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Isolation transformer; how hot do they run?

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bobk

bobk

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48' MOTOR YACHT-Series I (1981 - 1984)
I posed this question earlier in a mature thread and guess it got lost. The isolation transformer on my boat is almost too hot to touch. I'd guess 55C when running the four AC's etc.. Is that typical or do I need to get concerned? And if so, where can they be had?

Bob
 
If it gets too hot to touch that is too hot. That really goes for most electrical item i.e. motors, plugs etc. If you have in IR gun, ideally it should be below 120 degrees F.

Ted
 
Fully loaded, very warm is ok, but not burning hot.
 
Bob
With 4 a/c's and everything else running mine gets hot but you can just stand to hold your hand on it. I think we have similar boats.
Chris
 
chris said:
Bob
With 4 a/c's and everything else running mine gets hot but you can just stand to hold your hand on it. I think we have similar boats.
Chris
Click to expand...
Chris, yes our boats are similar. And your description of the temperature of the transformer is about like mine. I can just keep my hand on it. Thanks

Bob
 
On our 36C, the isolation transformer is only on the ship's power line. The other 30 amp circuit is devoted to the 2 AC units and doesn't go through the transformer. To explain the transformer better is tough, but here goes. They have 2 windings, one inside the other, but not touching. One is energized by the shore power, which energizes the other. Remember, the wiring is seperate, which is why it can take care of wiring errors ashore.
 
If you are running half of your boat on an isolation transformer and the other half on the shore power. You are defeating the purpose of the transformer.



BILL
 
An Isolation transformer is, in reality, the boat's power source. This is so since shore power is fed into the primary winding of the transformer and then it is transferred to the secondary winding magnetically. The only part of the IT that is grounded ashore is the shield of the primary winding. In other words, there is NO direct connection between the shore electrical supply and the boat's AC circuits.

Threrefore, any AC current leaks in the boat's AC circuits will go as far as the IT and not to shore supply. Thus, leaks can't find a path to ground thru the water and create havoc in the underwater gear.

Since the grounding wire from the shore supply is NOT connected to the boat's grounding, with the use of an IT, it means that a path for galvanic currents does not exist between the boat and the shore electrical supply. In other words the boat is isolated from the shore electrical supply and thus the name.

As to why the IT is hot to touch, I would check the shore voltage reaching the IT, electrical connections and damaged diodes.

CapetaniosG
 
This is a complicated topic now, and i really don't worry about these things unless there is a concern that someone will do something electrically which can hurt someone. We're getting close.

Typically, transformers cannot be used to protect one half of a power supply. The reason for this is that the neutral in a transformer is not on neutral with respect to ground or in this case the marina neutral. It is a "manufactured" neutral depending on how the transformer was wound. Bonding the two neutrals as they most certainly are in the boat would usually blow a breaker as there can be as much as 70 volts difference in the neutrals.

BUT this does not apply to most isolation transformers as the manufacturer expects partial circuit protection, i.e. computers or other sensitive equipment. Good isolation transformers are made with balanced circuits, that is why they cost more and bonding the neutrals together is expected and causes no problems.

What concerns me is that if the heat is excessive, there MIGHT be a small difference in neutral voltage causing it. A high resistance bond (green wires) on the boat could get hot in that instance and that would be bad obviously.

The way to check if there is a problem is to measure the AC voltage from the neutral (wide prong) in any outlet on the boat to the neutral on one of the air conditioning unit neutrals or the frame of the AC unit for that matter. If there is any voltage neutral to neutral that is not good and MUST be corrected.

This kind of problem is one that only industrial trained electricians are familiar with. The house guy, no matter how good, doesn't run into these issues and will not give you good info.

Ted
 
If you have two shore feeds, one going through an IT and the other not, you have a potentially dangerous situation on board. That was PROBABLY not original Hatteras.......

There's some good info in this thread and some not-so-good.

A quick primer on "split-phase" power...

