By Ronald (Dick) Hardie, Pr. Eng. (Retired) B Sc (Eng), FSAIEE
The actual current rating of a cable – derating factors are real
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 in close proximity to each other.
There are also other standard conditions for altitude, soil resistivity, depth of burial, etc. If any of the actual site conditions are worse than standard or can become worse during the life of the cable, then 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 cables’ current rating can be appropriately re-rated upwards.
Single core cables are very different from three or four core cables and should be installed differently
When the cable’s conductor size is required to be quite large, often there is 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. These cables need to be installed with care and treated differently from three or four core cables for the following reasons:
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, within the metallic screen or armour of each core, there is an electromagnetic field arising from only one phase, and 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. In the case of single core cables, this is not always recommended, 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 with an expert, in order to arrive at an optimum solution and avoid embarrassing cable failures as a result of these induced voltages or circulating currents.
No amount of testing of an electrical cable will improve its quality
The quality of an electric cable is a given. It has been 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 practice to retest the cable. Over-testing the cable by using an excessively high voltage or applying this voltage for a longer time than is necessary will not in any way improve the quality of the cable but may very well damage an otherwise acceptable cable.
The real reason for the retest should be simply 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 a relatively low voltage, applied for a fairly short duration, and this is particularly important when testing XLPE cables. There are published recommendations for such tests.
You need to know the fault level at the position in the network you are installing a cable, otherwise you may just be installing a long (and expensive) fuse
Very often, only a small amount of power is required by a load, and it seems logical to assume that only a small cable would be needed to carry this load. However, we need to 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 where the cable is installed.
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.
A cable just big enough to do the job is not always the cheapest cable
An electric cable selected for a particular 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. Sometimes it is selected slightly larger to accommodate future load growth.
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 necessary to allow the cable to lose the conductor heat losses via natural cooling to the environment. The heat loss to the surroundings, although necessary, represents wasted energy.
Using a larger conductor would lower the conductor temperature and reduce this 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. 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 an excessively high initial cost.
The coloured stripe on an electric cable does not indicate fire resistance
Within SANS 1507 there is mention of coloured stripes on the outer cable sheath to indicate certain fire performance properties.
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 is not a very nice cable to have burning in a fire situation and should not be used in a confined space where people may be present. The HCl gas may also attack any metal 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 burning 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. Certainly, this is the only cable to be used underground, and also in confined spaces like tunnels etc 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 discontinuation of this product type.
All the above cables are FR, ie flame retardant and will not burn for more than a certain length for a defined period as specified in IEC 60332.
Installing a number of parallel cables does not necessarily give a cheaper installation
There are many installations where the designer elected to install two or more parallel cables, rather than a correctly sized and much more expensive larger conductor. For example, although a 25 mm2 copper LV cable is rated 110 Amps in air, and a 150 mm2 copper conductor is rated 330 Amps in air, you cannot assume that three parallel cables of 25 mm2 would be able to carry 330A in air. You would need to apply the ‘group’ derating factor, and furthermore check if each cable is able to individually handle fault conditions.
It is interesting to note that the current density within a cable’s conductor is not a constant figure. The current density gets lower as the conductor size increases. This is due to the surface area of a conductor increasing more slowly than the cross-sectional area, for example, the current rating of a 1.5 mm2 copper conductor is about 20 Amps per phase. The current density works out at about 13 Amps per mm2. With larger conductors, we see that a 10 mm2 conductor has a rating of about 60 Amps, ie the current density has dropped to only 6 Amps per mm2. The largest conductor in a table of current ratings is 300 mm2, and for this size the current rating is approximately 510 Amps. The current density has now reduced to just above 1.5 Amps per mm2!
The reason for the above 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 a number of parallel cables to arrive at the required load current, we need to remember that each cable in the system needs to 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.
The importance of a cable’s outer sheath (sheath integrity testing after installation)
It’s wrong to believe that the conductor or the insulation is the most important component of an electrical cable. Whilst 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 the ingress of water 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 crew is 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 a hole, or holes, in the outer sheath. These must be located and repaired. The eventual leakage current may be recorded for future reference. On important cables this test may be repeated six-monthly or yearly.
Is it worthwhile to use aluminium conductors?
The price per ton of aluminium metal is cheaper than copper. It does, however, have a much lower density than copper, so you get a greater volume per ton than copper. Unfortunately, it does not have the same good conductivity as copper, so you need to use more when making a cable of equivalent current rating. Despite this, the cost in Rands per Amp carried, is cheaper to use aluminium conductors than copper conductors.
However, this is not the end of the comparison. Most electrical connectors are designed for copper conductors, and when used with aluminium conductors, there may well be a compatibility problem where these dissimilar metals meet. The chemical compatibility is easily handled with special greases etc, but the thermal differences are more problematic. In particular, aluminium has a greater coefficient of expansion with temperature than copper, so after a few load cycles, an aluminium conductor inside a copper lug or ferrule would become loose. This would become a ‘hot’ connection, and failure would result.
When using aluminium conductors, it is therefore necessary to use aluminium connectors, and special bi-metallic lugs and ferrules when connecting the aluminium conductors to copper busbar 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.
Generally, users tend to 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, on a separate project for example, or on a long and large feeder cable.
Covered conductors: The voltage rating of, and use of transformer ‘tails’
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 to be ‘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, and their plastic covering does not provide insulation, only a possible short-term protection in the event of accidental contact by rodents, snakes etc. If they do come into contact with each other, or earth, the covering will suffer electrical breakdown, and a flashover will probably occur shortly thereafter.