The benefits of Marine Isolation Transformers.
Running generators in marinas is noisy and pollutes the air. Hooking up to shore power is convenient and ecologically correct but unfortunately also the cause of numerous serious accidents and incidents.
- Fatal electric shocks to persons and pets on board and in the water around the boat caused by ground fault leakage due to faulty installation or defective cables and equipment.
- Electrical fires, mostly caused by ground fault stray currents, including ignition of fuel vapors or hydrogen leaking from batteries.
- Corrosion of propellers and other metal parts due to DC currents returning to earth via the green safety conductor in the shore cable, in worst cases holing steel- or aluminum hulls.
- Nuisance tripping of the GFI circuit breaker in the pedestal, especially since 2017 when NEC lowered the ground fault limit from 100mA to 30mA.
National electric systems worldwide utilize earth ground so any fault current must travel back to its source through earth driven by the difference in potential. The voltage differential is rarely life threatening on land, but on water a local difference in voltage between metal parts on the boat and earth ground may be fatal. A related hazard is faulty polarization by installers mistakenly swappingneutral and hot conductors. – Since the amount of fault current passing through a person is proportional to the voltage, in most foreign countries the risk of fatal electrical shock is much higher due to 230/400V AC distribution systems, virtually doubling the importance of safety precautions in Europe as compared to North America.
The bottom line is that whereas universal grounding promotes personal safety on land, it can be a serious liability on a boat, and as a result owners and builders increasingly understand that the solution for eliminating or mitigating electrical incidents is an isolation transformer.
According to ISO 13297:2018 (boats up to 80 ft). ”The function of a shore power isolation transformer is to electrically isolate all the normally live conductors and the protective conductor on a craft from the electrical power grid on shore”. In short. Installing a transformer severs the connection between the boat and shore ground, rendering the green conductor in the shore cable obsolete.
On land isolation transformers are used for personal protection in safety critical areas such as hospital operating rooms, emergency suites and intensive care units where isolated power systems are mandatory, subject to IEC/UL 60-601 Standard, limiting the max leakage current to ground to 0.5 mA. This degree of protection is much higher than that of 5-10 mA residential GFI devices and the 30 mA limit now required in marina electrical systems. – Medical isolation transformers feature several layers of primary to secondary insulation, each exhibiting a dielectric strength of 5-10 kilovolts, resulting in a total isolation resistance in the order of 25 megohms. There’s no galvanic path, so only capacitive leakage can occur. Iso-Puck™ brand isIEC/UL 60-601 certified medical grade isolation transformers, resin potted in non-metallic enclosures for maximum structural integrity.
Marine isolation transformers have been around since the mid-nineties and originally featured traditional architecture including E-I laminated cores and straight helical coils. The advisory ABYC standard prescribes a metallic shield located between the primary and secondary windings connected to earth ground via the green wire in the shore cable and furthermore, that the transformer casing must be metallic and bonded to boat ground. – Unfortunately, in a transformer constructed along those lines the risk remains of internal wires working loose due to engine vibration and either inadvertently re-connecting boat ground and earth ground or arcing against the case creating a dual fire and touch hazard.
In contrast Marine-Puck™ UL marine certified boat isolation transformers emulate Iso-Puck™ IEC/UL 60-601 certified medical isolation transformers. The non-metallic enclosure cannot corrode and does not require grounding, and transformers can be mounted onto any surface on board without reference to earth- or boat-ground. With these precautions it would be counterproductive to bring the shore ground conductor on board and into to the transformer case, but since the green conductor is present in most shore cables, for reasons of safety it should at least be capped off and insulated from boat ground where it enters the vessel.
Of course, after adding an isolation transformer, wiring on board may still comply with the voluntary ABYC standards, including bonding the transformer secondary neutral to hull/engine ground, but it is interesting that an increasing number of boat owners in Europe and lately also in Canada, choose to emulate UL/IEC 60-601 standards by not connecting the secondary winding to boat ground. The action represents a step further towards maximum safety, since with no bond in place a person can stand in water on a metal deck or touch boat- grounded metal parts with one hand while grabbing a live conductor with the other hand in perfect safety. In the “Floating IT System” appliances are still protected in case of shorts or overload by the panel circuit breakers, but any galvanic leakage current to boat ground or to the water is blocked by the isolation transformer
Fellow subcommittee members.
I was happy to participate in the meeting on Tuesday and am looking forward to seeing the revision proposal.It will be useful if we all get a chance to comment in writing priorto the next zoom meeting.
I hate to bore you again but have attached a package of material to further support my argument that our construction is safer in all respects than the solution advocated in current ABYC E-11.
When we have chosen to base our product on our earlier program of Iso-Puck medical isolation transformers it is because mostsafety requirements of UL/EIS 60-601 are tougher than those required by standards covering boat transformers. UL added tests specific for marine use and issued our ULmarine listing to testify that Marine-Puck are safe for use as isolation transformers on boats. Our product is selling well in the US, Canada and overseas.
As promised I included pages from UL/EIS 60-601 describing the alternative provided between d) a protective earthed screen and e) reinforced or double insulation separating primary and secondary windings. The HI-pot test required at 250VAC service voltage is 4KV.
I will try to explain again why I consider the screen a safety hazard on a boat hooked up to shore power regardless of whether the transformer is toroidal or traditional:
It requires current of at least 50-60 A to trip a shore breaker momentarilyand in theory a primary/secondary short willcause the pedestal breaker. The breaker trips and the touch hazardis gone. However, the amount of fault voltage depends on the location in the windings where the arc through occurred. The difference in potential can well be much lower than the service voltage and unable to support 30A,in which case the current will keep flowing and the touch hazardsustained indefinitely. Furthermore, at those current levels there’s a risk of wires and components including loose connections heating up,creating a fire hazard. It is not a good idea to depend on a current of these magnitudes to trip a safety system on a boat
In my opinion it is a far safer solution to add one or more layers of safety in the form of reinforced insulation, and of course a non-metallic case. Cable connections to the cable inlet point and bonding metal cases etc. to boat ground are vulnerable to chafing or severing. Much safer to eliminate those hazards with a class II device which does not require shore ground connection or boat ground bonding.
Some membersmentionedVictron isolation transformers,but Victron does not claim compliance with ABYC in its literature. Strangely, the schematic shows a shield connected to PE (ship) not to shore ground. It could be an electrostatic, interference barring shield. Definitivelynot a safety shieldas per ABYC. Also, Victron are ventilated Class 1transformers in metal enclosureswithout ignition protection. Not suitable for installation in an engine compartment.
In conclusion, I attached my abbreviated article dealing with the issues we discussed. As always, any comments are very welcome.