METERING FOR LINEMEN

 

Electrical Metering for Linemen and Techicians

 

Basic Mechanical Meter Parts

Base

The base is made of an insulating material to isolate the internal components from the different electrical potentials in the meter socket. It also provides insulation between sources of electrical potential entering the meter from the meter socket.

Cover

The cover protects the meter from the weather and helps to deter tampering for the purpose of energy theft.

Frame

The frame provides a rugged mounting surface to correctly align the various parts inside the meter.

Potential Coil

The potential coil may be compared to a voltmeter. It tells the meter how much voltage is present in the electrical circuit being monitored. It performs this task by creating a magnetic field (flux) proportional to the applied voltage. Potential coils are always connected in parallel. (Energy passing through the circuit being measured does not pass directly through the potential coil).

Current Coil

The current coil may be compared to an ammeter. It tells the meter how much current is present in the electrical circuit being monitored. It performs this task by creating a magnetic field (flux) proportional to current flowing in the circuit. Current coils are always connected in series. (Energy passing through the circuit being measured passes directly through the current coil).

Disk or Rotor

The magnetic fields developed in the potential and current coils cause eddy currents to flow in the meter disk. These eddy currents cause the disk to rotate. When a meter is properly calibrated, the disk (sometimes called rotor) will turn at a speed that is proportional to the voltage and current being measured. Anti-creep holes in the disk prevent the disk from turning when a load is not present.

Damping Magnet

The damping magnet provides a retarding effect to control the speed of the meter disk. Without a damping magnet, a meter would slowly coast to a stop after a load was removed. A more desirable response would be for the meter disk to stop immediately when a load is discontinued. The damping magnet provides the force to accomplish this task.

Stator

A stator consists of one potential coil and one or more current coils wound on a single laminated iron core. The key portion of the previous statement is that a stator only has one potential coil. The number of stators in a meter may be determined by counting the number of potential coils in a meter.

Register

The register provides a means of recording energy consumed by the customer. The register is driven by a worm gear cut into the disk shaft, which meshes with a first takeoff gear on the register. The complete gear train consists of two basic relationships. First, the relationship between the disk’s worm gear and the first takeoff gear on the register. This relationship is referred to as the "Rs" or "shaft reduction". If the shaft reduction (Rs) is 100/1 it means that the disk must make 100 revolutions in order to make the first take off gear on the register make one revolution. Second, the relationship between the first take off gear on the register to the far right hand dial on the front of the register must be considered. This relationship is referred to as the "Rr" or "register ratio". If the register ratio (Rr) is 13 8/9, it means that the first takeoff gear must make 13 and 8/9 revolutions to make the far right hand dial on the front of the register make one full revolution. (Remember, when the far right hand dial on the front of the meter makes one full revolution, it goes from zero all the way around and back to zero again. Not just from zero to one). Knowledge of both the shaft reduction (Rs) and the register ratio (Rr) values are required when calculating a dial multiplier. The register ratio (Rr) is clearly stamped on register. Unfortunately, the shaft reduction (Rs) is rarely stamped on meters. A table of common shaft reduction (Rs) values is included in the "Reference Data" section of this web site.

Potential Lights

Potential lights are present on three phase instrument rated meters. Potential lights indicate that approximately the correct voltage is reaching the potential coil and that the potential coil is functioning. They do not indicate metering has been correctly connected to the circuit being measured.

Detent

A detent is a small mechanical ratchet device that prevents the meter disk from running backwards. A detent is required when metering loads such as oil and gas wells, which have pump jacks that may momentarily cause the meter disk to rotate backwards. Without the detent, energy recorded by the meter moments earlier will be backed off the register by this inappropriate reverse rotation of the meter disk.