INSTALLATION OF SINGLE CONDUCTOR CABLE

TO

PREVENT EDDY AND CIRCULATING CURRENTS

    Although some of the installation practices outlined in this Technical Document would apply to medium and high voltage cables, these installation notes apply only to single conductor (1/C ) armoured and unarmoured equipment products (building wire ) cables rated up to 1000 volts.

   Beside ampacity which the Canadian Electrical Code ( CEC ) covers in Tables 1 and 3 as well as Appendix B and D, there are basically three areas of concern when installing 1/C cables.  First is the eddy currents that will be generated in any steel that may be surrounding a single conductor.  Second is circulating currents that may be generated in any metallic armour that's over the single conductor and third is the orientation or configuration of the single conductors when running more than one conductor per phase for the same circuit.  For more information on single conductor cable configurations and equal load sharing, see the Technical Newsletter on this subject.

Eddy Currents

  Eddy currents are generated in any ferrous materials ( steel ) surrounding an individual 1/C cable  carrying  alternating current.  Cables can be armoured or non-armoured and eddy currents will be generated where these cables pass through steel entry plates on enclosures, connectors, locknuts, ground bushings, cable clips, structural steel, conduits etc.  Eddy currents generally are not considered to be of concern in cables carrying less than 200 amperes and the 2006 edition of the  CEC makes reference to this limit in Rule 12-106(4) & (5).  Rule 12-106(4) makes a specific reference to the armour on 1/C cable.  It requires that the armour be of non-ferrous material.  Nexans manufacturers all of it's single conductor Teck90, Corflex RA90 and ACWU with aluminum armour only.  Rule 12-106(5) requires that 1/C cables carrying more than 200 amps not be surrounded by ferrous material.  Rules 12-3022(7) & (8) make specific reference to precautions to be taken at enclosures to prevent eddy currents and sheath/armour circulating currents.  We will cover circulating currents later in this newsletter.

  Previous editions of the CEC allowed "slotting" of a steel entry plate adjacent to the connector used on a 1/C cable.  This practice is no longer allowed. Now the Code requires that for 1/C cables carrying more than 200 amps non ferrous entry plates be used.  Most of the time these plates are aluminum which are mounted over one common hole cut through the original steel enclosure.  All three phases (plus the neutral if present) enter through this one entry plate.  This applies as well to cases where there is more than one cable per phase.  All cables enter through one common plate.  Since all cables enter through one hole cut in the steel of the enclosure, eddy currents are not generated in the surrounding steel because individual magnetic fields from each of the three phases cancel each other and the net field the steel sees is zero.

  This magnetic field cancellation effect also applies where cables pass through structural steel or are run in steel cable tray supported by steel or are run on steel strut that is supported by steel.  As long as the three phases are run through the same steel "window", the net field and therefore eddy current should be zero.  Where 1/C cables run through re-bar that may be imbedded in a concrete floor slab my advice is that the same general rules should be followed but since the re-bar is imbedded in the concrete there's less of a problem from heating.  This is because the re-bar has a small cross-section so there will be little eddy current induced and second is that any heat produced by these eddy currents will be taken away by the large heatsink action of the concrete.  My advice here is to make sure cables passing through a floor do not come in direct contact with the re-bar and where ever possible follow the guidelines above.

  In the case of cables in parallel, each set of three phases can run through a separate steel window, but proper cable to cable spacing and configuration must be followed.  This is necessary to maintain ampacity and equal load sharing between cables of the same phase.  We will detail these requirements later in this Technical Document.

Circulating Currents

  Circulating currents are generated in a metallic layer of 1/C cables when that cable's insulated conductor is carrying alternating current and that metallic layer is bonded to ground at both ends.  It doesn't  matter what metal the layer is.  If it's metal and grounded both ends there will be a current induced into it.  The magnitude of this circulating current depends on the current in the conductor and the resistance of the loop formed by this layer and the ground path.  The metallic layer can be the 1/C cable's interlocked armour or concentric bonding conductor as found in Teck90, AC90, and ACWU90), or it could be the aluminum sheath in RA90 as with Nexans' Corflex.

  As with eddy currents, the CEC has rules that cover the circulating currents and how to safely deal with them.  Rules 4-008, 10-304(2) and 12-3022(7) & (8) as well as notes in Appendix B for some of these rules apply is this case. Specifically Rule 4-008 and the Appendix B notes for 4-008 are the best references here.

