Isolation transformer for residential dock power

[smithlakeelectrical]

That's how I read it also. How are the marine transformers getting by this? Is it due to the fact that they are "intended" to be installed on the vessel? I came across this attached proposal for code change from a Georgia Tech EE. This is an issue that can be solved on site and needs to be addressed by the NEC. There are thousands of power company customers on just this lake that can't protect their own swim piers due to the bonding verbage, and the POCO resists installing the blockers at all costs.

 

[synchro]

... I came across this attached proposal for code change from a Georgia Tech EE. ...

From what I found, this is probably the proposal from Georgia Tech to the NFPA and at least part of the panel's response to it:

"Carson Day, Gerogia Institute of Technology - NEETRAC
19-107
The following text would be a new paragraph under section 555. Below are the changes
recommended to the original change proposal submitted in November 2011.

555.16 Mitigation of Neutral Related Stray Voltages and Currents
To provide protection for neutral related stray voltages and currents, a suitably rated isolation transformer (a separately derived system) at the branch circuit service panel supplying the shore power shall be permitted.
The following shall be required configuration is recommended for the isolated system:
(1) The isolation transformer should be double insulated or its equivalent and shall have an internal shield between the windings that is rated to carry full fault current.
(1)The isolation transformer shall have overcurrent protection on the supply side as required in 450.3.
(2) The isolation transformer shall be provided with a ground fault protection device on
the load side not exceeding 30mA (UL943C Class B Ground Fault Circuit Interrupter)
(3) Metal enclosure and internal shield conductor of the transformer shall be connected to the supply side neutral and grounding system as required by 250.4 (A).
(4) The load side neutral and equipment grounding conductors shall be connected together and grounded on the secondary side of the transformer as required by 250.20(8). To provide adequate isolation, the installed grounding electrode shall should be located at least 6' from the nearest grounding electrode of the supply side and shall should be connected to the transformer by an insulated grounding conductor.
(5) The location of the isolation transformer shall be on the load side of the service disconnecting means panel containing breaker and/or disconnecting means and shall not be below the electrical datum plane."

And the apparent response:
"As previously written in CMP-19's action and statement on Proposal 19-107, the installation of a transformer intended to isolate stray voltage (current) is not presently prohibited. The NEC is not intended as a design guide.
CMP-19 is very sympathetic to the evident safety concerns surrounding electrocution hazards at marinas and boatyards."

Note that the proposed isolation transformer is on the load side of the service disconnect.

 

[winnie]

I don't get the panel response. The primary circuit requires an EGC, and that EGC is connected to (among other items) the transformer chassis and core. Similarly the secondary requires a connection to the transformer chassis and core, in addition to any required earthing. Thus the primary grounding electrode system will automatically be connected to the secondary grounding electrode system. This defeats the desired isolation.

I suppose you could design a transformer that has a non-metallic enclosure some sort of split core, with sufficient insulation between primary and secondary side that the EGCs do not need to be joined. Such a beastie might be installed to isolate primary and secondary grounding systems yet still be NEC compliant.

-Jon

 

[winnie]

This is a link to the Georgia Tech / NEETRAC slide set showing their measurements at various docks using their proposed isolation arrangement:

https://www.neetrac.gatech.edu/publications/IEEE_PES_Boat_Docks-January2013.pdf