The power in your house (but not necessaily at your office!) is what is commonly called "single phase." That's not really quite accurate - it would be more accurately called "split phase" for the outlets in your home.

The feed in your house has two "hots", each of which is 120V referenced to ground (earth), but they are 180 degrees out of phase. That is, when one is going positive the other is going negative. Thus, if you use both hots, you get 240V.

Ok, so how do you get "neutral". Neutral is actually not "neutral". It s technically called "grounded conductor". At the main service entrance panel in your house you have an earth ground which is a big fat wire or strap connected to the earth - usually via a rod driven (literally) into the ground. At that point - and only at that point - the white wire ("neutral") and ground are connected together. This is because in the us, power feeds actually use the earth as part of the system - on purpose! This was a design decision made when the US energy system was very young and it is not the case everywhere - Europe has a (mostly) "bifilial" system where earth is not intentionally used as a return path.

Now how does this relate to a boat?

In a number of ways.

If you have a system without an isolation transformer, neutral is carried from the main panel at the marina - it must not be connected to ground (safety ground) anywhere else, including on board. Neutral - or grounded conductor - carries significant current (potentially the same as either "hot" leg!) and as a consequence of wire resistance there can be material voltages on that line. Thus, EXCEPT at the main service panel for the marina, you can cause quite-large currents to flow if you bridge those two anywhere else AND create a serious safety hazard.

An isolation transformer normally carries only three wires in from the pedestal (assuming a 240V/50A system) - the two hots and the safety ground. "Neutral" is not connected at all! Safety ground goes only as far as the primary of the isolation transformer, and is connected to a "shield" around the primary windings, with the intent of causing the inlet fuse to blow if the primary were to short against the case. In this case shore "ground" is NOT carried on board to the boat's ground system - this is the true benefit of an isolation transformer, in that it makes the return path for any fault current LOCAL TO THE BOAT. That prevents you from both energizing the water (and possibly killing someone swimming near the boat) AND causing electrolytic damage to your (and someone else's) underwater metals. Without an isolation transformer a fault in ANY boat connected to the same marina's dock power can be carried back to the marina ground via YOUR boat, creating both a safety and electrolysis risk. The largest electrolysis risk comes from DC leaks, with the primary risk from AC leaks being death.

If you're running your generator - or you have an isolation transformer - then that device is an energy "source". As such a genset - or the secondary of an isolation transformer - must connect both "Grounded conductor" (neutral) and safety ground together at that point. In this circumstance you have no electrical connection to the other boats at your marina - faults in your boat cannot affect them, and faults in their boats cannot affect you.

This is why your breaker on the boat's AC panel (if you have a slide-lockout) for the genset has THREE breakers ganged in it rather than two, even though there are only two hots. Neutral must be correctly connected so that it either (1) is carried back to the dock if you have no isolation transformer and are on shore power or (2) is BOLTED to safety ground on board if you DO have an isolation transformer or are running on your genset.

If you have a system on your boat where an isolation transformer feeds house loads but you have a second feed line for certain things (e.g. AC units) you have a potentially dangerous condition on board AND you have completely defeated the purpose of the isolation transformer. While it is possible to set up such a "secondary feed" safely, doing so always destroys the purpose of the isolation transformer - so if you're going to do that, you may as well remove the big hunk of iron as it does nothing for you.

Instead, I'd replace the isolation transformer with one that can carry the entire load and run it all through there instead, and rewire the AC side to properly handle the loads on board.

Isolation transformers are such a safety "plus" that IMHO they should be mandatory for boats with shorepower inlets. Yes, they can be heavy, but being totally isolated from the dockside electrical system is both a safety and underwater metals protector, and IMHO is well worth it.
 
So, Karl, the "neutral" conductor of the incoming shore lines should never be connected to ground, or to the boat's bonding system, isn't that right? On my boat, which has two incoming 120vac, no 240vac, I would either have to have two transformers, or rewire the boat for 240 and then split the output of one transformer with a 240 vac primary and secondary to drive two 120vac legs...right?

