System Down or Ride Right Through?

We recently reviewed a set of power monitor data from an MR (Magnetic Resonance) site. The facility was plagued by severe voltage sags; we ended up with a rather copious collection of classic but nonetheless ugly event waveforms. And in the course of analysis, we noticed that some sags caused system shut-down, some rode right through, and some perhaps caused an error or lock-up which the customer attempted to reset by powering down the system.

Reviewing equipment response to severe events in this way can help to calibrate system sensitivity when manufacturer or factory data about sag susceptibility is not available.

Example #1: System Rides Through Voltage Sag

Sag RMS No Shut Off

Despite a fairly serious sag, no sign of direct impact on the imaging system. Current levels shift during the sag event itself, but remain at about the same level before and after the sag.

Sag Waveform No Shut OffExample #2: System Shuts Down During Voltage Sag

Sag RMS System Down

At the time of a severe voltage sag, load current drops to a lower, standby or system-off level, and remains there.

Sag Waveform System DownSag Waveform Customer Shut OffExample #3: System Shuts Down During Second Voltage Sag

Sag RMS System Down Second Sag

During these sag events, the system appears to ride through a severe voltage sag; but shuts down during a subsequent sag 15 seconds or so after the initial sag event.

Sag Waveform System Down Second SagSag Waveform System Down First Sag

Example #4: System Current Drops Following a Voltage Sag; Customer Shuts System Down

Sag RMS Customer Shut Off

Following a severe sag event, current drops partially. Suspect that one or more subsystems shut-down and resulting system alarm or errors results in customer shutting down the system, 30 seconds after the sag event. Note drop in current not directly related to a voltage event.Sag Waveform Customer Shut Off