The control cables can be divided into the current transformer circuits; potential circuits; the power control circuit; the pilot contactor circuit, fault control and auxiliary circuits, SCADA input circuits and the start alarm circuit.
The current transformer circuits must have a burden on the current transformers of a maximum of 1VA to achieve this 2.5mm2 cables are selected which have a burden of 0.34VA/m this equates to a maximum length of 2.94metres. The cables must also have a 5A current rating at the starter design temperature and must be capable of withstanding the maximum current that can be produced by the current transformers under short circuit or overload conditions.
The potential circuits are from the main switch fuse, or where potential fuses are fitted from the load side of the potential fuses to the supply failure relay and the transducer voltage inputs including their protective MCBs. The cables must have a 6A current rating at the starter design temperature to achieve this 1.5mm2 cables are selected.
The power control circuit is protected by a 10A MCB, it follows therefore that the power control circuit cable has to have a current rating at the starter design temperature of 10A with an I2t rating greater than the MCB I2t cut off rating.
A 2.5mm2 cable is selected to minimise volt drop within the power control circuit which is subject to high inrush currents when the contactor(s) are energised.
The pilot contactor(s) circuit, fault and auxiliary circuits are protected by a 4A MCB the cable must have a minimum current rating of 4A at the starter design temperature with an I2t rating greater than the MCB I2t cut off rating.
A 1.5mm2 cable is selected as it permits reasonable length for the control system, it is also the preferred size of the control system field wiring.
The SCADA system wiring is protected by a 2A MCB the cable must have a minimum current rating of 2A at the starter design temperature with an I2t rating greater than the MCB I2t cut off rating.
A 1mm2 cable is selected as the SCADA system signal levels are extremely low.
The start alarm circuit is protected by a 6A MCB the cable must have a minimum current rating of 6A at the starter design temperature with an I2t rating greater than the MCB I2t cut off rating.
A 1.5mm2 cable is selected as it permits a reasonable length for the start alarm circuit, it is also the preferred size of the start alarm system field wiring.
To maximise the use of the design 105oC tri-rated switchgear cables to BS6231 type CK; which also conforms with European harmonised cable type HO7V2-K; are selected for the starter control cabling, these cables also meet the UL requirements of UL and of CSA for sizes from 0.5mm2 to 120mm2.