
Hooking up to shore power is convenient and ecologically correct, but unfortunately also the cause of serious accidents.
- Fatal electric shocks to persons and pets on board and in the water due to faulty installation or defective cables and equipment.
- Electrical fires.
- Corrosion of propellers and other metal parts due to DC currents returning to earth via the green safety conductor in the shore cable.
- Nuisance tripping of the GFI circuit breaker in the pedestal, especially since 2017 when NEC lowered the ground fault limit from 100mA to 30mA.
THESE HAZARDS ARE ELIMINATYED BY INSTALLING AN ISOLATION TRANSFORMER IN YOUR BOAT.
Bridgeport Magnetics Marine-Puck™ isolation Transformers are:
- Compact and easy to install.
- UL marine certified for personal safety. A US Coast Guard requirement.
- They are ignition protected, certified by Imanna Laboratories.
- Toroidal, lighter than conventional models.
- Efficient and cool running.
- No annoying hum.
National electric systems worldwide utilize earth ground so any fault current must travel back to its source through earth driven by a difference in voltage. The differential is rarely life threatening on land, but on water voltage differences between metal parts on a boat and earth ground may be fatal. A related hazard is faulty polarization by installers mistakenly swapping neutral 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 than in the US, 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 while 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: 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, limiting the max leakage current to ground to 0.5 mA. This degree of protection is much higher than that of residential GFI devices and the 30-mA limit now required in marina electrical systems. Bridgeport Magnetics brand Iso Puck ™- Medical isolation transformers feature several layers of primary to secondary insulation, each exhibiting a dielectric strength of 5-10 kilovolts. Iso-Puck™ are IEC/UL 60-601 certified medical grade isolation transformers, resin potted in non-metallic enclosures.
Shore Power isolation transformers originally featured E-I laminated cores with straight coils. ABYC recommends a metallic shield separating the primary and secondary windings and 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 fire and chock hazard.
Marine-Puck™ ULmarine certified boat isolation transformers emulate Iso-Puck™ 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 inherent in most shore cables, for safety reasons it should at least be capped off and insulated from boat ground where it enters the vessel.
When installing a Marine-Puck isolation transformer US boaters may still follow ABYC recommendations including bonding the transformer secondary neutral to hull/engine ground, but most builders and owners elsewhere choose to eliminate any bond connecting hull ground with shore ground. Appliances on board are protected towards shorts and overload by the panel circuit breakers, and any 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 prior to 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 most safety 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 momentarily and in theory a primary/secondary short will cause the pedestal breaker. The breaker trips and the touch hazard is 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 hazard sustained 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 members mentioned Victron 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. Definitively not a safety shield as per ABYC. Also, Victron are ventilated Class 1 transformers in metal enclosures without 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.