Force guided cables demand special attention during their initial selection. Whether they are intended to supply power,
control, voice, data or other signals, they must fulfill a large number of electrical and mechanical criteria.
The mechanical criteria of the force guided cable is the primary issue, as a cable that fails mechanically will surely also fail electrically.
However, the correct electrically based selection of the cable is also essential as it allows the mechanical functions to be achieved effectively and economically.
The designer must consider his electrical selection carefully as the force guided cable is a more complex and expensive design than that of conventional fixed installation type cables. An incorrect selection may cause significant impact on the cost and performance of the cable and the cable handling system as well.
This section is intended to assist the designer by highlighting electrical factors that can have significant impact on the economy and reliability of the finished cable handling system.
Correct sizing of flexible force guided cables has great impact on the cost and successful operation of force guided cable systems. A single cable size reduction will reduce the size, weight and cost of the cable and the cable handling equipment to a degree that often will produce significant economic benefit.
Perhaps the single most important point for sizing of power cables is the correct calculation of the R.M.S. current required by the material handling equipment.
Typically, cranes and material handling equipment have very cyclical loads with regeneration during deceleration.
The basis for ampacity tables is the thermal capacity of the conductor at continuous R.M.S. current, therefore a true R.M.S. load should be calculated for each case.
When both cyclical and regenerative portions of the duty cycle are properly considered, the result is often a surprisingly small R.M.S. value when compared to the total connected load. The Intermittent load duty factors can assist in this calculation.
The selection of the optimum system voltage and the correct conductor size is the next step
Mid range conductor sizes (ie. #2 AWG to #2/0 AWG) offer the best overall performance with regard to their ability to be reeled, regardless of voltage rating.
Cables smaller than #2 AWG may have strength to weight ratios that do not allow unrestricted reeling use. The larger conductor sizes (above #2/0 AWG) often become too large and heavy to allow a practical and economical cable handling system to be designed.
It is interesting to note that a conductor selection in this size range has economic as well as technical benefits. In fact, there are applications where the cost of transformers to change existing voltages can be recovered by the savings in the costs of the reeling cable, reel, and associated hardware.
The selection of the correct cable size is important in other respects also. For example, when round cables are used in festoon systems, it is desirable to keep all cables in a similar size range to promote a balanced system.
Very small (e.g. data) cables and very large (e.g. power) cables can be troublesome on the same festoon systems and should be avoided.
Another aspect of cable sizing that can have significant impact on cost and performance is the issue of spare conductors.
We feel that including spares as a precaution in the event of a system change or cable damage is a valid approach.
However, it must be considered that when purpose built cables such as CORDAFLEX (K), CORDAFLEX (SM) or SPREADERFLEX® are used in a properly designed and installed force guided system, there are virtually no cases where any conductor failure will occur in normal operation. Proper consideration of this issue may lead to a more economical and effective cable choice.
Medium voltage reeling cables are generally used in a symmetrical star point circuit. In the case of a phase to ground fault
it is important to limit the phase to ground current to avoid injury to personnel and damage to equipment. An impedance grounded
circuit ensures that any phase to ground fault will produce limited current flow which will be detected by the protection
circuit which in turn will interrupt the power supply.
The grounding of the neutral conductor through an impedance is in fact required by the NEC in article 250-154 (a).
The NEC requires, in article 250-154(d), that the integrity of the ground path through aground conductor be continuously monitored and in the event of a loss of integrity (or signal) that the power supply be automatically de-energized. This requirement does not specifically require the use of a ground check conductor in the cable. The continuity of the path to ground through the cable can be monitored through commercially available ground check circuitry.
Establishing the correct R.M.S. current required for material handling equipment is the fundamental starting point for correct cable selection.
Many types of material handling equipment have very cyclical loads that may also produce significant regeneration. When these factors are properly calculated, the R.M.S. requirement is often surprisingly small when compared with the total connected load. The table that follows can be used as a guide to allow for the typical intermittent load characteristics of material handling equipment. However, it cannot substitute for a properly calculated R.M.S. load cycle.
Ampacity conversion factors are based on intermittent loads with a 10 minute duty cycle and total duty factor as a function of operating time (%). These factors should be used in conjunction with the values given in the application chart.
