What are the rules for making joints inside a junction box or connection box?
A joint should preferably not be made at all; if it is unavoidable it must be made inside an accessible box, with a crimped connection or a terminal. The gland on the cables entering the box must suit the seal diameter, unused holes must be closed with blanking plugs, and a breathing ePTFE plug must be used so that moisture does not build up inside the box. The joint point should give a drop below 0.2 V under load.
Every joint is a new potential fault point. If it is unavoidable, minimise the risk with these rules:
- Accessibility: the joint should always be inside a box that can be opened. A joint buried inside conduit and impossible to find later is the worst choice.
- Type of connection: in a vibrating environment use a crimp butt splice or a screw terminal; fit a ferrule on stranded conductors. A soldered joint fatigues and breaks at the point where the solder ends.
- Strain relief: leave slack for the cables inside the box and fix them just outside the entry so that pulling force does not reach the terminal.
- Sealing: choose the gland to suit the cable diameter and close unused holes with blanking plugs. Position the entries facing downwards and give the cable a drip loop before the box.
- Condensation: a fully sealed box also keeps moisture inside. As the temperature changes, the humid air drawn in condenses when it cools. The solution is a breathing plug or breathing gland with an ePTFE membrane that equalises pressure but does not pass water.
- Labelling: mark every strand; it markedly shortens the later diagnosis time.
When the job is done, measure the joint point under load: below 0.2 V is good, above 0.5 V is unacceptable.
The full topic: Installation, redundancy and field continuity