The tables of current rating of electric cables, as published by cable manufacturers, give the value of “standard current rating”. This is the maximum safe continuous current that the cable can carry under standard conditions. In South Africa, the standard conditions assume an air temperature of 30°C; that the cables are shaded from the sun; that the maximum soil temperature of buried cables is 20°C and that there are no multiple cables touching or close to each other.
There are also other standard conditions for altitude, soil resistivity, depth of burial and others. If any of the actual site conditions are worse than standard or can become worse during the life of the cable, an appropriate derating of the standard current rating is necessary so that the electric cable does not overheat. The opposite also applies. For example, a cable buried in swampy soil will probably have a soil thermal resistivity much better than standard, and the cable’s current rating can be appropriately re-rated upwards.
Single-core should be installed differently
When the cable’s conductor size is required to be quite large, there is often no possibility of obtaining the cable as a three or four-core cable as it would simply be too big to handle. In such cases, it’s necessary to use single core cables.
Within a three or four-core cable, there is a balanced electromagnetic field within the armour and, as a result, there can be no voltage induced into this armour. In the case of single core cables, there is an electromagnetic field within the metallic screen or armour of each core arising from only one phase.
This will induce a voltage into any surrounding metallic sheath. These metallic sheaths include copper tape screens, lead sheaths and armour.
In the case of a three or four-core cable, it’s common practice to bond the armour on both sides of the cable to earth. This is not always recommended for single-core cables because the induced currents can drive large circulating currents in the armour loops.
Where the metallic sheaths of single core cables are single point-bonded (earthed on one side only), induced voltages will appear at the other end (the unearthed end) of the cable, between the metallic sheaths and earth, and between the metallic sheaths themselves.
When installing single core cables, whether LV or MV, it’s advisable to consult an expert to arrive at an optimum solution.
Testing will not improve quality
The quality of an electric cable is a given. It was made under stringent manufacturing control and is probably the most reliable part of any electrical installation. However, after installation, and certainly after any repair, it’s common to retest the cable.
Over-testing the cable by using an excessively high voltage or applying this voltage for a longer time than necessary will not in any way improve the quality of the cable, but may damage an otherwise acceptable cable.
The retest should simply be done to ensure that there is no blatant problem with the repair, and to prove that the cable is probably safe to re-energise. This can be achieved with relatively low voltage, applied for a fairly short duration, and is particularly important when testing XLPE cables. There are published recommendations for such tests.
Avoid “long, expensive fuses”
You must know the fault level at the position in the network where the cable is to be installed, otherwise you may simply be installing a long (and expensive) fuse.
Only a small amount of power is often required by a load, and it seems logical to assume that only a small cable would be needed to carry this load. However, we must be aware that, under fault conditions, even a small cable would be required to carry the full fault current offered by the electrical system at the point of installation.
In other words, every cable within an electrical system must be able to survive any fault current that can arise at that point – both short-circuit faults (carried by the conductor) and earth faults (carried by the screens and armour). This will ensure that the cable is still usable after a fault.
“Big enough” not always the cheapest
An electric cable selected for a task is normally just larger than necessary to carry the maximum required load current, to comply with volt drop legislation and to safely carry fault currents.
In carrying the rated current, the conductor of the cable would heat up to a temperature just below the maximum allowed conductor temperature for that type of cable. This would result in an increase in the temperature of all the cable’s components, including the outer sheath. This is to allow the cable to lose the conductor heat via natural cooling to the environment.
Using a larger conductor would lower the conductor temperature and reduce wasted energy, but just how much larger should the conductor be? Making it too large would result in an excessively high initial cable cost.
Somewhere, there is an optimum size, and this is the subject of “minimum life cycle costing”, where we select a cable size to result in the lowest total cost of cable price and cost of future heat losses.
Software programs using discounted cash flow techniques are available to optimise the cable size, taking into account the cable’s initial cost; the cost of borrowing money (to buy the cable); the expected cost increases in the price of electricity (cost of future heat losses), and other factors.
When using such software, the optimum cable size resulting is usually much larger than expected. Even if this size is not used, the calculation will indicate just how much larger you can safely select the cable size without unjustifiably incurring excessive initial cost.
A coloured stripe does not mean fire resistance
A red stripe indicates flame retardant cable. The portion of cable that does burn will, however, give off 30% of the PVC plastic mass as hydrochloric acid gas (HCl). The cable will also give off dense smoke. This type of cable should not be used in confined spaces where people may be present.
A blue stripe indicates flame retardant and low halogen cable. When burning, the cable gives off only 15% HCl and dense smoke. Again, the presence of HCl and smoke results in a very dirty cable when buning in a confined space.
