Open Circuit
This is the interruption of the current path at some point; it is caused by a broken strand, a backed-out pin, a blown fuse or an oxidised contact. The typical picture is that voltage is measured on the line yet the load does not work at all, because with no current flowing there is no voltage drop either.
On an open circuit the voltmeter is misleading: a high input impedance instrument can show supply voltage ahead of the break. When the load is connected, the voltage collapses instantly. For that reason, when an open circuit is suspected the measurement must be made with the load connected.
- Continuity test: an ohm measurement between the two ends of the line on a de-energised circuit. On a sound short strand the reading is typically below 1 ohm.
- Halving: take a measurement at the middle of the line to identify the faulty half, then split that half in two again.
- Most common causes: a pin backed out inside a connector, wires broken inside a crimp, a blown fuse, a loosened terminal screw.
- Hidden break: the insulation may be intact while the strand is broken inside; measuring while flexing the cable reveals this.
If a device gives no response at all, start the diagnosis by verifying the continuity of the supply and return paths; a significant share of faults is not in the device itself but in the line.
Context in which this term is used: Installation, redundancy and field continuity