Junction Box
A junction box is a connection enclosure in which cable joints are made over terminals and whose lid can be removed for access inside. It gathers the joints at a single documentable point and protects the connections from mechanical load and from the outside environment. Outdoors it is used with a gasketed lid and correctly sized cable glands.
Making a joint in an installation can be unavoidable: the cable comes up short, a consumer is added later, or the machine is transported in two parts. Where the joint is made, however, is entirely under your control, and the right answer is always inside an accessible box.
- Accessibility: the junction box is mounted where its lid can be opened; a box buried under bodywork is useless during maintenance.
- Mechanical load: the cable enters the box through a gland; the pulling force is taken by the gland, not by the terminal.
- Labelling: every terminal is numbered and marked on the circuit diagram.
- Moisture management: the entries face downwards, unused holes are closed with blanking plugs and a breathing plug is added if required.
Using a junction box has a side benefit for diagnosis as well: when you are looking for a fault you gain a natural measuring point that splits the circuit in two. A box in the middle of a long run lets you narrow down which half the fault is in with a single measurement.
Context in which this term is used: Installation, redundancy and field continuity