Each major component of the motor control system, that is, the starter and where fitted each control panel, will have an internal wiring system normally installed at the factory where they are built. They will also have external wiring systems which will be installed on site that external wiring is normally referred to as field wiring.
A simpler definition of field wiring is that cabling and wiring installed and connected on site as opposed to wiring installed within the MCC or control panel by the MCC or control panel manufacturer.
Field wiring for the starter is defined as that wiring between the starter and the motor and it’s local control station and where fitted the local isolator, the emergency stops, the thermistors, start alarm system, travel limits and the first control panel.
Field wiring for the control panels is defined as that wiring between the control panel it’s preceding connection point, either a starter or a control panel, it’s succeeding connection point and any other devices connected to the control panel.
By using a standardised control system it is possible to design a standard field wiring system for a starter. The design will have variations due to the motors being either uni-directional or bi-directional or multispeed.
The main cables from the starter to the motor must have a minimum current rating of the motor full load current for the chosen installation method at the ambient temperature of the installation and the applied group factor for the installation.
The main cables from the starter to the motor must have a minimum current rating of the overload setting current divided by 1.45.
The main cables from the starter to the motor must have a minimum I2t (which equates to the cable K2S2) rating that exceeds the fuse I2t rating and the fuse time current curve must be below the cable short circuit rating at 0.2seconds.
Having determined the minimum cable size it is then necessary to calculate the volt drop at the motor terminals for both running and starting and to adjust the cable size as necessary to limit the volt drop to 2.5% at the motor full load current and 10% at the motor starting current.
It should be noted that for star delta starting the cables from the MCC to the motor are one cable per phase to the motor and one cable per phase back to the MCC for the star connection and two cables in parallel per phase for the delta connection.
Once the cable size has been determined it is then essential to ensure that the cable(s) can be terminated at the starter and motor, or isolator where included. It should be noted that motors can usually be supplied with a variety of termination boxes to cater for different cable sizes.
Where the termination cannot be made with a single cable to each termination point parallel cables should be considered. Where parallel cables are used it is essential to keep the length of both cables the same.
The final checks are to ensure that using the selected cable(s) a short circuit at the motor terminals will cause the fuses to ‘blow’ within the time laid down by regulations or that an earth fault at the motor terminals will cause the earth fault relay to trip within the time laid down by regulations, and where necessary to increase the cable size to ensure compliance.
In the pdf version of this article the current rating and volt drop tables for standard three core copper conductor cables with steel wire armouring and 90oC thermosetting insulation are appended to this section. It should be noted that the maximum size of cable in the table is limited at 2x240mm2, this is the maximum cable that can be terminated to a standard starter designed by Keemag Ltd.; larger cables may however be used if the main terminals are changed for a higher capacity terminal.
Previously in section 02.09.19.04 entitled starter design – control circuit voltage determination the optimum control circuit voltage was determined as 110Vac with the optimum size being 1.5mm2 which allows a maximum route length of the control cable of 422metres. That maximum length is made up of the starter to the local control station distance plus the cable lengths to any local isolators, travel limit switches and emergency stop buttons plus the cable lengths to any control panels; plus the cable length from the starter to the MCC relay panel or PLC compartment. If the MCC internal cabling from the starter to the relay panel or PLC compartment and the actual starter compartment wiring is assumed at 22metres then the maximum route length of the field control wiring is 400metres. The tables appended to the pdf version of this section show main cables of up to 500metres this is for illustration only, MCCs should be sited carefully to avoid excessive cable lengths.
Where necessary consideration should be given to increase the number of MCCs to keep the field cables to sensible sizes for the motors served by a MCC. If in a particular circumstance the distance is exceeded then interface relays between the control panels can be included. It is important that the field wiring from the starter to the local control station and associated local equipment is directly connected to the starter any interface equipment to increase the distance of the control system should be between control panels.
A sample of a MCC compartment with ident 09/916054 single line diagram is included in the pdf version of this article. The compartment houses a star delta starter for a 55kW motor located 150metres from the MCC together with it’s start alarms, local control station and it’s associated control panels. The tables in section 02.09.10.00 motor load data show a 55kW motor has a full load current of 103A.
It is a separate file entitled single line diagram.pdf with an original paper size of A1 (841mm x 597mm) printing on to an A1 sheet will give a scale of 1:1, on an A2 sheet a scale of 0.707:1 and on an A3 sheet a scale of 0.5:1. To be able to read the drawing a recommended minimum paper size of A3 should be used, at the stated scaling the general text on the drawing has a minimum height of 0.9mm, it should be noted that the push button indicators located in the control panels have 0.75mm high engraving at A1 which will reduce to 0.375mm at A3.
A sample field wiring diagram for that same compartment is included in the pdf version of this article, it adds the connection detail to the single line diagram described above.
It is a separate file entitled starter field wiring diagram.pdf with an original paper size of A0 (1189mm x 841mm) printing on to an A0 sheet will give a scale of 1:1, on an A1 sheet a scale of 0.707:1, on an A2 sheet a scale of 0.5:1 and on an A3 sheet a scale of 0.353:1. To be able to read the drawing a recommended minimum paper size of A2 should be used, at the stated scaling the text on the drawing has a minimum height of 0.9mm.
The drawings are the copyrighted property of Keemag Ltd.
It is included so that readers of this article can appreciate the complete integration of all the various sections of this article. It must not be used for commercial gain.
Drawings and specifications for commercial use for named clients, projects and project engineers are available for purchase.
It should be noted that the cable glands shown on the field wiring diagram are standard metric size cable glands to BS 6121, however it must be remembered that the size across the hexagon flats of the gland are not standard metric sizes in fact they are what in the trade is known as bastard sizes. That is they are specials. It is most disappointing that those who write standards cannot think beyond the narrow band of the subject they are writing the standard on; they should look to incorporate such other standards that are applicable to the subject. It would be a good idea if those who wrote BS 6131 actually fitted a gland made to that standard, something I bet none of those on that standards committee have ever done.