Click here for the intermittent load duty factors tableWhen the R.M.S. value has been calculated, the conversion table ampacity table can be used to select the correct conductor size at a conductor temperature of 900C. However, it should be noted that 900C PROTOLON-EP insulated cables may be operated at an emergency overload temperature of 1300C for a maximum of 100 hours per year and a total maximum of 500 hours in the life of the cable. (ICEA S-68-516, NEMA WC8, Appendix D)
The Ampacities presented in the following tables are extracted from West German VDE Standards. Ampacities in the tables are based on an ambient temperature of 300C and multi-conductor cables with three loaded conductors in free air. These ampacities allow for a final conductor temperature of 900C for PROTOLON EP insulated cables and 700C for PROTODUR PVC insulated cables.
These ampacity and rating tables are not intended to supersede other ampacity tables such as NEC or ICEA. However, they are intended to provide a comprehensive means of conductor size selection and verification of metric cross-reference.
Click here for the Ampacity and metric conversion tableThe following table lists rating factors for the ampacity of various conductor counts. These factors may be applied to the values given in the Ampacity table.
Only the total number of conductors carrying load simultaneously need to be considered to determine the total thermal effect on the cable.
Number of Conductors Simultaneously at Full Load
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 9 | 12 | 16 | 18 | 20 | 24 | 30 | 36 | 44 | 54 |
| 1.43 | 1.14 | 1.00 | 0.80 | 0.75 | 0.69 | 0.65 | 0.58 | 0.52 | 0.48 | 0.45 | 0.43 | 0.40 | 0.38 | 0.35 | 0.32 | 0.30 |
These ambient temperature rating factors are applicable to the ampacities of PROTOLON EP insulated cables at a conductor temperature of 90° C
| Ambient Temperature |
°F °C |
50 10 |
59 15 |
68 20 |
77 25 |
86 30 |
95 35 |
104 40 |
113 45 |
122 50 |
131 55 |
140 60 |
149 65 |
158 70 |
167 75 |
| Correction Factor |
1.18 | 1.14 | 1.10 | 1.05 | 1.00 | 0.95 | 0.89 | 0.84 | 0.77 | 0.71 | 0.63 | 0.55 | 0.45 | 0.32 |
These ambient temperature rating factors are applicable to the ampacities of PROTODUR - PVC insulated cables (e.g. SPREADERFLEX) at a conductor temperature of 70° C
| Degrees F | Degrees C | Correction Factor |
| 86 to 95 | 30 to 35 | 0.94 |
| 95 to 104 | 35 to 40 | 0.87 |
| 104 to 113 | 40 to 45 | 0.79 |
| 113 to 122 | 45 to 50 | 0.71 |
| 122 to 131 | 50 to 55 | 0.61 |
These ratings should be applied to installations where full load currents are expected with cable layered on the reel. The correction factor of 0.76 applies to round cables on single layer reels and mono-spiral reels, to allow for thermal effects of adjacent coils on the reel. When reeled on a mono-spiral reel, flat cables level wind must be derated by a factor of 0.47.
| Number of layers | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Correction factor | 0.76 | 0.58 | 0.47 | 0.40 | 0.36 | 0.34 | 0.32 | .030 |
There are often applications where both power and control must be handled through a single force guided cable.
However, most composite combinations of dissimilar sized conductors do not allow a successful composite cable design to be developed.
In these circumstances electrically paralleling conductors is common and is the only technically and commercially viable alternative. Although this practice is generally discouraged in codes, it is interesting to note that NEC article 620 allows paralleling of conductors in traveling cables under certain circumstances. Making your local inspection authorities aware of this clause can lead to approval of paralleled conductors in force guided cables.
When selecting a cable fob this type of application, the ampacity must be debated to allow fob the mutual heating effect of multiple conductors in a single cable.
Please refer to Derating factors for multi-conductor cables to allow the proper conductor size to be selected.
Where the short circuit bating governs conductor size selection, the cost of an increase in conductor size must be carefully considered.
In most cases, the cost impact on both the reeling cable and the reeling system are sufficient that even a single conductor size increase would make alternate methods feasible.
