ABB Distribution Protection Unit 1500R
1-25
Protective Functions
9/99
website:www.abb.com/papd
The 79 Cutout Time (79-CO) function allows for the detection of low-level or intermittent faults prior to the resetting of
the reclose sequence. At the end of the selected cutout time period, all overcurrent functions are re-enabled based on
the 79-1 settings. For example, if the 79 Reset Time is set for ten seconds and the 79 Cutout Time is set for five
seconds, the first five seconds after reclosing, the DPU1500R follows the overcurrent function settings for the reclose
sequence, but the second five seconds (after the cutout time has expired) it follows the 79-1 settings. The 79-CO
setting (in Primary, Alternate 1 or Alternate 2 settings) is enabled by programming a time period from 1 to 200 seconds.
When enabled, the 79-CO setting must be less than the 79 Reset Time.
In fuse-saving applications
involving large downstream
fuses, the 50P and 50N
instantaneous functions are set
below the fuse curve to detect
faults on tapped laterals. These
functions are blocked after the
first trip in the reclose sequence.
The 51P and 51N time
overcurrent functions are set
above the fuse curve. This
results in the upstream
protection being less sensitive
to an intermittent or low-level fault during the subsequent reclose operations.
If the reset time is too short, the reclosing relay may reset before the fault is detected again. If the reset time is too
long, the intermittent or low-level fault is not cleared fast enough by the upstream protective device. In schemes using
discrete reclosing relays, blocked instantaneous overcurrent functions are placed back in service only after the reclosing
relay has reset. However, the 79-CO function in the DPU1500R re-enables the instantaneous functions at the end of
the selected cutout period. Set the time for the 79-CO function according to how long it takes a downstream fuse or
other protective device to clear downstream faults. The typical time setting is between 10 and 15 seconds. If an
intermittent or low-level fault exists, it will be detected at the end of the 79 CO cutout time period, and the DPU1500R
will trip and continue through the reclose sequence until the fault is permanently cleared or lockout is reached. The 79-
CO function allows the reset time to be set beyond 60 seconds without jeopardizing sensitivity to intermittent or low-
level faults.
Single Shot Reclose Logical Input 79S (O->I1)
The 79S logical input is used to initiate a single shot of reclosing when the circuit breaker is opened by an external
source. Logical input 79S must be mapped to a physical input contact for activation by an external device (see
Programmable I/O Section). The 43A (recloser enable) logical input must also be active for the 79S function to operate.
If 43A is not mapped to a physical input it defaults to enable. The 79S operates as follows:
If the breaker is opened by an external source and the 79S logical input is active, the circuit breaker will close in the 79-
1 open time.
If the breaker is opened by an external source and the 79S logical input is not active but is made active after the circuit
breaker is opened, the circuit breaker will close in the 79-1 open time.
If the circuit breaker is opened before the reset time expires, 79S will not operate again until the breaker is manually
closed back in and the reset time expires.
The 79S function can be made active or inactive by the mapping a User Logical Output (ULO) to the 79S
logical input through the feedback logic (see Programmable I/O Section).
Figure 1-15. 79 Cutout Time
Protection based on the settings of
the reclose sequence (79-1, 79-2, 79-
3, or 79-4) presently occurring
Protection based on 79-1 settings
79 Reset Time
79-CO expires
0 Seconds
Reset
Cutout Timer (O->I-CO)
