AT&T System 25 - Manual (Page 245)

MAINTENANCE
Power Supply Protection
Power supply outputs are protected from damage when they are subjected to
either momentary or sustained short circuits. “Shutdown” current protection
is provided at 125 percent (plus or minus 5 percent) of full load for each dc
output. Current foldback is provided for the 90 V ac output.
Output power is removed when the output voltage exceeds an internally set
trip point. The trip point is set at the factory at +6.25, +0.75 V for the +5.1 V
output for all rated line and load conditions.
Output power is removed when the power supply overheats for any reason
(for example, fan failure or operation outside ambient temperature limits).
Once the cause of the shutdown is corrected, remove ac power for about 5
minutes. The green LED (visible through the fan slots on the front of the
cabinet) will light when the power supply recovers. If the green LED is not
lighted, remove ac power a second time.
If green LED is still not lighted,
remove all ac power. Repeat procedure a third time; if the LED is now
lighted, there is a short. Find the short, and either repair it or have it
repaired.
Although the power circuits are designed to withstand lightning surges on the
ac line with proper grounding, it is advisable to provide supplemental
protection with external protectors in high lightning activity areas.
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