MPS Doc: M359_USER_MANUAL REV G January 17, 2010 Page 18 of 30
5.7. UPS Response to Overload Condition
The M359 has two distinct overload protection functions. The first is the linear (continuous)
current limiting of the output current and other is an overload protection that shuts down the
UPS Output if the output power (in watts) is exceeded. In case of a shutdown due to an
overload, the green OUTPUT OK LED (21) will turn off and the red OUTPUT FAIL LED (19)
and the red O. LOAD SHUTDOWN LED will both be On.
5.7.1.
Output Current Limiting
The DC-to-AC Inverter of the UPS that generates the AC Output (see Figure 1) has a
current limiting circuit that limits the output current to about 42Amp (peak). It means
that the maximum undistorted sine-wave current that the M359 can provide is about
30Arms, limiting the output VA to about 3.4KVA. If the load on the UPS output exceeds
this rating, the current waveform will be clamped at 42Amp peak and the output current
will be a sine-wave with a “flat top”. When the overload approaches short circuit, the
current wave form will look like a trapezoid (see the Figure 6).
A short circuit or an Overload that tries to exceed the current limit threshold of the UPS
and as a result pulls down the output voltage to below 80Vrms and persists for more
than about 0.7 second will shutdown the Output of the UPS (see Figure 7).
5.7.2.
Overload Protection
The Overload Protection circuit of the M359 monitors the real output power into the load
(in Watts). If the output power exceeds the rating of the M359 (2KW) a timer is started
and if the Overload condition persists, the UPS output will shutdown. The allowed
overload duration before the UPS Outputs shuts down depend on the depth of the
overload. For an overload of up to 150% (3KW) the allowed duration (before the output
will shutdown) is about 3 seconds (see Figure 8). At full short the duration will be
reduced to about 0.7 seconds.
5.7.3.
Recovery From an Overload Shutdown
To recover from a shutdown caused by an Overload condition (the O. LOAD
SHUTDOWN red LED (22) and OUTPUT FAIL red LED (21) are On):
1)
Turn off the PWR ON/OFF switch (2), or the RMT PWR ON/OFF switch.
2)
Remove the overload (or short circuit) from the UPS output.
3)
Turn on the PWR ON/OFF switch (2) or the RMT PWR ON/OFF switch (as
applicable, since both must be On in order to enable the UPS).
Note: The output may turn on immediately or after a short delay, depending on the
duration the unit was off. During this delay the OUTPUT STDBY yellow LED (18)
will blink.
