Installation: Power service must be interrupted to install an S- or
A-type kilowatthour meter. If service cannot be interrupted, clamp-on
split core transformers may be used. Core transformers are generally
used as a temporary measure or for spot monitoring.
2.2.1 S- and A-Type Kilowatthour Meters. The S- or A-type designation on a
kilowatthour meter refers to the counting configuration. An S-type meter is a
socket-mounted meter. A socket is hard wired into position and the u eter is
then plugged into the socket. The meter can then be removed any time without
disconnecting wiring. The A-type meter is hard wired into position at the
time of installation and requires complete disconnection of wiring for removal.
2.2.2 Transformer-Rated Kilowatthour Meters. If connected load exceeds 200
amperes and/or voltage of a circuit exceeds 600 volts , a transformer-rated kWh
meter must be used in conjunction with instrument transformers. For safety
reasons, transformers should be used when voltage exceeds 277 volts, even
though meters without transformers are supplied for 480 and 600 volt service.
Transformers reduce the voltage and line current to meter operating range.
Both current and potential transformers are used for this purpose.
Transformers can be either an integral part of the meter or a separate
component. In using transformer-rated meters, it is important to know that
two values of Kh may be shown on the meter nameplate. The first Kh which
is a small numeric value is the Kh of the meter itself. The second Kh
which may be shown as Kt, in which the subscript "t" stands for test, is the
value used when the meter is being tested. There is another term which should
be understood; this is the primary Kh abbreviated Pri Kh or PKh which is
followed by a numerically large number. This value is the number of watthours
of primary energy for one revolution of the disk and is equal to the Kh (of
meter) x Transformer Factor. The transformer factor is equal to the product
of CT and PT ratios. The CT ratio is always the rating of the CT divided by 5
and the PT ratio is the voltage of the primary circuit divided by 120.
2.2.3 Three-Phase Circuits. Polyphase watthour meters are applied to both
delta ( ∆ ) and wye (Y) circuits.
Three-Phase, Four-Wire Y Circuits. A three-phase, four-wire circuit
is equivalent to three single-phase, two wire circuits having a common return
circuit with potentials 120 degrees apart. It can, therefore, be metered with
three two-wire meters or with a three-stator polyphase meter as shown in
Figure 9-9. Because the voltages of the three phases are balanced, it is
common practice to use a 2-1/2 stator self-contained watthour meter to meter
this circuit. Connections for a self-contained watthour meter are shown in
18.104.22.168 Three-Phase, Three-Wire Circuits. Metering of a three-phase,
three-wire circuit is accomplished in accordance with Blondel's theorem by
means of a two-stator meter having current coils connected in two lines and
corresponding potential coils connected from these lines to the third line.
The connection of a transformer-rated meter is shown in Figure 9-11.