Boat Dock Exposure Voltage Mitigation

Transcription

1 Jodie Lane National Conference Boat Dock Exposure Voltage Mitigation Frank C. Lambert December 9, 2013

2 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. 2

3 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 3

4 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 With Iso Tx Loc V exp-oc V exp-cc * V exp-oc V exp-cc * Fr-W Fr-W Deck-W *Closed circuit it voltage = exposure current Isolation Tx had ground on secondary All measurements in Volts (V)

5 Circuit Configuration - 1 5

6 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 Tx 2 Loc V exp-oc V exp-cc 1 V exp-oc V exp-cc 1 Fr-W / /0.1 Fr-W / /0.5 Light 2-W / /0.3 1 Closed circuit voltage = exposure current 2 For Iso Tx readings: (with secondary grounded/with secondary floating) All measurements in Volts (V)

7 Circuit Configuration - 2 7

8 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 With Iso Tx Loc V exp-oc V exp-cc * V exp-oc V exp-cc * Fr-W Fr-W Deck-W *Closed circuit voltage = exposure current All measurements in Volts (V)

9 Neutral Exposure Voltage Magnitude at Site 3 Approximately 48Volts NEV is related to running of a local pump storage hydro plant 9

10 Boat Dock Testing at Site 3

11 Stray Voltage Testing Open Circuit Voltage Open Circuit Volt tage (Volts s) Isolation Transformer Bypassed Isolation Transformer In with Secondary Grounded Isolation Transformer In with Secondary Lifted/Floating 0 Site 1 Low Site 1 High Site 2 Low Site 2 High Site 3 Low Site 3 High * * *Configuration not tested 11

12 Stray Voltage Testing Closed Circuit Voltage 25 ltage (Volts) rent (ma) d Circuit Vol posure Curr Closed Exp Isolation Transformer Bypassed Isolation Transformer In with Secondary Grounded Isolation Transformer In with Secondary Lifted/Floating 0 Site 1 Low * Site 1 High * Site 2 Low Site 2 High Site 3 Low Site 3 High *Configuration not tested 12

13 Contact Voltage Testing Open Circuit Voltage age (Volts) Open Circuit Volt Isolation Transformer Bypassed Isolation Transformer In with Secondary Grounded Isolation Transformer In with Secondary Lifted/Floating *Configuration not tested; results would have been similar to sites 1 and Site 1 Low Site 1 High Site 2 Low Site 2 High Site 3 Low Site 3 High * * 13

14 Contact Voltage Testing Closed Circuit Voltage Clo osed Circuit Vol ltage (Volts) Exposure Curr rent (ma) Isolation Transformer Bypassed Isolation Transformer In with Secondary Grounded Isolation Transformer In with Secondary Lifted/Floating *Configuration not tested; results would have been similar to sites 1 and Site 1 Low Site 1 High Site 2 Low Site 2 High Site 3 Low * Site 3 High * 14

15 Change Proposal Language (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 (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 dby 2504(A) (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 (B). 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. 15

16 NEC Change Proposal Results Submitted Changes for: Article 555 Marinas and Boatyards (Code Panel 19) Results Reject Panel Statement: The installation of a transformer intended to isolate stray voltage (current) is not presently prohibited. The proposed method of connection and grounding of a separately derived system is not in accordance with Article It was noted that Class B ground fault circuit interrupters are no longer permitted by UL 943. The panel requests that this panel action be provided to Panel 5 for their information with reference to Proposal 5-92.

17 Change Proposal Language (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 (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 (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 (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

18 NEC Change Proposal Results Submitted Changes for: Article 682 Natural and Artificial Made Bodies of Water (Code Panel 17) Results Reject Panel Statement: The Code is not a design document. The Code does not preclude such installations in general. Note that Class B GFCIs are not currently listed.

19 Comments / Contact Info Frank C. Lambert, P.E. Georgia Tech / NEETRAC frank.lambert@neetrac.gatech.edu a eet ac.gatec.edu