The use of fast acting current limiting type fuses in this type of application may eliminate the need to increase conductor size.
The table shows the thermally-permissible 1 second short-circuit current fob cables with PROTOLON insulation. The values are based on an initial conductor temperature of 80°C and a final conductor temperature of 200°C. This nominal final temperature is based on the melting point of tin at 231°C. Although PROTOLON EP insulation is suitable fob a final conductor temperature of 250°C during short circuits, the nominal maximum temperature of 200°C prevents possible damage to the conductor caused by melting of the tin coating.
Other thermal restrictions may also apply to cables with PROTOLON insulation (e.g., softsoldebed splices used as a cable repair method). In this case, the maximum short circuit temperature should be restricted to 160°C by applying a factor of 0.85 to the maximum short circuit current. Such restrictions would not apply with CADWELD® conductor splices
Click here for the short circuit ratings tableThe following formula may be used to convert the values in the table to other short-circuit durations of up to 5 seconds.
Where: I(t) = Thermally permissible short circuit current in kA.
I(1s) = Thermally permissible 1 second short circuit current in kA.
t = Short circuit duration in seconds.
A factor which may influence the conductor size selection of a reeling cable is voltage drop. The designer should try to make allowance in other parts of the power distribution system to avoid increasing the size of the reeling cable.
For example, an increase in the operating voltage may reduce conductor size. Other alternatives are to increase the conductor size of the cables in the fixed runs that form part of the circuit. These may be increased in size at a fraction of the cost of the reeling cable/reeling system.
Correct selection of the distribution transformer may also eliminate the impact of voltage drop from being a factor in conductor size selection.
When the optimum conductor size has been calculated utilizing the ampacity table, the result may be verified for voltage drop by using the following formulas. The ampacity table will enable conversion of AWG to mm2 to assist in completing this calculation.
Click here for the voltage drop formulasThe electronic revolution has had considerable impact on the material handling industry in recent years. In this information age, increasing levels of automation, electronic data interchange (EDI), closed circuit television monitoring (CCTV) and other electronic and voice communications are becoming integrated into most facets of industry
For such information based systems to operate effectively, a network of wire and cable (LAN) must link the various components within the system and provide reliable transmission of information.
However, the mechanical demands imposed on force guided cables have not allowed these increasing data communications rates to be achieved without compromise.
Typically, co-axial, twin-axial, or twisted shielded pair cables, are utilized for data transmission in fixed installations.
When installed in a fixed route and segregated and/or shielded to avoid the effects of Electromagnetic interference (EMI) and radio frequency interference (RFI), such cables can easily accommodate data transmission rates into the Megahertz range over considerable distances with high reliability and minimal losses.
However, the mechanical requirements of force guided cables do not allow use of such standard cable types.
Force guided cables must achieve the same electrical tasks while still remaining flexible, retaining tensile strength and performing the mechanical tasks without loss of reliability.
Active pursuit of these often conflicting goals has led to the development of a standard range of products that can cater to the needs of the local area network (LAN) designer while still maintaining the mechanical integrity of the force guided cable.
The most simple of these is the unshielded control cable. A large standard range of CORDAFLEX (K), CORDAFLEX (SM), PLANOFLEX and SPREADERFLEX control cables is available which can handle many electronic signals, low level data transmission and voice communications.
To maintain the mechanical integrity required of force guided cables, these control cables are generally not available below #16 AWG. Low capacitance insulation materials are generally excluded as they do not provide sufficient flexibility and mechanical integrity for force guided applications. This class of cables is usually restricted to relatively short runs of low frequency information in areas where EMI and RFI are not a factor.
A number of standard versions of CORDAFLEX (K) and PLANOFLEX cables are available with individually shielded conductors. Individually shielded conductors are also available as an option in CORDAFLEX (SM) and SPREADERFLEX cables.
The individually shielded conductor has similar mechanical characteristics to the unshielded control conductor, but it utilizes a high coverage (90% minimum) tinned copper wire braid shield over the conductor insulation to maintain the integrity of the data transmission from ambient EMI or RFI.
Individually shielded conductors have been successfully used for various electronic signals, voice communications and data transmission at rates of up to 19,200 baud over considerable distances.
