Lessons · Electrical · when one branch opens
One branch opens: what the others feel
In parallel, an open branch simply stops carrying its current and the others carry on unchanged; in series, an open anywhere stops everything.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
Half the receptacles in a room go dead and the other half work. That pattern alone tells you whether the fault is an open in a shared path or one branch on its own, before you have opened a cover. The old string of lights that all went dark when one bulb blew was a series circuit; a modern room is parallel.
How to think about it
Ask: is the thing that failed in the only path, or in one of several? If one of several, the voltage on the rest does not change, so their currents do not change; only the total in the feed drops. If it was in the only path, the current everywhere is zero.
Worked example
Three 60 Ω lamps in parallel on 120 V: each draws 120 / 60 = 2 AEach lamp sees the full 120 V.
Feed carries 2 + 2 + 2 = 6 AThe branch currents add.
One lamp burns out (opens): the other two still see 120 V, still 2 A eachNothing changed for them. A parallel branch cannot feel its neighbour.
Feed now carries 2 + 2 = 4 AThe only thing that changed is the total. The breaker sees less, not more.
Same three lamps in series: one opens, current = 0 A everywhereOne path, one break, no current. All three go dark.
Your turn
Five 1 A branches in parallel, and two of them open. Write the current left in the feed.
I = 1 + 1 + 1 = A
Solve one, graded on the server
The trap
Expecting the surviving lamps to get brighter or dimmer when a parallel neighbour fails. They see the same volts as before, so they do exactly what they did before. A change in brightness means the fault is in the shared path, not in the branch.