A
ELECTRICAL CONSIDERATIONS
A.1 GENERAL
Proper grounding procedures are essential to the operation of an uninterruptible
power system (UPS), personnel safety, and critical load protection.
It is the
customer/customer electrician’s responsibility to ensure compliance with the
National Electrical Code (NEC, ANSI/NFPA 70) and applicable local codes.
A.2 TERMS AND DEFINITIONS
1.
Grounded conductor: The branch of the circuit (usually the neutral) that is
intentionally connected to ground. This grounded conductor is part of the current
carrying circuit.
2.
Grounding conductor: The conductor(s) that connects all exposed metal parts of
the equipment to the ground. This grounding conductor is NOT part of the current
carrying circuit.
3.
Separately derived source: Article 250-5(d) of the NEC defines a separately
derived source as:
A premises wiring system whose power is derived from generator, transformer,
or converter windings and has no direct electrical connection, including a solidly
connected grounded circuit conductor, to supply conductors originating in another
system, if required to be grounded as in Article 250-5(a) or (b), shall be grounded as
specified in Section 250-26.
4.
UPS classification: Digital’s UPS inverter is connected to an isolation transformer,
where its secondary windings are connected in WYE configuration. The UPS requires
the bypass ac input to be fed from a WYE-connected source. The neutral of this
source is directly connected to the neutral of the output isolation transformer. So,
the UPS may or may not be considered a separately derived source, depending on the
arrangement of the bypass ac input neutral.
5.
Specific provision: The NEC prohibits connecting the grounded circuit conductor
(neutral) to the grounding conductor at more than one point. The reason is that if
the neutral were connected to the grounding conductor at more than one point, some
of the neutral current would be allowed to flow in the grounding conductor circuit
between the points of connection. Besides being a safety hazard, this practice defeats
ground fault protection schemes.
A–1
Summary of Contents for HA3000
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