Liebert Series 600T manual Static Bypass, Fuse Protection, Shorted SCR Monitoring

2.2.5Static Bypass

A static bypass is an integral part of the UPS System Control Cabinet (SCC). Refer to Figure 7. The static bypass consists of solid-state switching devices working in conjunction with the motor-operated System Bypass Breaker (SBB). An automatic transfer control circuit senses the status of the operator controls, UPS logic signals and alarm messages, and critical bus operating conditions. If the UPS modules can no longer supply the critical load, the static bypass switch (in conjunction with the SBB and UPS Output Breaker) automatically transfers the critical load to the bypass source without interruption.

Figure 7 System Control Cabinet (SCC) Block Diagram

Gate

Driver

Static

Bypass

Switch

Static Switch

Disconnects

Static Bypass Breaker

Bypass Line
		Sync &
		Transfer
	Control	Transfer
UPS	Power
	Supply	Control
System
Modules		System
		Monitor
		Voltage
		Adjust
		Alarms
		UPS Output

To Critical Load

Fuse Protection

The static bypass switch path uses two back-to-back SCRs per phase. Each phase is individually protected by a fuse sized to clear only in the event of a catastrophic fault. This is a more reliable method than depending on external protection devices. The fuses are in the circuit to protect the critical bus distribution equipment against catastrophic faults. The static switch SCRs are oversized to easily handle any current surges that may blow the fuses.

Shorted SCR Monitoring

The static bypass system has redundant shorted SCR sensing circuits and disconnects. This will prevent UPS out- put power backfeed to the distribution system even if two component failures exist simultaneously. If a shorted SCR is detected, the static bypass switch is isolated and annunciated at the SCC control panel, and the critical load remains on UPS output power.

Static Switch Isolation

The motor-operated system bypass circuit breaker (SBB), wired in parallel with the static switch, automatically closes in approximately 200 milliseconds after the load is transferred to the bypass power source, removing the static switch from the power flow.

Theory of Operation 19

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