Much higher data transmission rates are possible with individually shielded conductors where route lengths are limited and ambient EMI and RFI are not extreme. Where high data transmission rates are applied to individually shielded conductors, the electronics manufacturer's instructions regarding system capacitance and impedance must be observed and shield grounding and segregation from power cables may also have to follow specific guidelines.
Where slip rings are employed, specific manufacturer's guidelines may also apply. However, most established reel manufacturers offer a range of precious metal plated slip rings or mercury wetted slip rings that are suitable for data transmission.
Although there are some limitations to the electrical performance of the individually shielded cable, these are offset by the mechanical benefits. The individually shielded conductor behaves mechanically in a fashion very similar to that of an unshielded conductor and therefore it can be cabled into a wide variety of standard force guided cable constructions with virtually no loss in mechanical performance.
This characteristic allows the individually shielded conductor to become the standard choice where a compact and proven flexible cable design is required. The individually shielded conductor is often the most economical choice for low to medium speed data transmission of voice or other electronic signal applications.
The following table allows the system designer to validate the suitability of the electrical characteristics of the individually shielded conductor for specific applications.
CORDAFLEX (K) and PLANOFLEX cables are available in standard versions with twisted shielded pairs. Optional CORDAFLEX (SM) designs also available.
The twisted shielded pair is based on #16 AWG conductors to provide the optimum balance between mechanical integrity and electrical performance. It utilizes a high coverage (90% minimum) tinned copper wire braid shield over the twisted pair to maintain the integrity of the data transmission from ambient EMI or RFI.
Twisted shielded pairs are successfully used for applications where electronic signals, voice communications and data transmission at rates up to 100 K baud must be transmitted over considerable distances.
Even higher rates are possible with short routes where ambient EMI and RFI are not extreme.
Where high data rates are applied to twisted shielded pairs, the electronics manufacturer's instructions must be carefully implemented. System capacitance and impedance become increasingly important as data transmission rates increase and such factors as impedance matching of fixed and force guided cables in the same circuit must be considered.
Correct shield grounding and segregation from power cables must be carried out to meet the circuit's requirements.
Specific requirements relating to the use of slip rings in the circuit may also apply.
Established reel suppliers offer a range of precious metal plated or mercury wetted slip rings that accommodate successful data transmission.
While the twisted shielded pair cable allows considerably higher data rates than the individually shielded cables, this is not without cost. The twisted shielded pair generally results in a bulkier, heavier and more expensive cable which `does not lend itself to the same broad variety of cable designs.
The twisted shielded pair also does not tolerate cable tension as well as the individually shielded conductor.
The following table allows the system designer to validate the suitability of the electrical characteristics of the twisted shielded pair for specific applications.
The trend to ever higher rates of data transmission continues. Manufacturer's Automated Protocol (MAP), for example seeks to integrate different hierarchy's of data transmission while high level protocols continue to reach higher than ever before into the materials handling industry.
Protocols such as Ethernet are beginning to make their presence felt at high data transmission rates and even industrial programmable logic controllers are beginning to communicate in the megahertz bandwidth.
Such developments have been swiftly moving the force guided cable's twisted shielded pair out of consideration for the LAN designer.
In fixed applications, special low capacitance twin-axial and co-axial cables are able to handle these increased speeds, but even there, the optical fiber cable is beginning to make its presence felt.
In force guided applications, the step to fiber optics is even more logical, as special high data rate co-axial and twin-axial cables cannot be adapted to the mechanical requirements of force guided application, while the optical fiber's characteristics allow this step to be taken with relative ease.
Optical fiber has advantages over copper conductor based cables in the following respects:
These characteristics have enabled a large standard range of optical fiber based forced guided cables to be developed that are beneficial to the materials handling industry.
In composite form, fiber optics are now available with all products in this catalog. Also a new fiber optic product designated OPTOFLEX® has been developed for fiber only applications.
Complete information on these available products and their application in force guided systems is available in the separate catalog entitled - "Flexible fiber-optic cables for use on cranes, hoists, and bulk materials handling equipment". Catalog number P9204 April 1989.
Copies are available from your nearest Siemens sales office or distributor
