Stray Current Investigations

Stray Current InvestigationsA Method of Troubleshooting Stray Current on Underground Residential Distribution (URD) Loops

Stacey Mighty MalcolmDistribution System ReliabilityCenterPoint EnergySWEDE 2012Introduction - What Is Stray CurrentStray Current is defined as "A current resulting from the normal delivery and/or use of electricity that may be present between two conductive surfaces that can be simultaneously contacted by members of the general public and/or their animals.

Common causes of stray current:Primary and/or secondary return current on the neutralBreaks in the neutral that causes the current to take an unintended pathImproper grounding practicesInduced voltages from high voltage lines

Stray current is NOT related to power system faults or direct contact with energized lines.Stray voltage as defined by IEEE is..[read definition]

It is important to understand utility/customer grounding requirements when there is an incident of possible stray current

2Introduction - Utility Design FactsCenterPoint Energys system is designed with 3f 4-wire Wye circuits. The system is designed with a solidly grounded neutral.The neutral, from the substation, is grounded multiple times throughout the distribution system, and grounded at every equipment pole in accordance with Section 9 of the National Electric Safety Code IEEE C2-2007 .CNPs system is a 4-wire system with a grounded neutral. This system neutral conductor is common throughout the system and grounded on the primary and secondary of equipments3Introduction - NESC Grounding RequirementsCNP system design is in accordance with the NESC, IEEE C2, Section 9: Grounding Methods:

092(B)(2) Grounding connections shall be made at the neutral of the source. Additional connections may be made, if desired, along the length of the neutral, where this is one of the system conductors.097(B)(2) The grounding conductors of primary and secondary may be interconnected.096(C) Multi Grounded Systems The neutral, which shall be of sufficient size and ampacity for the duty involved, shall be connected to a made or existing electrode at each transformer location and at the sufficient number of additional points with made or existing electrodes to total not less than four grounds in every mile of the entire line, not including grounds at individual services.

LOADCNPs grounding methods are in accordance to Section 9 of the National Electric Safety Code.

This is a visual representation of a 4-wire circuit from the substation until service is delivered to a customers home. What we can see here is the neutral conductor being grounded throughout the system until service is delivered.

4Introduction NESC Grounding RequirementsAn Illustration of What is PermittedCenterPoint Energy Transformer Connections

In a typical installation, the system neutral is connected to the transformer primary, secondary and also to the earth.

This is a visual representation of a single phase transformer connection. The grounded neutral conductor and two un-grounded conductors comprise the service type. This is the typical service arrangement to a single family home.5Introduction NESC Grounding Requirements, ContdAn Illustration of What is PermittedTypical Open Delta Equipment bonding on a poleConnections

All equipment on the pole is bonded together and connected to the neutral. Connection is made to the grounding electrode conductor and then to the ground rod to earth.All equipment on the pole is bonded to the neutral. 6Introduction National Electric CodeCustomer Requirements for GroundingSingle Phase 3 wire Service2011 NEC Article 250.24(A)(1)The customer grounding requirements are in accordance with the National Electric Code Article 250.A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service.The grounding electrode conductor connection shall be made at any accessible point from the load end of the service drop or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means.

2011 NEC Article 250.26 requires the customer to bond to the utility neutral at their service entrance.Source: 2011 NEC HandbookIntroduction Typical Grounding at the Service EntranceService Entrance Single Family HomeGrounding Electrode Conductor connected to the ground rod

Introduction Typical Connections inside a Main Distribution PanelConnections inside of a Main Panel Single Family HomeNeutral Terminal block

NeutralUn-grounded ConductorsMain Bonding JumperNeutralIntroduction Typical Grounding at Service EntranceService Entrance Single Family HomeGrounding Electrode Conductor connected to the ground rod

All grounds are bonded to the grounding electrode at the service entrance.Neutral and Grounding Connections and Stray CurrentStray current can occur:If there is an open connection in the neutralIf neutral-ground connections are compromisedIf there is a failed ground rod clampIf all of the premise grounding electrodes are not bonded together to form the grounding electrode system Note: grounds must be bonded together to eliminate possible difference in potentialStray Current on Underground Residential Distribution LoopsCase: A residential customer who is served from a URD loop contacts CNP to complain about being shocked while swimming in the pool.The customer complained that after being in the pool, there is a slight sensation of electricity by one of the pool lights.The customer is not aware of what length of time over which this is occurring as it is not evident unless he is swimming for a long period of time.Typical Underground Residential Distribution (URD) Loop

A typical CNP URD Loop is served from two overhead sources with a normal open point to split the load on the loop.The configuration allows for heightened reliability and can reduce the impact of extended outages through switching.

