Stray Voltages and Public-Exposed Lighting Installations�

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					“Stray Voltages and Public-Exposed Lighting
               Installations”




           Massimo Mitolo, Ph.D.
           IEEE Senior Member
 Outdoor lighting installations

Lighting fixtures along with their supply
            circuits, including:
   transformers, breakers, reclosers,
  switches, manholes, and whatever is
      functional to the installation
               performance.

          All publicly exposed!
                 Stray Voltages
  “Stray”: Element or occurrence not desired in
  theory, but unavoidable in a practical realization.


   Term coined in the 1970s: Stray Voltages In Dairies

  “Elevated metal object-to-ground or neutral-to-ground
                        voltages”

Current, and not Voltage, is the proper criterion of
                   shock intensity

                                                          3
              Stray Currents

             Permanent current
   other than momentary fault current
         circulating over the earth

      objectionable and undesirable




    Concerns for pedestrians and their
                 animals!
                                         4
  TT earthing system per IEC

                          T T
T= direct connection of
                           T= direct electrical connection of
       one point of the
                              exposed-conductive-parts to
       power system to
                              ground, independently of the
                ground.
                              grounding of any point of the
                              power system
               Utility        Customer



                                                           5
           TT earthing system
              To 480V, 3 Phase Utility Transformer




The intensity of the ground fault current is limited
by the series resistance RL and RN of the grounds.
                   RCD is crucial!
   Drawbacks with the RCDs
            Nuisance trips
e.g. transient (10 μs) overvoltages during
              thunderstorms
  RCD trips the breaker in 10-40 ms: it
 operates when the impulse has already
                  expired.

 RCD with intentional tripping delay is
allowed, better if with high immunity to
             disturbances.
  RCD with automatic reclosure
          capability?

 Not required, so far, by IEC standards
because of the risk of repetitive shock to
    persons due to a persistent fault.

Professional Engineers must decide on an
          individual case basis!
          TT Equivalent fault circuit
                                     Person’s body resistance:     788Ω
                                     @125V, 700 Ω @220 V, etc.



                                     VOLTAGE DIVIDER!


                                     Person’s resistance to ground in the
                                     absence of floor: 2ρ.
                                     Person assumed shoeless as per:
                                     IEC 60479-1; 1994-09, 3rd Ed.,
                                     “Effects of current on human beings
                                     and livestock - Part 1: General
                                     aspects”.


Persons in contact with the energized pole, and standing in
its proximity, are in an area at a potential other than zero.
  This circumstance limits the source touch voltage VST!
Independently grounded poles in TT Systems

                             What if the
                             neutral
                             conductor is
                             faulted?




       the RCD is desensitized!
Independently grounded poles in TT Systems




            No, thank you!
             TN-C-S Earthing System

           T                  N                -C                 -S
T= direct connection of
   one point to ground.
                                                       S= protective function
                                                          provided by a conductor
                                                          separate from the grounded
 N= direct electrical connection of                       conductor
   the ECPs to the grounded
   point of the power system.


                                      C= neutral and protective
                                         functions combined in a single
                                         conductor


                                                                               12
        No-Fault Condition




Stray current as a result of unbalanced loads:
                                                 13
                 Permanent!
                  Fault Condition




The fault current’s return path to the power-supply winding
     TN-C-S Earthing systems
                                                                           B
                                                                         Z eq
                                                V B  V ph 
                                                                Z eq  Z ph
                                                                     B          B




                                                            V ph
                                                     IF              Ia
                                                            Z Loop


Ia  is the current causing the automatic operation of the
overcurrent protective device within the safe time ta
            Vph (V)      Disconnecting time ta (s)

                                                                DOSE!
             120                   0.8
             230                   0.4
             277                   0.4
             400                   0.2
             >400                  0.1
    Extraneous-conductive-part (EXCP)
                   Conductive part:

   not forming part of the electrical system

        liable to introduce a “zero” potential
          (local/remote earth potential) or

                 an arbitrary potential
                                                  16
Whole potential between the faulted pole and the fence




 Should we bond?                        EXCP


EXCP




 Bonding Jumper between pole and fence (only if Extraneous-
                    Conductive-Part)

        Drawback: Ground Potential Rises are tranferred!
         Class II equipment
Double insulated (or reinforced insulated)
equipment:
incorporates a supplementary insulation, in
addition to the basic one.
The two insulations are physically separated
(and tested)
they cannot be subject to the same
deteriorating factors (e.g. temperature,
contaminants, etc.) to the same degree.
           Class II equipment
It is an equivalent protection against indirect
contacts if applied to the entire outdoor
installation (i.e. light fixtures, conductors, splices,
and terminal strip).


According to IEC, class II equipment is not
permitted to be grounded.


 According to NEC, double insulated equipment is
not required to be grounded.
    Measuring stray voltages
                            Persons are sensitive to
                            currents, and not to voltages.
                            Stray voltages, thus, cannot
                            point out, per se, dangerous
                            situations.

                             The human body resistance RB
                             is variable with the touch
                             voltage.

                            It is important to assess the
                            capability of the stray voltage
                            to impress a dangerous current
                            and not the magnitude of the
                            voltage itself.

 Touch-voltage must be measured with reference to a
standard human body resistance value: 1 kΩ as per IEC
     Is the faulted pole found?
Poles’ enclosures are connected together.


The presence of stray voltage at the pole
under investigation does not necessarily
          mean a fault on it!




                                       21
                    Conclusions
Analysis of the fault-loops is crucial to understand the causes of
stray voltages.

TT and TN-C-S distribution systems have fault loops of different
natures, the first one comprising the actual earth.

Both distribution systems require an effective partnership between
the protective device and the grounding system, in order to protect
persons by automatic disconnection of supply.

Class II installations are an efficient solution to protect persons
from electrocution and preserve the continuity of the service,
especially in areas at high pedestrian and/or vehicular circulation.
     “Stray Voltages and Publicly Exposed Lighting
                     Installations”
                                      References
1.    M. Mitolo, “On Outdoor Lighting Installations Grounding Systems”, IEEE-
      IAS “Industry Application Society 41st Annual Meeting”, Tampa, Fl, October 2006.
2.    IEC 60364-1; 2002-06, 2nd Edition, “Electrical installations of
      buildings”.
3.    M. Mitolo, “Protective bonding conductors: an IEC point of view”, I&CPS
      Technical Conference 2005, Saratoga Springs, NY, Proceedings.
4.    IEC 60479-1; 1994-09, 3rd Edition, “Effects of current on human
      beings and livestock - Part 1: General aspects”.
5.    IEC 60364-7-717; 1996-04, 1st Edition, “Electrical installations of
      buildings”, part 7. “Requirements for special installations or locations -
      Section 714: External lighting installations”.
6.    NFPA 70, National Electrical Code 2005, National Fire Protection
      Association, Quincy, Massachusetts.
7.    IEEE Std. 142-1991, “IEEE Recommended Practice for Grounding of
      Industrial and Commercial Power Systems”.
8.    British Standard BS 7671; 2001, 16th Edition “IEE Wiring Regulations”.
9.    “Stray Voltages, Concerns, Analysis and Mitigation”, NEETRAC Project,
      Number 00-092, September 2001.
                                                                                         23

				
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