Specifications

December 2008 Page 14–7
Chapter 14. Protective Relay Interface (PRI/PRS) Module
14
14.2.4 Security Delay
The “Security Delay” setting lets you indepen-
dently configure each of the four functions within
the PRI module for different levels of security.
The setting choices are 1, 2, 4, or 8 milliseconds
security delay. A setting of 1 millisecond requires
8 consecutive frames of correct information to be
received before a trip output is initiated, while a 2
millisecond setting requires 16, a 4 millisecond
setting 32, & an 8 millisecond setting, 64 frames.
In general, when you are using a channel for
directional comparison relaying, we recommend a
shorter security delay of 1 or 2 milliseconds.
When you are using a direct transfer trip scheme,
the 4 or 8 milliseconds delay would be more
appropriate to add additional security against false
tripping. The longer the security delay setting, the
greater the security of the system. The shorter the
security delay setting, the greater the depend-
ability of the system. Spurious trip input times
(plus contact bounce setting) lasting less than the
security delay setting are ignored. Any setting
above the minimum 1 millisecond delay adds to
the overall channel trip time.
14.2.5 Unblock Timer
You can set the “UnBlocking” setting to either
“On” or “Off” for each of the four output
functions. When it is set to “On,” a 150
millisecond trip output is initiated whenever the
PRI module detects a loss of channel. This feature
should never be applied on a direct transfer trip
scheme and should only be applied to permissive
schemes where there is a chance of losing the fiber
optic channel at the same instant that a fault could
occur on the transmission line. The application of
this logic is analogous to a power line carrier
unblocking scheme, where the power line carrier
may be significantly attenuated during an internal
transmission line fault. Use of this setting signifi-
cantly improves the system dependability and
decreases system security.
14.2.6 Guard Before Trip Logic
There are three Guard Before Trip (GBT). settings
for each function: GBT with override (“W/Over”),
GBT without override (“WO/Over”), and GBT off
(”Off”). Guard Before Trip logic is a carry over
from analog pilot channels such as audio tones.
These systems generally use Guard Before Trip
logic to enhance the security of the channel. A
digital channel, such as the PRI, is inherently
more secure than a traditional FSK (Frequency
Shift Keyed) channel using microwave, PLC, or
leased circuits.
For most applications, you may set the GBT
setting to “Off.” The security of the channel,
however, is increased when Guard Before Trip
logic is used. When using it in a permissive
scheme, you should set it to the “with override”
setting. When using it in direct transfer trip
schemes, you should set it to the “without
override” setting. The GBT “with override”
feature allows you to use open breaker keying in a
permissive scheme after a loss of channel.
Providing that trip has been received for at least
500 milliseconds prior to a loss of channel
condition, the channel is permitted to return
directly to the trip output state when the loss of
channel is cleared. For this example, the GBT
“Off” option will perform the same as above. This
is not the case for the GBT “without override”
option, however, which only permits a trip output
if a legitimate guard signal is received immedi-
ately prior to the trip command, regardless of
channel status.
The guard output is active when the PRI is
receiving good data without a trip being received
on any of the four functions. For example, if trip
one, two, three, or four outputs (or any combina-
tion) are active, the guard output will not be
present. The guard function for GBT logic is
handled a little differently. A “pseudo-guard” is
created whenever any channel is receiving a trip
output. Either the “pseudo-guard” or “real” guard
can provide the guard logic input for the GBT
logic.
14.3 Installation
As with other FOCUS modules, we recommend
that you initially install and test the Protective
Relay Interface (PRI/PRS) Modules and inter-
faces in “test” chassis (i.e., chassis that are not
part of an operating network). This way, you can