1A terminal screw left loose
When the house cable was fitted, the screws on the line-out terminal were left short of their specified torque. The copper strands touch the brass at only a few points, so the joint has a resistance of a few tenths of a milliohm instead of under 0.1. Tonight the water heater, hob and air conditioner pull 30 amps through those few points.
2Heat, creep and oxide
Every time the load comes and goes, the joint heats and cools. Copper and brass expand by different amounts, and soft copper slowly flows away from the screw under pressure (creep), so the clamp keeps loosening. Hot copper oxidises, and copper oxide conducts poorly. The resistance climbs to several milliohms, and the heat, which grows as current squared times resistance, climbs with it.
3The heat spreads
At 8 milliohms, 30 amps makes about 7 watts in a joint the size of a sugar cube, enough to push it past 80 °C. The cable’s PVC insulation, rated for 70 °C, softens and gives off fumes, and the terminal block plastic around the joint starts to discolour. Copper carries the heat up the busbar to the temperature sensor clipped onto it.
4Caught: the meter raises an alarm
The thermistor on the busbar passes 100 °C. The chip logs an over-temperature event with the time and the current and sends it straight to the utility over the radio, without waiting for the next scheduled reading. The display shows the alarm.
5The meter opens its own relay
At 115 °C the meter pulses its relay open. The current stops, so the heating stops, and the joint slowly cools. The meter will not reconnect by itself or on a remote command: an engineer has to visit, replace the damaged terminal or the whole meter, and tighten the new joint properly. Without the sensor this joint would have kept heating until it arced, charred the block and could have started a fire.
6How loose terminals are caught and prevented
- Tighten terminal screws with a torque screwdriver to the maker’s figure (often 2.5 to 3.5 newton metres), and check them again after the cable has settled.
- Two screws per terminal share the clamping force, so the joint stays tight if one relaxes.
- Strip the insulation to the right length, so only bare copper sits under both screws and no PVC is trapped in the joint.
- Terminal temperature sensors let the meter raise an alarm and open its relay before anything burns.
- Terminal blocks are made of plastic that passes a 960 °C glow-wire test, so a hot joint chars rather than catching fire.
- Utilities look for warm meters with thermal cameras, and for unusual temperature reports across the network.