Control cable is the least glamorous item in an installation and the one that causes the most
puzzling faults. It carries commands to actuators, statuses back from switches, and analogue
signals from instruments - and unlike power cable, where the failure modes are obvious, control
cable fails subtly. A signal picks up interference and the reading wanders. A screen is earthed
at both ends and a circulating current appears. A pair is shared between a 4-20 mA loop and a
relay coil and the loop twitches every time the relay operates.
The three decisions that matter are screening, armour and fire performance, and each follows
from the circuit rather than from a catalogue.
Screening is about the electrical environment. An industrial site is full of variable-speed
drives, contactors and radio transmitters, all of which induce voltages into any conductor
running nearby. A low-level analogue signal - a thermocouple, a 4-20 mA loop, a strain gauge -
has to be screened, and the screen has to be earthed at one end only so it does not become a
conductor in its own right. Individually screened pairs matter where several signals share one
cable, because a common screen does nothing to stop them interfering with each other. Digital
signals and switched contacts are far more tolerant, so paying for individual screens on those
circuits is waste.
Armour is about mechanical risk. Steel wire armour is specified where the cable can be
walked on, driven over, dropped on or chafed, and it also provides an earth path and a degree of
rodent protection. Where the run is inside trunking or conduit in a clean area, it is
unnecessary weight and cost - and it makes glanding more involved at every entry.
Fire performance is about consequences. Flame retardant cable resists spreading a fire along
its own length. Low smoke halogen free matters in enclosed and manned spaces, because ordinary
PVC insulation produces dense acrid smoke that fills an escape route. Fire resistant cable -
which keeps a circuit working for a defined period in a fire - is a different category again, and
is what emergency and life-safety circuits require. Confusing the three is a common and serious
specification error.
Beyond the type, the things that decide whether a control installation behaves are settled at
design stage: segregation from power circuits, conductor size against volt drop over the run,
core count with spare cores allowed for future use, and the gland and termination detail. We
specify and supply against your cable schedule with the certificates the installation needs,
rather than offering a single part and calling it control cable.
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