Construction and Components
Domain 4: Construction and Components
This domain tests the parts of induction and static meters and how each part affects registration, safety and sealing.
Core rules
- Induction meter: voltage coil with many turns of fine wire, current coil with few turns of thick wire, aluminium disc, brake magnet, register, terminal block, cover and seal.
- The brake magnet controls disc speed; moving it changes calibration and must not be done in the field.
- Static meter: current sensor, voltage divider, metering IC, microcontroller, EEPROM, LCD, optical port and communication module.
- The terminal block carries supply and load; poor torque causes heating and voltage drop.
- Seals and calibration marks are evidence of authorized adjustment.
Worked example
If the brake magnet is moved closer to the disc, eddy-current braking increases, so the disc runs slower and the meter under-registers. If the magnet is moved away, the meter runs fast. The reasoning is that braking torque depends on magnetic flux and disc speed; calibration sets the balance between driving torque and braking torque.
Traps that cost marks
- Confusing voltage and current coils.
- Assuming a static meter has a disc.
- Tightening terminals without the specified torque.
- Breaking a seal to adjust a meter in the field.
- Ignoring a damaged terminal block that can overheat.