Appendix B Rule 4-008
Induced voltages and sheath currents in metal sheaths and interlocked armour of single-conductor
When an alternating current flows in the conductor of a single-conductor metal-armoured cable, continuous or interlocked, an alternating magnetic field is created around the entire cable, which induces a voltage in the metal armour sheathing. The magnitude of the induced voltage is proportional to the magnitude of the current in the conductor and its length.
Sheath current will flow along the sheath as a result of the induced voltage, provided that the sheath circuit is completed, e.g., by grounding both ends of the sheath (or by contacts made along the sheath to the sheaths of adjacent cables or to metal building members). If the sheath circuit is not completed, no sheath current will flow. If the sheath circuit is completed, the magnitude of the sheath current is a function of the induced voltage and sheath impedance. Sheath current increases in magnitude with increased spacing between single-conductor cables and decreases with reduced spacing (within the range of typical spacings).
Sheath currents can be large and result in considerable heating of the sheath. Coupled with the heat resulting from the passage of current through the conductor, the conductor insulation will subjected to temperatures that can cause failure or a serious reduction in the life expectancy of the cable.
The phenomenon of sheath currents is common in varying degrees to single-conductor cables enclosed in ferrous metals, as in galvanized conduit, and in non-ferrous metals, such as copper, aluminum, and lead employed as cable sheaths, and will occur whether the enclosure is of the continuous tube or the spiral armoured type.
Single conductors in free air — mitigating the effects of sheath voltages and currents
For cables carrying currents up to and including 425 A, sheath losses can be reduced to tolerable levels, without the need to apply derating, by spacing cables approximately a diameter apart. This reduces the induced sheath voltage (by virtue of the three-phase field cancellation effect at close spacing). For cables carrying currents greater than 425 A, it is generally necessary to apply derating to avoid overheating of the cable unless sheath currents are eliminated. To eliminate the flow of sheath currents, it is necessary to ensure that all paths through which they may circulate are open-circuit.
Cable sheaths should be grounded at the supply end only and thereafter be isolated from ground and each other. Sheath isolation may be attained by installing the cables in individual ducts of insulating material, by employing cables jacketed with PVC or other insulating material, or by mounting unjacketed cables on insulated supports. The sheath or sheaths should be isolated from any metal enclosures or other terminations at the load end that might bridge them or cause them to contact ground. The cable sheaths in such circumstances cannot be used for bonding the electrical system, and a bonding conductor of adequate size for this purpose must be provided [see Rules 10-610 5) and 10-616].
Sheath currents are allowed to flow in single-conductor mineral-insulated cables, with phase conductors grouped to minimize the flow of sheath currents [see Rule 4-004 10)] without Table 5B derating. The sheaths are bonded to ground at both ends to provide an effective equipment bonding conductor.
Single conductors — external effects
The magnetic field around a single-conductor cable or single insulated conductors can create heating in
ferrous materials through which it passes, e.g., the wall of an enclosure, steel supports, connectors,
glands, locknuts, or clamps that encircle a cable or insulated conductor. There are two effects. The magnetic field causes currents to flow in the wall of the enclosure (as an example) through which a single-conductor cable or single insulated conductor passes. These are known as “eddy currents” and create heating. The other effect is that the wall itself has magnetic properties, and a magnetic field will be set up within the wall. The energy expended in setting this up causes heating in the wall known as hysteresis heating. This type of heating occurs only in ferrous materials, and only when the encircling ferrous material is in close proximity around the cable or insulated conductor.
Single insulated conductors or single-conductor cables in free air — mitigation of external effects
Cables or single insulated conductors carrying currents 200 A and less, with typical spacings in air, do not constitute a problem because of the low current levels. If the opening in an enclosure is large — e.g., coming up underneath equipment through a large opening from the floor beneath — the cancellation effect can be taken advantage of by grouping the phase conductors close together to reduce the external magnetic field to the point where there is no impact on surrounding ferrous materials that are not closely encircling the group. For all other cases, the preferred option is to cut out a section of the enclosure a little larger than the opening required for the insulated conductors or cables, and replace it with a non ferrous plate through which the single insulated conductors or cables pass.
Single conductors in underground locations
The wider spacings generally employed with underground single-conductor cables or single insulated conductor installations, in comparison with cables in free air, can be expected to generate circulating currents of greater magnitude in metallic sheaths or armours when these are bonded together or grounded at both ends. It is necessary to correct all such installation ampacities to avoid overheating of the cable if sheath currents are not to be eliminated. If derating is the desired course, the cable manufacturer should be consulted because this factor depends on the type and size of cable and installation arrangements and could be more favourable than the factor given in Subrule 1) a).
Source: Appendix B Rule 4-008, CSA C22.1:21, Canadian Electrical Code, Part 1 (25th Edition), Safety Standard for Electrical Installations. © 2021 Canadian Standards Association. Please visit store.csagroup.org