Polarization, Isolation Transformers, and galvanic isolators

[REBrueckner]

I recently observed the survey of an Aluminum hull boat’s electrical system and the surveyor and I were unsure exactly what the “isolation’ transformer aboard [ISOLATION was stamped on the transformer case] provided in terms of galvanic isolation from unwanted shore currents. On board testing revealed there were no corrosion currents from shore aboard.

[ I think my former 1972 Hatt 48 YF had a “polarization” transformer, but I can’t remember if the shore power ground and neutral were isolated from the boat or not. I did make tests and recall there was no leakage [corrosion] current.]


Synopsis: A “polarization transformer’ [and of course an isolation transformer] MAY be used to provide ‘isolation’ from unwanted shore power corrosion currents.


In both types transformer, neutral [white conductor ] from a shore power cord is typically not brought aboard the boat…it is not connected to the secondary side of the transformer, only the primary side. The major difference between a polarization transformer and an isolation transformer is that in properly wired isolation systems there IS a break, that is no continuity, in the grounding [green] wire between the primary and secondary sides of the transformer. The shore power grounding system is ‘isolated’ from the boat grounding system providing corrosion current protection aboard from any nasty currents ashore.

With “polarization systems” [meaning transformer plus the wiring] the grounding wire continuity IS maintained from the shore grounding connection to the secondary side of the transformer. So wired this way, there is the possibility of unwanted corrosion currents from shore power ground. [Well that can’t be how an Aluminum boat transformer is wired; it is too subject to corrosion.]

Solution, I am guessing: eliminate the ground connection between primary and secondary of a polarization transformer and in effect create the desired isolation. Can anybody confirm this?

Testing: [You can check for isolation this way aboard any boat.]
[This was a three wire 125/250V 50A shorepower cord, neutral white, two hot, one red,one black.]
Shut off any inverters and generators aboard, leave the main breakers and at least a branch breaker on to be used for testing; then check the continuity [resistance] between the neutral at the on board shore power cord connection point [primary side of the transformer] and neutral aboard [secondary side of the transformer] say at a receptacle, and also the shore power cable ground on the ring of the shore power cable to ground aboard. “Isolation” means these should be open, no continuity, infinite resistance. So No shore power corrosion currents can get aboard.

Note[1]: The second source below discusses the fact that on board equipment with high frequency [HF] circuits may render galvanic isolators ineffective. [They may cause the diodes to continually conduct.] For example, such HF oscillators may replace transformers such as in battery chargers: STATPOWER battery chargers use such technology. So the article recommends isolation transformers over galvanic isolators. The article also claims in certain circumstances isolation transformers are not entirely safe in protecting swimmers. Calder claims swimmers WILL be safe. It is not to me obvious if Rifkin’s argument is sound or not. Rifkin claims an isolation transformer is safer than a polarization transformer better protecting swimmers.

Note[2]: An isolation transformer must have a metallic shield installed between the primary and secondary windings that is electrically insulated from all other portions of the transformer and must meet certain ABYC standards. A polarization transformer may not have a shield or may have a shield that does not meet all ABYC requirements.

Sources:
Calder, Boatowners Mechanical and Electrical Manual, under “Isolation Transformers” and the following pages [pg 103 second edition]

Rifkin, Isolation or Polarization,
Which is Safest?
http://www.iamimarine.org/iami/IAMI_OCT06.pdf

Any comments, corrections, welcome.

[drburke]

Rob, interesting post.

In the case of a pure isolation system, what is the outcome of a scenario where an electrical device faults shorting one power leg to boat ground which is now fully floating with respect to shore?

Doesn't this introduce a personnel hazard condition that the polarization scheme does not?

DAN

[REBrueckner]

Dan,
Rifkin discusses some fault scenarios of concern. I have not studied them to
determine what if any opinion I might have.

If I understand your question correctly, any fault from hot to ground immediately trips a
circuit breaker shutting off power regardless of the transformer type, even if there is no transformer. Neutral and ground are connected at any power source....genny, inverter or transformer secondary, regardless of type, when providing power. Nothing 'floats'unless the connection to ground is lost; That is how correct polarity is provided aboard even if improperly reversed from shore

edit: In the above example, a GFCI would also trip the circuit off , if there is one, as an imbalance between hot and neutral will occur.