Years ago I put galvanic isolators on the green (ground) legs of the incoming AC lines. I don't know how effective this has been but the underwater fittings aren't glowing brightly, and I don't eat up zincs at a huge rate, so she is probably okay. I would rather have an isolation transformer, but the weight and cost have kep me from doing it. Well, the weight anyway.
 
jim rosenthal said:
So, Karl, the "neutral" conductor of the incoming shore lines should never be connected to ground, or to the boat's bonding system, isn't that right?
Click to expand...
That is correct with respects to ON BOARD, when one is on shore power. That is, if you unplug the shore cord, you should never see continuity between the neutral "feed wire" from the shore side and the ground feed wire from the shore side. If you do, something is BADLY screwed up.

With an isolation transformer there is no direct continuity between anything in the shore cord and anything in the boat. Without one, the Neutral is passed through the shore cord to the boat, but is NOT bolted to ground in the boat. EVER.

If the selector is set for GENERATOR, then neutral and ground ARE bolted inside the boat - AT THE GENERATOR. But this had better be invisible from the shore power connection (since the selector switch for shore must be open in that circumstance!)

If, however, you had the generator STOPPED and the switch set to GEN, and took an ohmmeter and measured between ground and neutral, you SHOULD see a short. That's normal. With an isolation transformer you'd see the short even with the switch set to "SHORE", since neutral and ground are bolted at the output of the transformer. WITHOUT a transformer (and the cord DISCONNECTED - you can be KILLED screwing around on an energized AC circuit!) you should see an OPEN - if there's a short then someone has done a very bad thing in wiring the boat's AC side.
On my boat, which has two incoming 120vac, no 240vac, I would either have to have two transformers, or rewire the boat for 240 and then split the output of one transformer with a 240 vac primary and secondary to drive two 120vac legs...right?
Click to expand...
Correct. And I would do the latter for a whole host of reasons, not the least of which is that a 240/50 feed has almost 4 times the wattage capacity of a 120/30 one. BUT - proper wire sizing and such is very important, so you have to make sure you do this SAFELY.
Years ago I put galvanic isolators on the green (ground) legs of the incoming AC lines. I don't know how effective this has been but the underwater fittings aren't glowing brightly, and I don't eat up zincs at a huge rate, so she is probably okay. I would rather have an isolation transformer, but the weight and cost have kep me from doing it. Well, the weight anyway.
Click to expand...
Galvanic isolators are a blessing and a curse.

They are basically big diodes. They will stop SMALL voltage offsets from getting through but not big ones (they can't stop big ones, since the entire point of a ground is to pass the big ones so the fuse blows!)

I personally find galvanic isolators to be a huge safety risk. If they fail open - and they can - you have no safety ground protection whatsoever. Failing shorted means you have no electrolysis protection. And if someone (including you) is dumping more than about 0.7v into the water, it'll run through the isolator ANYWAY and still do the damage! So if you have a 12V bilge pump wire end up loose in the bilge, the isolator won't help.
 
Genesis said:
If you have two shore feeds, one going through an IT and the other not, you have a potentially dangerous situation on board. That was PROBABLY not original Hatteras.......

There's some good info in this thread and some not-so-good.

A quick primer on "split-phase" power...

The power in your house (but not necessaily at your office!) is what is commonly called "single phase." That's not really quite accurate - it would be more accurately called "split phase" for the outlets in your home.

The feed in your house has two "hots", each of which is 120V referenced to ground (earth), but they are 180 degrees out of phase. That is, when one is going positive the other is going negative. Thus, if you use both hots, you get 240V.

