Boat Dock Exposure Voltage Mitigation
Working Group on Voltages at Publicly and
Privately Accessible Locations (P1695)
January 2013
Frank Lambert and Shashi Patel
Georgia Tech / NEETRAC
Problem
Statement
Swimming or working in the water around docks with customer
owned electrical circuits and equipment installed in accordance with
current applicable standards can be unsafe because the water around
a dock can be at a different potential than the dock for a variety of
reasons.
Proposed Solution
• Create a new separately
derived source that is ground
isolated to feed the dock’s
loads.
• The secondary side ground
would be bonded to dock
equipment as it normally
would, but this ground would
have no reference to the
distribution system neutral.
• This would reduce or
eliminate the neutral to water
voltage at docks.
Dock Isolation Unit
Test Site 1
•
•
•
•
Permanent isolation transformer installation.
Only tested with transformer secondary grounded.
NEV hazard was mitigated by use of the isolation transformer
Contact voltage hazard was also mitigated
NEV Measurements
Without Iso Tx
Loc
With Iso Tx
Vexp-oc
Vexp-cc*
Vexp-oc
Vexp-cc*
Fr-W1
11.9
9.8
1.3
1.0
Fr-W3
16.1
13.1
1.7
1.3
Deck-W3
15.8
3.6
1.7
0.6
*Closed circuit voltage is same as exposure current
Isolation Tx had ground on secondary
All measurements in Volts (V)
Circuit Configuration
5
Test Site 2
•
•
•
•
Test site with lowest NEV measured (under 10V).
Tested with transformer secondary grounded and floating
NEV hazard was mitigated by use of the isolation transformer
Contact voltage hazard was also mitigated
NEV Measurements
Without Iso Tx
With Iso Tx2
Vexp-oc
Vexp-cc1
Vexp-oc
Vexp-cc1
Fr-W1
3.6
3.0
0.7/0.1
0.6/0.1
Fr-W4
5.0
4.3
0.8/0.7
0.7/0.5
Light 2-W
4.7
2.5
0.8/0.1
0.4/0.3
Loc
1 Closed
circuit voltage = exposure current
2 For
Iso Tx readings: (with secondary
grounded/with secondary floating)
All measurements in Volts (V)
Circuit Configuration
7
Test Site 3
• Testing was performed at an additional site where the highest NEV
measured was over 50V.
• NEV hazard was mitigated by use of the isolation transformer
• Contact voltage hazard was also mitigated
NEV Measurements
Without Iso Tx
Loc
With Iso Tx
Vexp-oc
Vexp-cc*
Vexp-oc
Vexp-cc*
Fr-W1
19.7
15.2
0.6
0.6
Fr-W2
22.5
17.6
1.0
0.6
Deck-W1
20.3
5.5
0.8
0.7
*Closed circuit voltage = exposure current
All measurements in Volts (V)
Neutral Exposure Voltage Magnitude at Site 3
Approximately 48 Volts
NEV is related to
running of a local
pump storage hydro
plant
9
Boat Dock Testing at Site 3
Stray Voltage Testing – Open Circuit
Voltage
*Configuration not tested
11
Stray Voltage Testing – Closed Circuit Voltage
*Configuration not tested
12
Contact Voltage Testing – Open Circuit Voltage
*Configuration not tested;
results would have been
similar to sites 1 and 2.
13
Contact Voltage Testing – Closed Circuit Voltage
*Configuration not tested;
results would have been
similar to sites 1 and 2.
14
NEC Change Proposal
• Submitted Changes for:
– Article 555 – Marinas and Boatyards (Code Panel 19)
– Article 682 – Natural and Artificial Made Bodies of
Water (Code Panel 17)
• Change Proposal Process (dates for NEC-2014):
Change Proposal
Submission
(by 11/4/2011)
Code Panel Report on
Proposal (ROP) Meeting
Panel 17: 1/19-21/2012
Panel 19: 1/9-14/2012
Code Panel (ROP)
Final Ballot Results
3/9/2012
Comment
Submission
(by 10/17/2012)
Code Panel Report on
Comments (ROC) Meeting
Panel 17: 12/3-5/2012
Panel 19: 12/6-8/2012
Code Panel (ROP)
Final Ballot Results
3/22/2013
Change Proposal Language (555.16)
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 configuration is recommended for the isolated system:
(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.
(3) Metal enclosure 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(B). To provide adequate isolation, the installed grounding electrode shall
be located at least 6' from the nearest grounding electrode of the supply side and
shall 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 and shall not be below the electrical datum plane.
16
Change Proposal Language (682.16)
682.16 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 configuration is recommended for the isolated system:
(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.
(3) Metal enclosure 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(B). To provide adequate isolation, the installed grounding electrode shall be
located at least 6' from the nearest grounding electrode of the supply side and shall
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 and shall not be below the electrical datum plane.
17
Comments / Contact Info
Frank C. Lambert
Georgia Tech / NEETRAC
404-675-1855
frank.lambert@neetrac.gatech.edu