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The cable calculator for ships determines the short-cycle duty rating and intermittent service rating for cables used in fixed electrical systems on board ships, operating at voltages up to 15 kV, in accordance with the international standard IEC 60092-352.
Posted 2025-04-10
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IEC 60092 forms a series of International Standards concerning electrical installations in sea-going ships and fixed or mobile offshore units, incorporating good practice and co-ordinating as far as possible existing rules. Part 352 of the series covers choice and installation of electrical cables. This standard IEC 60092-352 provides the basic requirements for the choice and installation of cables intended for fixed electrical systems on board ships at voltages up to and including 15 kV. This does not include sub-sea and umbilical cables.
The procedure for cable selection employs rating factors to adjust the current carrying capacities for different ambient temperatures, for the mutual heating effects of grouping with other cables, methods of installation and short time duty. Guidance on the use of these factors is given below.
Continuous service for a cable is to be considered, for the purpose of this standard, as a current-carrying service with constant load and having a duration longer than three times the thermal time constant of the cable, i.e., longer than the critical duration (see Figure 2).
The current to be carried by any conductor for sustained periods during normal operation shall be such that the appropriate conductor temperature limit is not exceeded. The value shall either be:
If a cable is intended to supply a single motor or equipment operating for periods of half an hour or one hour, its current rating, as given in the relevant table (see Annexes A and B), may be increased using the relevant correction factors obtained from Figure 1. These correction factors are only applicable if the intermediate periods of rests are longer than the critical duration (which is equal to three times the time constant of the cable), obtained from Figure 2, as a function of the cable diameter $D_e$.
$$T = 0.245 \times D_e^{1.35}$$The correction factors given in Figure 1 are approximate and depend mainly upon the diameter of the cable. In general, the half-an-hour service is applicable to mooring winches, windlasses, heavy cargo winches and bowthrusters. The half-an-hour rating might not be adequate for automatic tensioning mooring winches and bowthrusters of specialised vessels.
$$F_{\text{scd}} = \sqrt{\frac{1.12}{1 - \exp\left(-\frac{t}{T}\right)}},$$ Intermittent serviceFor cables supplying a single motor or other equipment intended to operate in an intermittent service, as is generally the case for cargo winches (except heavy cargo winches), engine room cranes and similar devices, the current ratings as given in Annexes A and B may be increased by applying the correction factor obtained from Figure 3.
The correction factor given in Figure 3 has been calculated for periods of 10 min, of which 4 min are with a constant load and 6 min without load.
$$F_{\text{is}} = \sqrt{\frac{1 - \exp\left(-\frac{t}{T}\right)}{1 - \exp\left(-\frac{\text{ratio} \times t}{T}\right)}}$$ Different ambient air temperaturesThe current-carrying capacities tabulated in Annexes to the standard assume a reference ambient air temperature of 45 °C. This temperature is generally applicable to insulated conductors and cables in any kind of ship and for navigation in any climate, irrespective of the method of installation.
Where the ambient temperature in the intended location of the insulated conductors or cables differs from the reference ambient temperature, the appropriate correction factor specified in Table 3 shall be applied to the values of current-carrying capacity.
The correction factors in the following table do not take account of the increase in temperature, if any, due to solar or other infrared radiation. Where the cables or insulated conductors are subject to such radiation, the current-carrying capacity shall be derived by the methods specified in IEC 60287.
Table : Correction factor for various ambient air temperatures| Conductor temp. $\theta_c$ |
Ambient air temperatures $\theta_a$ | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 35 °C | 40 °C | 45 °C | 50 °C | 55 °C | 60 °C | 65 °C | 70 °C | 75 °C | 80 °C | 85 °C | |
| 60 °C | 1.29 | 1.15 | 1.00 | 0.82 | - | - | - | - | - | - | - |
| 65 °C | 1.22 | 1.12 | 1.00 | 0.87 | 0.71 | - | - | - | - | - | - |
| 70 °C | 1.18 | 1.10 | 1.00 | 0.89 | 0.77 | 0.63 | - | - | - | - | - |
| 75 °C | 1.15 | 1.08 | 1.00 | 0.91 | 0.82 | 0.71 | 0.58 | - | - | - | - |
| 80 °C | 1.13 | 1.07 | 1.00 | 0.93 | 0.85 | 0.76 | 0.65 | 0.53 | - | - | - |
| 85 °C | 1.12 | 1.06 | 1.00 | 0.94 | 0.87 | 0.79 | 0.71 | 0.61 | 0.50 | - | - |
| 90 °C | 1.10 | 1.05 | 1.00 | 0.94 | 0.88 | 0.82 | 0.74 | 0.67 | 0.58 | 0.47 | - |
| 95 °C | 1.10 | 1.05 | 1.00 | 0.95 | 0.89 | 0.84 | 0.77 | 0.71 | 0.63 | 0.55 | 0.45 |
Following points are only covered here briefly and we refer to the standard for more details.
Cable groupingIn the case of a group of insulated conductors or cables the current carrying capacities tabulated are subjected to the group correction factors given in the relevant annex.
The group correction factors are applicable to groups of insulated conductors or cables having the same maximum operating temperature. For groups containing cables or insulated conductors having different maximum operating temperatures, the current carrying capacity of all the cables or insulated conductors in the group shall be based on that of the lowest maximum rated conductor temperature of any cable in the group together with the appropriate group correction factor.
Parallel connection of cablesThe current carrying capacity of cables connected in parallel is the sum of the current ratings of all parallel conductors but the cables must have equal impedance, equal cross-section, equal maximum permissible conductor temperatures and follow substantially identical routing or be installed in close proximity. Connections in parallel are only permitted for cross-sections of 10 mm$^2$ or above. When equal impedance can not be assured, a correction factor of 0,9 shall be applied to the current carrying capacity.
Let us try an example now.
| Parameter | Short cycle duty | Intermittent service |
|---|---|---|
| Correction factor | 1.554 | 1.450 |
| Permissible current | 310.9 A | 290.0 A |
| cable time constant | short cycle duty | intermittent service |
|---|---|---|
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| Figure 1 | Figure 2 | Figure 3 |
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