Description
2.1 Overview - applications and portfolio
3VA molded case circuit breakers with UL and IEC certification
Manual, 01/2019, L1V30435333-01
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Electric circuits with "straight" and "slash" voltages
In the USA, voltages are considered from the power supply company side or the secondary
side of the power supply company transformers. The connection (wye or delta) of the electric
circuit and the method of grounding play a major role here.
In electric circuits with a Grounded Wye, the circuit breaker only switches the full voltage
between the phases (e.g. 480 V). The phase-to-ground voltage is only 277 V in this case,
which explains the "slash" voltage of 480Y/277 V. Industrial Control Panels in which these
devices are installed must be marked with the following inscription: "For use on a solidly
grounded wye source only" (UL 508A Art. 54.12).
In ungrounded or high-resistance grounded wye or delta circuits, and in corner-grounded
delta circuits, only devices (e.g. circuit breakers) that are labeled with a "straight" voltage,
such as 240 V, 480 V or 600 V, may be used. These devices must be able to switch the full
voltage between the phases and between one phase to ground.
Grounded wye, 480Y / 277 V, 3 phase, 4 wire
Short circuit current rating of the control panel main (power) circuit
An industrial control panel must be marked with a short-circuit current rating (SCCR). In IEC
terms, this corresponds approximately to the I
cw
value of the panel. NEC 2017 Article 409
describes the specifications of the short circuit current rating markings on industrial control
panels (with reference to UL 508A, SB4). For the short circuit current rating, not only is the
short-circuit breaking capacity (e.g. of the circuit breaker) relevant, but also the short-circuit
current rating of each individual device. The SCCR-relevant components in the main circuit
include circuit breakers, contactors, overload relays, and solid-state switching devices, as
well as terminals, busbars, the line side of control transformers and frequency converters,
but not the internal wiring of the control panel.
The lowest value applies to the entire control panel. A short-circuit current higher than this
must not be available at the input terminals of the control panel.
