What is a series-circuit junction box?
The sealed box where the primary cable of a series loop is connected through — kept dry, earthed, labelled and ready to isolate for testing.
A series-circuit junction box is a sealed enclosure where the primary cable of a series loop is connected through to the next length. The cable enters through a gland, is joined at terminals or connectors, and leaves through a second gland. A removable link lets one section be isolated for testing, and the box is earthed and labelled so the circuit and cable ends can be identified.
A series loop has one path, so a junction box passes the current on; it never branches it. One wet or loose connection affects every lamp on the loop, and the CCR will keep driving current into an open circuit, so a box opened while live can put a dangerous voltage across the break. Open it only with the circuit isolated, locked out and proved dead.
- Isolate. CCR off, locked out and tagged; prove the circuit dead and earth or short it as the procedure requires.
- Open. Clean around the lid, remove it, and inspect the seal, entries and inside for water or damage.
- Connect. Make the terminal or connector joints to the specified torque and fit the removable link.
- Earth & label. Connect the earth, and label the circuit and each cable end.
- Seal. Refit the gasket and lid, and tighten the fixings evenly.
- Test & record. Continuity and insulation resistance through the box; log the readings.
Frequently Asked Questions
Q1What is a series-circuit junction box?
A sealed enclosure, usually in a pit or handhole or on a wall or plinth, where the primary cable of a series loop is connected through to the next length. It holds the connections, protects them from water and damage, and gives a place to isolate and test the circuit.
Q2Does a junction box branch the circuit?
No. A series loop has one path, so the same current passes through every connection. A junction box takes the cable in and sends it out; it does not split the current. A branch to a lamp is made through an isolating transformer, not a parallel tee.
Q3What is inside a junction box?
Cable entries or glands, terminals or connectors that join the incoming and outgoing cable, often a removable link to isolate a section for testing, an earth connection, a sealed lid, and a label that identifies the circuit and the cable ends.
Q4Why must a series circuit never be opened while it is energised?
A constant-current regulator keeps driving the set current even into an open circuit, so the voltage across the break can rise to a dangerous level. Always switch the CCR off, lock it out, prove the circuit dead and short or earth it as the procedure requires before opening a box.
Q5What problems do junction boxes cause?
Water ingress that lowers insulation resistance and causes an earth fault, loose terminals that overheat or open the loop, damaged seals, and mislabelled cable ends. Because every lamp shares the loop, one poor connection can put the whole circuit out.
Q6How is a junction box checked?
With the circuit isolated and proved dead, by inspecting the lid seal, entries, terminals, earth and labels, then testing continuity and insulation resistance of the circuit through the box and comparing the readings with the commissioning baseline.
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