The slip ring, the key component
For current to flow while the drum turns, the cable cannot be hard-wired. A set of rings and brushes provides the electrical connection continuously. It is what defines the number of conductors available, the admissible current and the service life of the reel.
Three characteristics are read together: the number of poles, which must cover phases, neutral and earth; the cross-section of the cable; and the admissible current per pole. A three-pole slip ring is not enough for a three-phase supply with neutral and earth, and that cannot be recovered after installation.
A wound cable heats up
A cable left on its drum dissipates the heat it produces poorly. Its admissible current is therefore markedly lower wound than unwound, and the gap reaches half on site reels. On a station where the cable remains partly wound at all times, it is the “wound” value that must be used for sizing.
Enclosed, open, motorised
The enclosed reel protects the cable and suits workshops and traffic areas. The open reel accepts greater lengths and larger cross-sections, at the price of a visible footprint. The motorised reel is required on on-board applications — aerial platforms, overhead cranes, telescopic booms — where manual pulling is no longer conceivable.
The spring pulls permanently, including at rest. That is the limit the choice between spring and motor must settle, and it is not only a matter of length. A spiral spring keeps the cable under tension whatever the position of the machine; on a large cross-section left unwound for hours, that constant pull fatigues the core. A motorised reel controls its effort and can bring it down to almost nothing once the machine is stationary.
A simple check: ask for two lengths, not one. A spring reel does not accept the same length depending on whether the cable runs out horizontally or descends vertically, since the weight of the unwound part then adds to the effort taken up by the spring. At STEMMANN, a size rated for 60 m in horizontal run-out falls to 27 m in vertical descent — more than half. A supplier who states only one value has not sized the descent case, and that is the one that breaks.
One case escapes this rule and deserves to be known: if the reel rises and descends with the machine and the supply stays at the fixed point, the weight of the cable no longer bears on the sizing of the spring. This arrangement allows lengths far greater than a classic descent, and it is too rarely considered.
The special case of earthing
Earthing reels do not distribute energy: they discharge electrostatic charges during a transfer of flammable liquid, between a tanker or a drum and the earth of the installation. They often include a clamp and, depending on the model, a continuity monitor that permits or prohibits the transfer. It is safety equipment in its own right, dealt with on our ATEX page.