MGE UPS Systems STS 600A - Manual (Page 45)

Installation and User Manual
3.9 Open SCR Protection
The STS design includes sensing circuits to detect an open SCR and in addition to the following actions, will initiate
audible and visual alarms:
In the event of an open SCR in the active source powering the load, the unit will alarm and immediately transfer to
the inactive source. Then the SCR isolation switch on the previously active source will be shunt tripped. Automatic
retransfer will be inhibited under such conditions until repair is made and the system is reset.
3.10 Source Cross-Connection Protection
The STS transfer logic will check that the two input sources, of any combinations, are never connected in a fashion
to allow current flow from one source to the other without first passing through the load. The logic circuitry has
protective sensing circuits that prohibit this conduction. Transfers and retransfers between sources will be break-
before-make to prevent any cross-conduction, even in out-of-phase conditions.
3.11 Bypass Procedure
Bypass Load to Source #1
1. The Source #1 SCR Switch #1 must be indicated closed on the display panel. If not, transfer the load to Source
#1 by turning the Manual Control key switch on the front display panel to the “ON” position and pressing the
Change Preferred Source Pushbutton. The STS should transfer to Source #1.
2. Open Source #2 isolation switch, CB2. (For 4-interlock system, lock and remove the key).
3. Lock Source #2 bypass switch, CB5, and remove the key.
4. Using the key(s), unlock and close Source #1 bypass switch, CB4. CB1, CB3A and CB3B (if installed) will shunt
trip, isolating the static switch section.
Return to Normal Operation (load on Source #1)
1. Close Source #1 isolation switch, CB1.
2. Wait for indication on the display panel that Source #1 SCR Switch #1 is closed.
3. Close Output isolation switch(es), CB3A and CB3B (if installed).
4. Open Source #1 bypass switch, CB4, and lock.
5. Close Source #2 isolation switch, CB2. (For 4-interlock system, remove key from CB4 lock to unlock CB2, and
close CB2).
Operation 3 —7 86-504004-00 B03