Ok, so how do you get "neutral". Neutral is actually not "neutral". It s technically called "grounded conductor". At the main service entrance panel in your house you have an earth ground which is a big fat wire or strap connected to the earth - usually via a rod driven (literally) into the ground. At that point - and only at that point - the white wire ("neutral") and ground are connected together. This is because in the us, power feeds actually use the earth as part of the system - on purpose! This was a design decision made when the US energy system was very young and it is not the case everywhere - Europe has a (mostly) "bifilial" system where earth is not intentionally used as a return path.

Now how does this relate to a boat?

In a number of ways.

If you have a system without an isolation transformer, neutral is carried from the main panel at the marina - it must not be connected to ground (safety ground) anywhere else, including on board. Neutral - or grounded conductor - carries significant current (potentially the same as either "hot" leg!) and as a consequence of wire resistance there can be material voltages on that line. Thus, EXCEPT at the main service panel for the marina, you can cause quite-large currents to flow if you bridge those two anywhere else AND create a serious safety hazard.

An isolation transformer normally carries only three wires in from the pedestal (assuming a 240V/50A system) - the two hots and the safety ground. "Neutral" is not connected at all! Safety ground goes only as far as the primary of the isolation transformer, and is connected to a "shield" around the primary windings, with the intent of causing the inlet fuse to blow if the primary were to short against the case. In this case shore "ground" is NOT carried on board to the boat's ground system - this is the true benefit of an isolation transformer, in that it makes the return path for any fault current LOCAL TO THE BOAT. That prevents you from both energizing the water (and possibly killing someone swimming near the boat) AND causing electrolytic damage to your (and someone else's) underwater metals. Without an isolation transformer a fault in ANY boat connected to the same marina's dock power can be carried back to the marina ground via YOUR boat, creating both a safety and electrolysis risk. The largest electrolysis risk comes from DC leaks, with the primary risk from AC leaks being death.

If you're running your generator - or you have an isolation transformer - then that device is an energy "source". As such a genset - or the secondary of an isolation transformer - must connect both "Grounded conductor" (neutral) and safety ground together at that point. In this circumstance you have no electrical connection to the other boats at your marina - faults in your boat cannot affect them, and faults in their boats cannot affect you.

This is why your breaker on the boat's AC panel (if you have a slide-lockout) for the genset has THREE breakers ganged in it rather than two, even though there are only two hots. Neutral must be correctly connected so that it either (1) is carried back to the dock if you have no isolation transformer and are on shore power or (2) is BOLTED to safety ground on board if you DO have an isolation transformer or are running on your genset.

If you have a system on your boat where an isolation transformer feeds house loads but you have a second feed line for certain things (e.g. AC units) you have a potentially dangerous condition on board AND you have completely defeated the purpose of the isolation transformer. While it is possible to set up such a "secondary feed" safely, doing so always destroys the purpose of the isolation transformer - so if you're going to do that, you may as well remove the big hunk of iron as it does nothing for you.

Instead, I'd replace the isolation transformer with one that can carry the entire load and run it all through there instead, and rewire the AC side to properly handle the loads on board.

Isolation transformers are such a safety "plus" that IMHO they should be mandatory for boats with shorepower inlets. Yes, they can be heavy, but being totally isolated from the dockside electrical system is both a safety and underwater metals protector, and IMHO is well worth it.
Click to expand...

Yes, Karl, Our '86 36C was wired that way from Hatteras and I have the original drawings to prove it. I believe that it is fine as the boat has 2 30 amp power inlets. One is devoted to the ship and goes through the isolation amp. The other is strictly for the 2 Cruise Air units and goes directly to them and to nowhere else.
 
Maynard Rupp said:
Yes, Karl, Our '86 36C was wired that way from Hatteras and I have the original drawings to prove it. I believe that it is fine as the boat has 2 30 amp power inlets. One is devoted to the ship and goes through the isolation amp. The other is strictly for the 2 Cruise Air units and goes directly to them and to nowhere else.
Click to expand...
Yes it does go somewhere else!

Maynard, in fact your boat has no isolation at all when it is plugged in.