A white stripe indicates flame retardant, non-halogenated cable. This type of cable doesn’t give off any HCl gas when burning. There is very little smoke and very low toxicity in the gases given off.
This is the only cable to be used underground and in confined spaces like tunnels, where people may be present.
Fire survival power cables are no longer manufactured in South Africa. Despite their superb performance in fire situations, their premium price resulted in customer resistance to their purchase and its discontinuation.
All the cables mentioned here are flame retardant (FR) and will not burn for more than a certain length for a defined period, as specified in IEC 60332.
Parallel cables don’t always make for a cheaper installation
There are many installations where the designer elected to install two or more parallel cables, rather than a correctly-sized and more expensive larger conductor. For example, although a 25 mm2 copper LV cable is rated 110 A in air and a 150 mm2 copper conductor is rated 330 A in air, you cannot assume that three parallel cables of 25 mm2 would be able to carry 330 A in air. You would need to apply the “group” derating factor and check that each cable can handle fault conditions individually.
Interestingly, the current density within a cable’s conductor is not constant. The current density gets lower as the conductor size increases due to the surface area of the conductor increasing more slowly than the cross-sectional area. For example, the current rating of a 1,5 mm2 copper conductor is about 20 A per phase. The current density works out at about 13 A per mm2.
With larger conductors, we see that 10 mm2 conductor has a rating of about 60 A, i.e. the current density has dropped to only 6 A per mm2. The largest conductor in a table of current ratings is 300 mm2 and, for this size, the current rating is approximately 510 A. The current, density has now reduced to just above 1,5 A per mm2.
The reason for this is that the conductor current is based on its ability to get rid of its resistive heat loss, which can only take place from the conductor surface. As already stated, surface area increases more slowly than the increase in cross-sectional area.
Even if one did the necessary derating of parallel cables to arrive at the required load current, we must remember that each cable in the system must be able to carry the full fault current on its own.
Fault current is not always shared between parallel cables. The cables may well share a through fault current but any fault on only one cable in the parallel group would need to be survived by that single cable on its own.
Integrity of the outer sheath
The conductor or the insulation is not the most important component of the electrical cable. While all the cable’s components are important, the integrity of the outer sheath will have the greatest influence on the reliability and future lifetime of the cable.
Damage to the outer sheath during installation may allow water ingress and contamination, which can run along the cable to joints and terminations and cause a reduction in reliability and result in premature failure.
It is relatively cheap to conduct sheath integrity testing after installation, when the cable testing and commissioning crews are still on site. A DC voltage is applied across the outer sheath, between the armour and the substation earth (the cable armour having been disconnected from earth on both sides). Any unusually high leakage current recorded will indicate holes in the outer sheath.
Are aluminium conductors worth using?
The price of aluminium is lower than that of copper. It does, however, have much lower density than copper, so you get greater volume per ton than with copper. Unfortunately, aluminium does not have the same conductivity as copper, so you must use more when making a cable of equivalent current rating. Despite this, the cost in rand per Amp carried is lower for aluminium conductors than copper conductors.
This, however, is not the end of the comparison. Most electrical connectors are designed for copper conductors and, when used with aluminium conductors, there may be compatibility problems where these dissimilar metals meet.
The chemical compatibility is easily handled with special greases and by other means, but the thermal differences are more problematic.
Aluminium has a greater coefficient of expansion with temperature than copper, so after a few load cycles, the aluminium conductor inside a copper lug or ferrule will become loose, potentially causing a “hot” connection. Failure will result.
It is therefore necessary to use aluminium connectors with aluminium conductors, and special bi-metallic lugs and ferrules when connecting the aluminium conductors to copper busbars or switches.
Bi-metallic lugs and ferrules are quite expensive and consist of friction-welded components of both copper and aluminium, which provide a compatible interface between the dissimilar materials.
Users generally stick with what they have been using. It would be bad practice to mix copper and aluminium cables on the same project because the wrong accessories will probably be used somewhere.
There is some justification, however, for considering using aluminium conductors when they cannot become mixed with copper conductors, for example on a separate project, or on a long, large feeder cable.
Covered conductors
Transformer tails are designed for use between transformer bushings and overhead lines. They do not have any specific voltage rating and must not be considered insulated. As a result, they must not come into contact with each other, with earth, or with anything else.
They should be treated as bare conductors as their plastic covering does not provide insulation, only possible short-term protection in the event of accidental contact. If they do come into contact with each other, or earth, the covering will suffer electrical breakdown and flashover may occur shortly thereafter.
Ronald (Dick) Hardie, Pr. Eng. (Retired) BSc (Eng), FSAIEE ♦
