Power Measurement Equipment 77
The simplified circuit of a clamp-on analog current meter is shown
in Figure 3-14. Current flow through a conductor creates a magnetic field
around the conductor. The varying magnetic field induces a current into
the iron core of the clamp portion of the meter. The meter scale is calibrat-
ed so that when a specific value of current flows in a power line, it will be
indicated on the scale. Of course, the current flow in the power line is pro-
portional to the current induced into the iron core of the clamp-on meter.
The clamp-on meter may also have a voltage and resistance function that
utilizes external test leads. Thus, the meter can be used to measure other
quantities.TELEMETERING SYSTEMS
When a quantity being measured is indicated at a location some
distance from its transducer or sensing element, the measurement pro-
cess is referred to as telemetering. Many types of metering systems fit this
definition. However, telemetering systems are usually used for long-dis-
tance measurement, or for centralized measurement systems. For instance,
many industries group their indicating systems together to facilitate pro-
cess control. Another example of telemetering is the centralized monitor-
ing (on a regional basis) of electrical power by utility companies. These
systems are similar to other measuring systems, except that a transmitter/Figure 3-14. Circuit diagram of a damp-on current meter