 What Nexans recommends in our installation notes and what's most often done in the field is that a single conductor cable carrying more than 425 amperes has it's armour/sheath/concentric bonding conductor grounded at one end and isolated from ground and from each other at the other end (usually referred to as the floating end).  For practical application, single conductor 90 °C rated cables this works out to be cables larger than 250 MCM  in copper  and 350 MCM and larger in aluminum.  Sizes smaller than these may be run at their Code rated ampacity but remember that aluminum entry plates are required for cables carrying over 200 amps.

  Bonding and grounding is done at the supply end, therefore the floating end is usually the load end of the run.  There's always exceptions such as in the case of services, where the floating end is most often the supply end.  Where one end of the circuit is floated off ground a separate bonding conductor, sized in accordance with Code Table #16, must be run to tie the non-current metal parts ( the enclosures ) together.  With services there's usually no bonding conductor but where the cables enter the service enclosure they enter through a non-ferrous entry plate and here ground bushings as well as locknuts are used on the connectors that fix these cables to the entry plate.  A suitably sized bonding conductor is connected through each ground bushing to a ground lug inside the enclosure. 

  In order to float the sheath/armour off ground an insulating mounting plate is used in place of an aluminum plate.  ( See the notes above regarding eddy current prevention and the use of aluminum entry plates.)  This insulating plate is usually at the load end of the run.   In the case of services, sheath isolation is sometimes achieved by not using a mounting plate at all.  For example where cables connect to a pole mounted transformer or where they enter the bottom of a disconnect switch and no entry plate is present, sheath isolation is obtained by just keeping the cables separated so their sheaths, armours or concentric grounds do not touch ground or each other.

  There may be cases where 1/C cables run to a splitter then on to another enclosure and possibly on again.  In these cases you must alternate entry plates.  For example leave the supply end through an aluminum plate, enter the next enclosure through an insulating plate, leave that enclosure through an aluminum plate, enter the next enclosure through an insulating plate and so on.  All enclosures must be tied together with a bonding conductor sized in accordance with Code Table #16.  Bonding of the cables is done at the aluminum plates only so isolation is maintained in each cable section.

  Another way to run armoured 1/C cables larger than 250 MCM copper and 350 MCM aluminum and larger is to ground them at both ends, but if this is done the Code requires a large derating.  Code Rule 4-008(1)(a) requires that cables must be derated to 70 % of their normal conductor current !  As you can see it's better to float one end and utilize the full ampacity.

  When the cable's armour / sheath / concentric ground  is floating at one end a standing voltage will be generated on this metallic layer.  The magnitude of this standing voltage is directly proportional to conductor current and to cable length.  It's also proportional to the spacing between cables and how they're laid out ( ie flat or triangular ), but current and length are the big factors.  Generally standing voltage is not of concern except if the runs are long and / or conductor current is high.  Although 1/C cables are not used very often in hazardous locations, we would recommend grounding the sheath at both ends and applying the 70% derating factor. 

  During installation care must be taken to ensure that cable jackets are not damaged.  If they are damaged the armour or sheath could make contact with ground along the run.  This  forms ground loops which would allows circulating currents to flow and this   causes cables to overheat.  Sparking and cable failures have also been observed at points where cable  jackets have been damaged and these circulating currents are allowed to flow to ground.

Summary

  To eliminate eddy currents and circulating currents when installing 1/C cables you have to meet two conditions :

  1) If a 1/C cable is carrying over 200 amperes do not surround it with ferrous metal. This includes entry plates to enclosures, connectors, locknuts, ground bushings, clips, etc.

  2) If a 1/C cable with sheath / armour is carrying 425 amps or more, the sheath / armour should be isolated from ground at one end in order to obtain C.E.C. Table 1 or 3 current ratings.

  For more information on 1/C copper conductor Corflex RA90 click here.

  For more information on 1/C aluminum conductor Corflex RA90 click here.

  For more information on 1/C copper conductor Teck90 click here.

  For more information on 1/C aluminum conductor ACWU90 click here.

  By : D.S. Reith - Applications Specialist, Equipment Products

  Technical Bulletin #7, Rev #1 - January 24, 2006