Service manual

Always On UPS Systems Inc

7. STATIC SWITCH

The static switch is composed of two pairs of back to back naturally commutated SCR’s,

which moves the load from inverter to reserve (utility) or from reserve to inverter without

interruption of power to the load.

Detection circuitry is included in the control circuit to achieve “0” dead time transfer.

Additional detection is employed to control the static switch transfer.

Dead Short: If a dead short circuit was to occur under normal mode operation, the UPS

would detect the short circuit and immediately stop the inverter until the problem has been

rectified. Due to the large amount of current a dead short requires, the static switch will not

transfer to reserve preventing the tripping of the reserve breaker.

Overload condition: The UPS will transfer the load to reserve (utility) path. The static switch

is designed to handle 110% of the capacity of the system but will switch to the reserve path if

the overload continues for any length of time. The load will still be operational, but the load

system will not be protected from a utility failure, as it has no battery capability until the

overload is removed.

The static switch will only be activated if the input levels are within parameters to ensure the

load is only supplied with acceptable power to protect against damage that may be caused to

the critical load. The system performs numerous checks on the transfer from inverter to

bypass and visa versa to ensure the exchange is smooth.

Figure 7.1

Static B

ass