Power Circuit Requirements - Trane Starters Engineering Bulletin

Electrical System—Design Guidelines
by a short circuit (fault current), it may be opened safely at any time. A low-voltage starter door may
be opened only if the circuit breaker is in the "off" or open position.
Circuit breakers of various interrupt ratings are offered as options.The interrupting capacity of a
circuit breaker must also be considered. An analysis of an installation is usually done by the
electrical engineer to calculate the maximum available fault current (AFC) that could be delivered
if a short circuit occurred.
When a low-voltage starter is ordered with a circuit breaker, the starter manufacturer furnishes a
standard circuit breaker sized at approximately 125 percent of compressor RLA or larger.The trip
current of a circuit breaker is adjustable so that it will not trip during starting and acceleration, but
will trip instantaneously (typically within one cycle) if a short circuit occurs.
Short-Circuit Current Rating (SCCR)
The disconnect means and the short-circuit protection discussed above are either supplied and
installed by the customer upstream of the starter or they can be supplied as options with theTrane
starter. However, the starter's SCCR must be communicated to the electrical engineer early in the
project to ensure that the starter short-circuit current rating will meet the requirements. If the SCCR
is not high enough, the electrical engineer will have to look for solutions upstream of the starter.
Each low-voltageTrane
size of the starter as well as the type of disconnect or circuit breaker included. See the appropriate
submittal drawings for the starter short-circuit current rating.
Note: Don't confuse short-circuit current rating (SCCR) with silicone controlled rectifier (SCR)—
Medium-voltage short circuit interruption
All medium-voltage starters contain current-limiting fuses for short circuit protection.The current-
limiting fuses and isolation switch, which is furnished as standard, fulfill the request for a fused
disconnect. If you need more detail on the fuses for a particular starter contact CenTraVac Field
Sales Support.
It turns out that the starter short-circuit current rating is not an issue with medium-voltage starters.
BecauseTrane
and provided as standard, and the starter short-circuit current rating no longer applies. Medium-
voltage starters fromTrane have an interrupt rating based on the standard current-limiting fuses.
At 4,160 volts, the NEMA E2 fused-interrupt rating is 50,000 amperes or 400 MVA.
Medium voltage fuse coordination suggestions
Proper sizing of fuses and circuit breakers upstream of the starter is the responsibility of the
customer or the electrical engineer.

Power Circuit Requirements

Starter Applications
When applying a starter to a chiller motor on a specified power circuit, the power circuit has to meet
minimum criteria to allow for a successful chiller start without an immediate shutdown or safety
trip.This applies to utility power, local or smaller-scale facility power, and backup or peak-shaving
generator power.
The following are some considerations for motor starter power quality requirements:
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starter or frequency drive has a short-circuit current rating, based on the
utilized in solid-state starters and AFDs.
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medium-voltage starters meet UL 347, current-limiting power fuses are required
Size of the power circuit
Source and/or total system impedance
System regulation
Inrush characteristics of the starter type
Acceleration time
Maximum voltage sag
Maximum voltage swell
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CTV-PRB004-EN