Here's why.

1. Your AC units have neutral and hot connected to them. They also have safety ground connected to the boat's grounding system. (This is connected to the bonding system thorugh the engines, but is DISTINCT from it.)

2. Your isolation transformer OUTLET has neutral and safety ground BOLTED at that point. At least it should have. Otherwise, there is no such thing as a safety ground on the isolation transformer side! Note that this practice (no "safety ground") is in fact used frequently in Europe and is perfectly safe (there's no return path except back down the other conductor) BUT the entire system must be designed this way to be safe and you have the (1) side which precludes this design.

3. The AC safety grounds and every other device on board's safety grounds are connected to the SHIP'S ground.

The unfortunate reality is that the ship's grounds are connected to the shore and to neutral on shore! How come? Because back at the main panel at the marina, neutral and safety ground are bolted together!

The isolation transformer thus does exactly nothing if the second cord (for your AC units) is plugged in, other than consuming a small amount of energy (which it rejects as heat) and adding to the displacement of your boat.

I'd either fix that or remove the transformer.

I'm surprised Hatteras did this, to be honest....... but heh, I guess you learn something new every day!
 
Genesis said:
Yes it does go somewhere else!

Maynard, in fact your boat has no isolation at all when it is plugged in.

Here's why.

1. Your AC units have neutral and hot connected to them. They also have safety ground connected to the boat's grounding system. (This is connected to the bonding system thorugh the engines, but is DISTINCT from it.)

2. Your isolation transformer OUTLET has neutral and safety ground BOLTED at that point. At least it should have. Otherwise, there is no such thing as a safety ground on the isolation transformer side! Note that this practice (no "safety ground") is in fact used frequently in Europe and is perfectly safe (there's no return path except back down the other conductor) BUT the entire system must be designed this way to be safe and you have the (1) side which precludes this design.

3. The AC safety grounds and every other device on board's safety grounds are connected to the SHIP'S ground.

The unfortunate reality is that the ship's grounds are connected to the shore and to neutral on shore! How come? Because back at the main panel at the marina, neutral and safety ground are bolted together!

The isolation transformer thus does exactly nothing if the second cord (for your AC units) is plugged in, other than consuming a small amount of energy (which it rejects as heat) and adding to the displacement of your boat.

I'd either fix that or remove the transformer.

I'm surprised Hatteras did this, to be honest....... but heh, I guess you learn something new every day!
Click to expand...


Karl, I have so much confidence in Hatteras that I am going to spend some time with the blueprints. As I will be on the water with the boat for the next month, I should have ample time to do the research. I am sure they have a way to make their wiring work, otherwise they wouldn't have wasted their money on an isolation transformer in the first place.
 
Maynard,
It seems to me that this design is only good when you do not have the 2nd cord connected. I have 2 isolation transformers aboard and either one can be configured so that it supplies any of the loads on the boat. This is great if one of the transformers should happen to fail.

If I had your setup I think would add a second iso transformer and then I would be totally protected. It would not be too difficult to do so, and then you would be sure that you would not have any problems related to dock wiring.

Regarding temps: My transformers are warm to the touch, but definately not hot.
 
I Think Hatteras is great also. BUT! All Engineers makes mistakes. That is why they have automotive recalls. Operating like that would scare the hell out of me. Karl is correct. One or the other has got to go. It may have been an after thought by two different engineers and know one caught the OH sheet. That is just asking for trouble.



BILL
 
Beware - misinterpreting that document could get you in trouble.

He is correct in that plug in isolation transformers (e.g. those sold by Tripplite and others for use in managing electrical noise in computer equipment, often with buck/boost functionality as well) are required to pass ground. However, hard-wired isolation transformers do not, and in the marine environment, must not, or they actually increase rather than decrease hazard.
 
Well when a secondary lays on ground (happens with some regularity), what is your opinion on what happens?

Thanks.

Ted
 
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