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 3.2.1.1 Mechanical Construction.
Current transformers are classified as
follows:
Window type: This type has a secondary winding completely insulated
q
and permanently assembled on the core, but has no primary winding.
This type of construction is commonly used on 600 volt class current
transformers.
Bar type: Same as the window type except a primary bar is inserted
q
into the window opening. This bar can be permanently fixed into its
position or be removable.
Wound (wound-primary) type: "This type has primary and secondary
q
windings completely insulated and permanently assembled on the core.
The primary is usually a multiturn winding.
3.2.1.2 Electrical Connection. The following are types of CTs based on
electrical connection:
Single primary: This term is frequently applied to current
q
transformers having a single primary electrical circuit and is
generally used to distinguish them from current transformers having
series-parallel primary windings.
Window type with one or more primary turns: A single conductor
q
straight through a window type transformer is a "one turn primary"
connection.
If this single conductor is taken through the "window" twice, the
nameplate primary rating is reduced by 1/2. (Multiply meter reading
by 1/2 the CT nameplate ratio).
If on a single-phase, three wire circuit, two different leads are
taken through the same window transformers, the nameplate primary
rating is reduced by 1/2 (multiply meter reading by 1/2 the CT
nameplate ratio).
Double ratio: Double ratio units are built either with a two-part
q
series parallel winding or with a tap on a secondary winding.
3.2.2 Potential Transformer. Potential transformers (PT) are designed to
have the primary winding connected in parallel with a circuit, the voltage of
which is to be measured or controlled. The secondary winding will then
deliver a voltage proportional to line voltage for operation of meters,
instruments, and relays. A PT is used to reduce line voltage to a level to
match the meter rating, generally 120 V. The ratio of primary to secondary
voltage is known as the PT ratio or PTR. The leads on a PT shall always be
fused and shall never be shorted together. Voltage transformers are available
in many accuracy classes; it is important to select one suitable for a
specific application. Figure 9-14 shows how a voltmeter would be connected to
a potential transformer in a high-voltage circuit. In this instance the turns
ratio would be 120 to 1. As with CT hookups, the manufacturer's manual should
be consulted and followed to ensure the meter is properly connected in the
circuit.
9-28
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