Emerson Static Transfer Switch user manual System Grounding, Control Wiring Connections

Power and Control Wiring

5.3System Grounding

Equipment grounding — Grounding is primarily for equipment and personnel safety, although proper grounding also enhances equipment performance.

All input and output power feeds must include an equipment grounding means as required by the NEC and local codes.

An insulated equipment ground conductor is recommended to run with each input and output power feed. The equipment ground conductors should be at least the minimum size conductor per the NEC based on the upstream overcurrent protection device.

! WARNING

If conduit is used as a grounding means, adequate electrical continuity must be maintained at all conduit connections. The use of isolating bushings with a metal conduit can be a safety hazard and is not recommended.

4-Wire-Plus-Ground Systems — When 4-wire-plus-ground input feeds are utilized, the input power sources must be properly grounded. Because the neutral is not switched by the Liebert STS2, the neu- trals of the two power sources are solidly interconnected. The NEC prohibits grounding a power source at more than one point. Connecting the neutrals of two grounded power sources together effec- tively grounds each of the sources at more than one point, which allows neutral current to flow on the ground system, defeats ground fault protection, creates a safety hazard, and violates the NEC.

Where possible, the two power sources should be located in close proximity and a single neutral-to-ground bond made (as shown in Figure 4 on page 12 or as is typical with a double- ended substation).

Figure 4 Typical one-line diagram of two PDUs and a static transfer switch

SOURCE 1		SOURCE 2
MICB1		MICB2
ISOLATION		ISOLATION
TRANSFORMER		TRANSFORMER
CB1	STS1	STS2
		CB2

K1

K2

K2 K3 CB4 CB3 K1 K3 CB5

225A 225A 225A 225A 225A 225A

5.4Control Wiring Connections

No control wiring is needed on the standard Liebert STS2. Only certain options require external con- trol wiring. See 6.0 - Options on page 14 for details

12