Typical Electrical One-line of a URD Loop

A typical CNP URD Loop is served from two overhead sources with a normal open point to split the load on the loop.URD Padmounted Transformer

Single Phase URD padmounted transformer19.9 kV Line to Ground Primary 120V SecondaryURD Padmount Transformer Typical Connection

X-FeedConcentric neutral along the primaryY-FeedConcentric neutral along the primaryGround RodAll Neutral conductors are grounded in the transformerSecondary-FeedNeutral is grounded in PMTMeasuring current along the loop

Current measurements are taken along every URD transformer at both primary and neutral connections.

Method of Investigation Initial CheckUtility Initial CheckpointsCustomer Initial CheckpointsEnsure that the URD transformer is properly grounded and that there are no loose connections. Ensure that there is a proper ground at the customers service entrance.Disconnect customers service to check if the stray current still exists.Ensure that the terminal poles at both ends of the loop have good neutral/ground connections.

Ensure that grounding is in compliance with Article 250 of NEC.All grounding electrodes shall be tied together to form grounding electrode system.

Note: Article 250.53(A)(2) Measure resistance on customers ground to ensure that no supplemental grounding electrode/rod/pipe is required (i.e. resistance must be less than 25 ohms.

In order to determine the cause/source of the stray current the entire loop and the customers service has to be checked.Investigation MeasurementsThe initial check of the customer/utility showed no apparent causes for the stray current in the pool.Additional Measurements had to be taken in order to detect the source of the stray current:Measure voltage at the pool light in reference to ground.Measure current along the primary and neutral at the terminal poles and at every transformer in the loop. Current measurements other than a fundamental frequency of 60Hz may not be a electric utility source.The sum of the current measurements along the loop should approach zero.19Investigation Data GatheringLocationTIMEPXPYNXNYTP X9:3016.211.3PMT 19:4315.49.4106.8PMT 29:506.542.4PMT 310:057.21.25.40.9PMT 410:0800.31.41.1PMT 510:2201.80.90.6PMT 610:303.96.22.11PMT 7* 10:404.85.50.42.9PMT 810:553.78.11.10PMT 911:1410.715.61.73.8PMT 1011:2217.217.121.7PMT 1111:3013.314.41.22.3PMT 1211:4115.713.54.13.1PMT 1311:5218.318.73.73.5PMT 1411:5916.518.54.56.6TP Y12:0419.18.2Current measurements were taken at all URD transformers terminal polesAnalysis of Field FindingsWe noticed that there was a change in the current along the neutral on the x and y of the transformer cable.

Location of PoolNormal Open PointAnalysis of Field Findings, ContinuedChanges in the neutral and primary current around transformer 7 showed unexpected results.The data along the neutral between transformers 6-7 and 7-8 showed that there was a higher change in the current than the other spans of the loop.

Location of PoolNormal Open PointCorrective Action TakenDirect buried cable along three sections of the loop was replaced and placed in conduitCable between PMT 6-7Cable between PMT 7-8Cable between PMT 8-9

Follow Up MeasurementsThe same measuring method was used to detect if there was any stray current at the pool light.No evidence of stray current existed at the time.If the customer complains of stray current in the pool again, cable in the entire loop needs to be assessed for replacement.ReferencesPower Distribution Engineering, James J BurkeNFPA 70, 2011 National Electric CodeIEEE C2, 2007 National Electric Safety CodeImpact of Transmission Lines on Stray Voltage, IEEE Nagy Abed, Sasan Salem, and James BurkeCNP Overhead Distribution StandardsDangers of Stray Voltage and Stray Current, Donald W Zipse, P.E.Electrical Shock Hazard due to Stray Current, Donald W Zipse, P.E.

Appendix National Electric CodeCustomer Requirements for Grounding3 Phase 4-wire Wye Service3 Phase 4-wire Delta Service

2011 NEC Article 250.26 requires the customer to ground the utility neutral at their service entrance. Source: 2011 NEC HandbookSource: 2011 NEC Handbook26