Stray Current Investigations
A Method of Troubleshooting Stray Current on
Underground Residential Distribution (URD) Loops
Stacey Mighty Malcolm
Distribution System Reliability
CenterPoint Energy
SWEDE 2012
Introduction - What Is Stray Current
• Stray 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 neutral
▫ Breaks in the neutral that causes the current to take an
unintended path
▫ Improper grounding practices
▫ Induced voltages from high voltage lines
• Stray current is NOT related to power system faults or direct
contact with energized lines.
Introduction - Utility Design Facts
• CenterPoint Energy’s 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 .
Introduction - NESC Grounding Requirements
CNP system design is in accordance with the NESC, IEEE C2, Section 9:
Grounding Methods:
LOAD
• 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.”
Introduction – NESC Grounding Requirements
An Illustration of What is Permitted
• CenterPoint Energy Transformer Connections
• In a typical installation, the system neutral is connected
to the transformer primary, secondary and also to the
earth.
Introduction – NESC Grounding Requirements, Cont’d
An Illustration of What is Permitted
Typical Open Delta Equipment
bonding on a pole
Connections
• 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.
Introduction – National Electric Code
Customer Requirements for Grounding
Single Phase 3 wire Service
2011 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.
Source: 2011 NEC Handbook
• 2011 NEC Article 250.26 requires the customer to bond to the utility
neutral at their service entrance.
Introduction – Typical Grounding at the
Service Entrance
Service Entrance – Single
Family Home
Grounding Electrode
Conductor connected to the
ground rod
Introduction – Typical Connections
inside a Main Distribution Panel
Connections inside of a Main Panel
– Single Family Home
Un-grounded
Conductors
Neutral Terminal block
Neutral
Neutral
Main Bonding Jumper
Introduction – Typical Grounding at
Service Entrance
Service Entrance – Single Family
Home
Grounding 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 Current
•
•
•
•
•
Stray current can occur:
If there is an open connection in the neutral
If neutral-ground connections are compromised
If there is a failed ground rod clamp
If 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 potential
Stray Current on Underground Residential
Distribution Loops
• Case:
▫ 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 transformer
19.9 kV Line to Ground Primary – 120V Secondary
URD – Padmount Transformer – Typical
Connection
X-Feed
• Concentric
neutral along the
primary
Y-Feed
• Concentric
neutral along
the primary
Secondary-Feed
• Neutral is
grounded in
PMT
Ground Rod
All Neutral conductors are grounded in the transformer
Measuring current along the loop
Current measurements are taken along every URD
transformer at both primary and neutral connections.
Method of Investigation – Initial Check
• In order to determine the cause/source of the stray current the
entire loop and the customer’s service has to be checked.
Utility Initial Checkpoints
Customer Initial Checkpoints
• Ensure that the URD
transformer is properly
grounded and that there are no
loose connections.
• Ensure that there is a proper
ground at the customer’s service
entrance.
• Disconnect customer’s 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 customer’s
ground to ensure that no
supplemental grounding
electrode/rod/pipe is required
(i.e. resistance must be less than
25 ohms.
Investigation – Measurements
• The 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.
Investigation – Data Gathering
Current measurements were taken at all URD transformers terminal poles
Location
TIME
PX
PY
NX
NY
TP X
9:30
16.2
PMT 1
9:43
15.4
9.4
PMT 2
9:50
6.5
4
PMT 3
10:05
7.2
1.2
5.4
0.9
PMT 4
10:08
0
0.3
1.4
1.1
PMT 5
10:22
0
1.8
0.9
0.6
PMT 6
10:30
3.9
6.2
2.1
1
PMT 7*
10:40
4.8
5.5
0.4
2.9
PMT 8
10:55
3.7
8.1
1.1
0
PMT 9
11:14
10.7
15.6
1.7
3.8
PMT 10
11:22
17.2
17.1
2
1.7
PMT 11
11:30
13.3
14.4
1.2
2.3
PMT 12
11:41
15.7
13.5
4.1
3.1
PMT 13
11:52
18.3
18.7
3.7
3.5
PMT 14
11:59
16.5
18.5
4.5
6.6
TP Y
12:04
11.3
19.1
10
6.8
2.4
8.2
Analysis of Field Findings
Normal Open Point
Location
of
Pool
• We noticed that there was a change in
the current along the neutral on the x
and y of the transformer cable.
Analysis of Field Findings, Continued
Normal Open Point
Location
of
Pool
• Changes 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.
Corrective Action Taken
• Direct buried cable along three sections of the
loop was replaced and placed in conduit
▫ Cable between PMT 6-7
▫ Cable between PMT 7-8
▫ Cable between PMT 8-9
Follow Up Measurements
• The 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.
References
•
•
•
•
Power Distribution Engineering, James J Burke
NFPA 70, 2011 National Electric Code
IEEE C2, 2007 National Electric Safety Code
“Impact of Transmission Lines on Stray Voltage”,
IEEE Nagy Abed, Sasan Salem, and James Burke
• CNP Overhead Distribution Standards
• “Dangers 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 Code
Customer Requirements for Grounding
3 Phase 4-wire Wye Service
Source: 2011 NEC Handbook
3 Phase 4-wire Delta Service
Source: 2011 NEC Handbook
2011 NEC Article 250.26 requires the customer to ground the utility
neutral at their service entrance.