Disconnect Non-Essential Loads (DNEL)

(Adapted from CRP 9408 - Recovery from DNEL)

"Disconnect Non-Essential Loads" refers to the spacecraft autonomously opening the Main Switch, thereby disconnecting the Payload from the Main Bus and making approximately 55W of additional power available to support the spacecraft housekeeping subsystems. DNEL occurs as a result of a malfunction in the Power Subsystem or a power anomaly in the science instruments. The spacecraft can initiate DNEL by one of two power failure detections. In each case, the conditions for entry are different and are summarized below.

1. Main Switch Overcurrent Detection.
If the Main Switch current (P007 M.SWITCH CUR) exceeds 3.70A for more than 5.7 milliseconds, the the Main Switch will be switched off.


2. Hot Case Heater Desaturation Detection.
If the Hot Case Heater becomes desaturated such that the heater current is below 0.31A for more than 24 seconds, the the Main Switch will be switched off.

Following a DNEL, the Data Handling Subsystem REC program, which continually checks the spacecraft configuration, will note that the Main Switch position has changed. REC will reconfigure the spacecraft to a safe power and thermal state that correctly distributes the 55W of extra power dumped onto the Main Bus. REC activation will occur within 64 seconds of DNEL, and the two events are usually thought of as a single operational event.


The spacecraft configuration following DNEL+REC is dependent on the safe configuration REC has loaded at the time. This safe configuration (called the Backup Table) is periodically updated by the Mission Operations Team with ground commands as the mission conditions change, but all such configurations will include:

  • Loss of X-band telemetry. The spacecraft will transmit carrier only, at a higher power.
  • Main Switch off (all science instruments powered down)...
  • ...a consequent increase of about 55W in the Internal Power Dumper (IPD) Power.
  • Both spacecraft receivers switched on.
  • Operational units in the Attitude and Orbit Control Subsystem (AOCS), and the operational Transmitter will be switched off; the corresponding redundant units will be switched on.
  • As a consequence of the AOCS switching, any manoeuvre in progress will be aborted

Recovery from DNEL

DNEL is an important safety precaution, but its consequences are highly undesirable - particularly the loss of science data as telemetry is interrupted and the science instruments powered down for a period. It takes the Mission Operations Team 2-3 days of intense activity, and the transmission of thousands of telecommands, to restore the spacecraft to its pre-DNEL condition. The recovery consists of the following stages:

  • Telemetry analysis to establish why a DNEL occurred.
  • Restore telemetry, assess spacecraft condition.
  • Thermal reconfiguration.
  • Main Switch on.
  • Re-enable spacecraft protection logic (REC).
  • Switch on experiment power converters. (Repeat diagnostic switching may be necessary at this stage).
  • Reconfigure the science instruments to their pre-DNEL condition.
  • Return to prime Attitude and Orbit Control units. Return to prime Transmitter. (Repeat diagnostic switching may be necessary at this stage).
  • Resume routine operations

Occurences of DNEL

DNELs have ocurred at the following times:

  1. 91.165.14:20:39 UTC.
  2. 93.045.00:15:57 UTC.
  3. 93.221.07:00:24 UTC.
  4. 93.331.18:16:57 UTC.
  5. 94.238.15:31 UTC.
  6. 95.344.13:15 UTC.
  7. 96.230.13:00 UTC.
  8. 97.091.19:23 UTC.