In this article, let us review issues
concerning induced current flow in the metallic
coverings of single-conductor cables. Single-conductor
cables have some advantages over multi-conductor cables,
especially in larger sizes. Tables 1 and 3 of the
Canadian Electrical Code allow us to use smaller wire
sizes for single-conductor cables than Tables 2 and 4
for multi-conductor cables. Naturally, the smaller
cables are always easier to handle and install than the
larger, heavier ones.
Despite these valid advantages,
single-conductor metal sheathed and armoured cables come
with some special problems in the form of sheath
currents and eddy currents. And Rules 4-008 and 12-3024
provide solutions for overcoming the overheating
problems thus created.
Sheath currents (also known as
circulating currents) flow in the metal sheaths or
armour of single conductor cables. They are induced by
the AC magnetic fields surrounding each circuit
conductor. Metallic sheaths and armour of cables are
often bonded to equipment at both ends, providing a
closed circuit path through metal cabinets, bonding
conductors and other metallic paths when sheath currents
are permitted to flow.
The induced sheath current flow
causes temperature rise in metal sheaths or armour of
single-conductor cables. Heat is transferred to the
cable insulation with harmful effects. And unless cable
loadings are reduced or the sheath currents are
prevented by other means, insulation temperature ratings
may be exceeded resulting in premature cable failures.
The Canadian Electrical Code
recognizes this problem and provides several possible
solutions to avoid sheath currents and their associated
problems. Rule 4-008 specifies the following for
single-conductor metal-sheathed or armoured cables when
electrical loads exceed 425 amperes:
• Derating the single-conductor
cables to 70 percent of the minimum conductor
ampacities permitted in Tables 1 or 3; or
• With an inspector’s
permission, derating the cables according to the cable
manufacturer’s recommendation; or
• Isolating the metallic sheath
or armour from ground at one end of the cable by
entering electrical equipment through a non-metallic
plate with separate bonding added to re-establish
electrical continuity between equipment.
To ensure that metallic sheaths or
armour are isolated from grounded metal, cables must
have non-metallic jackets to ensure that sheath currents
are not permitted to flow due to contact with
ground-fault return paths such as cable trays, racks or
structural steel.
The same magnetic fields that
surround single-conductor cables also produce eddy
currents in the steel enclosures which completely
surround the cables. Eddy currents can overheat iron or
steel cabinets, locknuts or bushings or any ferrous
metal that completely encircles the single-conductor
cables. This pre-sents no problem in multi-conductor
cables, where the magnetic fields tend to cancel each
other.
Rule 12-3024(7) and (8) of the
Canadian Electrical Code provides a solution to prevent
harmful eddy current heating effects:
• For loads up to 200 amperes, no
special precautions are required;
• For loads over 200 amperes,
cables must enter all metal enclosures through a
non-ferrous plate (normally aluminum), and any
connectors, bushings, etc., that completely surround
the conductors must be of non-ferrous materials.
At one time, slotting the wall of an
electrical equipment enclosure between the cable
openings was considered an acceptable way to reduce the
effects of eddy currents. This is no longer desirable,
since equipment enclosures may fail when weakened by
slotting.
Heavier electrical loads often
involve both sheath current and eddy currents. In this
case, Rules 4-008 and 12-3024 for single-conductor
cables can be summarized as follows:
• For loads up to 200 amperes, no
special measures are required by the electrical code;
• For loads over 200 amperes,
cables must enter electrical equipment through
non-ferrous plates and all locknuts, bushings, etc.,
must be of non-ferrous materials;
• For loads over 425 amperes,
single-conductor cables may be derated; or
1. Cable sheaths, armour and
concentric bonding wires are bonded to ground, and
cables enter the equipment through a non-ferrous plate,
usually at the supply end; and
2. Cables enter electrical equipment
at the load end through a non-metallic plate; and
3. All metal hardware encircling the
cables is non-ferrous metal; and
4. Cable armour, sheaths and
concentric bonding conductors are isolated from ground
by cable jackets, taping, etc.; and
5. The bonding continuity between
electrical equipment is re-established by a separate
bonding conductor sized to the minimum requirements of
Table 16.
As usual, you should consult the
electrical inspection authority in each province or
territory as applicable for a more precise
interpretation of any of the above.
Leslie Stoch, P.E. is principal of L.
Stoch & Associates, providing electrical engineering
and ISO 9000 quality systems consulting. Prior to that,
he spent over 20 years with Ontario Hydro as an
electrical inspection manager and engineer. Les holds a
B.S. in electrical engineering from Concordia University
in Montreal.
|
