends on an improperly installed isolation transformer.
3) You need to provide evidence that your failure scenario has ever actually happened, please provide links.
4) The same failure you describe can happen with a breaker/distribution panel. the hot conductor heats up, the insulation melts and touches ground.
5) An properly installed isolation transformer provides physical galvanic isolation, whereas a galvanic isolator depends on semiconductors, which have and do fail in a manner that no one knows it happened.
6) A shoreside pedestal with a ELCI breaker will trip if the shorepower hot touches the metallic case of a boat-grounded isolation transformer. My isolation transformer has an ELCI breaker ahead of the primary input.
7) An isolation transformer provides a means to step up/down voltage and/or create split phase power when none exist on the dock.
8) There is a reason expensive medical devices are installed behind an isolation transformer, they provide the best safety for the patient, workers, and the equipment.

1) My post stated 'a properly installed' isolation transformer.
ANSWER : A “properly installed” isolation transformer has the grounding conductor of its input power source bonded to the chassis (case) of the transformer, so that if the hot conductor shorts to the chassis, the grounding conductor will return the current to the input source and trip the CB.
2) Your failure scenario depends on an improperly installed isolation transformer.
ANSWER : See above.
3) You need to provide evidence that your failure scenario has ever actually happened, please provide links.
ANSWER : Scenario analysis does not depend on historical events.
4) The same failure you describe can happen with a breaker/distribution panel. the hot conductor heats up, the insulation melts and touches ground.
ANSWER : You have just stated the facts and answered your own information request in item #3 above. Thank you. You have described a common failure observed over and over in houses and plants. A loose connection on a hot wire terminal gets hot, melts the insulation and the conductor shorts to the chassis. Because the chassis of the distribution panel is bonded to the input power ground (per NEC) the CB trips and prevents a fire. If the CB does not trip, the event gets worse and typically makes the news. The same NEC regulation must apply to your isolation transformer on your boat for it to be properly installed and thus safe.
5) An properly installed isolation transformer provides physical galvanic isolation, whereas a galvanic isolator depends on semiconductors, which have and do fail in a manner that no one knows it happened.
ANSWER : American Boating and Yachting Counsel (ABYC) has recommended regulations that govern “fail safe” galvanic isolators and monitors.
6) A shoreside pedestal with a ELCI breaker will trip if the shorepower hot touches the metallic case of a boat-grounded isolation transformer. My isolation transformer has an ELCI breaker ahead of the primary input.
ANSWER : Several people have already pointed out in this post that the electrocution concern is mainly applicable to fresh water and brackish water marinas. Fatalities that I remember reading about occurred in fresh water marinas.
7) An isolation transformer provides a means to step up/down voltage and/or create split phase power when none exist on the dock.
ANSWER : Yes. Item #7 is correct. Thank you.
8) There is a reason expensive medical devices are installed behind an isolation transformer, they provide the best safety for the patient, workers, and the equipment.
ANSWER : We are not recommending galvanic isolators for patient safety in hospitals, just for diver/swimmer safety near boats.
Ahmet Erkan is offline  
Old 02-07-2018, 05:46   #14
Registered User

Join Date: Oct 2007
Posts: 114
Re: Onboard Isolation Transformer vs Safety

Quote:
Originally Posted by dfelsent View Post
As someone who has experienced very unpleasant electrical shocks (when I was young I learned slowly) I will say that I want no part of more experiences like those. I have also spent time designing, and getting fda and CE approval for, medical surgical equipment. Isolation transformers were common.

It is clear to me that an isolation transformer is a technically superior system in terms of protecting a boat. I also believe that the vast majority of issues are installation or inspection and maintenance problems as pointed out by Pelagic.
I’m rebuilding my shore power and will use an isolation transformer.
But a ground isolator is not a bad compromise either.


dfeltsent,
Here are a few thoughts about single point failures with isolation transformers and galvanic isolators.
A single point failure is not desirable for obvious reasons. There are also cases that a single point failure cannot be eliminated but the risk of occurrence is minimized by various means. For example a small airplane cannot have a redundant second engine (because it would be too heavy, too expensive etc) but a single engine airplane has a special engine with dual redundant spark plugs, magnetos, fuel tanks etc. Not only these dual redundant systems are built in but the pilot verifies each redundant system is operational prior to flight.
Lets look at an isolation transformer installed at the dock. The neutral of the transformer is not grounded. The hot, neutral and ground conductors are brought into the boat via the shore power cable. The underwater metals of the boat are electrically floating. (as well as floating on the water)
Now the system does not have single point failures. You do not have to connect the shore grounding conductor to the underwater metal of the boat. If any hot conductor shorts to the hull the diver will not get electrocuted because there is no return path to ground from the hot conductor.
The only way the diver can get electrocuted is if two failures occur simultaneously. For example if the hot conductor shorted to the hull and the neutral conductor shorted to the ground. A caveat with the systems that do not have single point failures is an effective warning system must exist when a failure has occurred.
So please do not think that an isolation transformer on board is going to make your corrosion problems go away and provide safety just like it does for the patient at the hospital.
People who have disconnected the shore power grounding conductor from the underwater metals of their boats because they have been misguided by the manufacturers of isolation transformers. Get a galvanic isolator. Your boat is a disaster waiting to happen,
especially in a fresh water marina.

Ahmet Erkan is offline  
Old 02-07-2018, 07:07   #15
Registered User

Join Date: Jun 2007
Location: SW Florida
Boat: FP Belize, 43' - Dot Dun
Posts: 3,834
Re: Onboard Isolation Transformer vs Safety

Quote:
Originally Posted by Ahmet Erkan View Post
1) My post stated 'a properly installed' isolation transformer.
ANSWER : A “properly installed” isolation transformer has the grounding conductor of its input power source bonded to the chassis (case) of the transformer, so that if the hot conductor shorts to the chassis, the grounding conductor will return the current to the input source and trip the CB.
That is not true. The shorepower ground is connected to the internal static shield of the transformer. The metallic case of the transformer needs to be bonded to boat ground, not shorepower ground.
__________________

DotDun is offline  
Closed Thread

Tags
safety

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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


Similar Threads
Thread Thread Starter Forum Replies Last Post
Isolation Transformer for Europe Charlie Electrical: Batteries, Generators & Solar 22 27-06-2011 07:55
Victron Isolation Transformer Voltage Increase Ultimarv Electrical: Batteries, Generators & Solar 5 06-09-2010 10:42
ELCI and Isolation Transformer Microship Electrical: Batteries, Generators & Solar 20 19-08-2010 16:54
Isolation Transformer and SSB Sailabel Marine Electronics 3 02-04-2010 17:30
Isolation transformer Pa La O La Construction, Maintenance & Refit 11 12-08-2008 14:14



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 22:57.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2018, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.