PGA-016A Littelfuse Inc, PGA-016A Datasheet - Page 51

• Manufacturer Part Number: PGA-016A
• Description: BULK / WATERTIGHT COVER FOR PGR-6200 / PGR-7200
• Manufacturer: Littelfuse Inc

Specifications of PGA-016A

• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

© 2009 Littelfuse
POWR-GARD
Ct application
III. CT APPLICATION
Current Transformers (CTs)
A current transformer is defined as a transformer that
produces a current in its secondary circuit that is in
proportion to its primary current.
Although there are other types of CTs, only the window (or
ring) type will be discussed here. Window-type CTs get their
name from their design that consists of a ring-shaped core.
This core is formed by a single length of strip ferromagnetic
material tightly wound to form the ring-shaped core.
A CT operates on a principle of flux balance, as shown
in Figure 13. If the primary winding is energized with the
secondary circuit open circuited, the transformer becomes
an iron-cored inductor. The primary current generates a
magnetic flux in the core as shown (flux direction can be
determined by the right-hand rule). When the secondary
winding is connected to a burden or is short circuited,
current flows through the secondary winding creating
magnetic flux in the core in opposition to the magnetizing
flux created by the primary current. If losses are ignored,
the secondary flux balances exactly to the primary flux. This
phenomenon is known as Lenz’s Law.
Lead Length
The secondary lead resistance of CTs cannot be ignored,
particularly with low Volt-Amperes (VA) CTs. For example,
let’s look at an electronic overload relay.
Now let’s solve for the maximum length of #14 AWG leads
that will result in a rated accuracy for a 10A secondary
current. Solving for maximum total impedance (Z
The relay’s CT input impedance or burden (Z
The maximum current (I) = 10 A
The CT rating (P) = 5VA
P = I²Z
Z
PRIMARY FLUX
T
= P / I² = 5 / 10² = 0.05Ω
•
POWR-GARD
T
®
Protection Relay Catalog
®
Protection Relays
FIGURE 13
T1
T2
SECONDARY FLUX
T
B
):
) = 0.01Ω
49
Solving for the maximum lead resistance (Z
CT Installation
A CT should not be operated with its secondary open-
circuited. If the secondary is opened when primary current is
flowing, the secondary current will attempt to continue to flow
so as to maintain the flux balance. As the secondary circuit
impedance increases from a low value to a high value the
voltage across the secondary winding will rise to the voltage
required to maintain current flow. If the secondary voltage
reaches the breakdown voltage of the secondary winding, the
insulation will fail and the CT will be damaged. Furthermore,
this situation presents a personnel shock hazard.
When a ring-type CT is used to monitor a single conductor
or multiple conductors, the conductors should be centred
in the CT window, as shown below, and should be
perpendicular to the CT opening.
In some applications it is difficult or impossible to install
the primary conductor through the CT window (example:
existing bus bar structure). For these applications a split core
CT is sometimes used. Performance of split core CTs may
be less than that of solid core CTs.
CT characteristics are normally specified at a single
frequency such as 50 or 60 Hz. Therefore the question
arises: What happens when CTs are used with variable
frequency drives (VFDs)? For CTs that are linear to
approximately 10x rated primary current at 60 Hz, the
Volts / Hertz ratio is approximately constant. That is, for all
other conditions held the same at 6 Hz, the CT will be linear
to only 1x rated current and at 30 Hz the CT will be linear
to 5x rated current. For a standard silicon steel-core CT, the
upper bandwidth frequency is approximately 5 kHz.
Z
Z
If we look up the #14 AWG resistance we find it equals
2.6 ohms/1000 ft.
Therefore, lead length = Z
Lead length = (0.04 x 1000) / 2.6 = 15.4 ft.
T
W
= Z
= 0.05 – 0.01 = 0.04Ω
W
+ Z
B
Incorrect
B
A
C
FIGURE 14
W
/ #14 AWG resistance
www.littelfuse.com/ProtectionRelays
C
Correct
A
B
A
B
W
C
):
SPACER
P