29.5 TELEPHONE SERVICE REQUIREMENTS
The recommended four-wire leased telephone voice-grade service for communications with the RFL 93B MO is
an "SPO-A" (Service Performance Objective Class A), as mentioned in IEEE Standard 487-1980, "Guide for the
Protection of Wire-Line Communication Facilities Serving Electric Power Stations". The SPO-A line must pro-
vide reliable service before,
during, and after a fault.
Appendix A is an example of a questionnaire form used by a telephone company which will give a feeling for the
relevant information required to provide SPO-A service. The RFL 93B MO modem module does not require a dc
connection to the remote terminal. The voice channel (300 Hz to 3300 Hz) is adequate.
29.6 MICROWAVE, RADIO, AND T1 ANALOG CHANNELS
If microwave, radio, or T1 communication is used, all equipment must be close together and on the same
ground mat. The connection between the radio and the RFL 9300 relay must be shielded twisted pairs, with the
shields grounded at both ends. There must be an independent solid ground bus connecting the chassis grounds
of both the RFL 9300 and the radio.
The communications system is responsible for maintaining continuity though a fault. If the S/N drops below +15
dB during a fault, the RFL 9300 will not be dependable.
29.7 CHANNEL ROUTING VS DEPENDABILITY
Higher dependability can be obtained if the transmit and receive lines are routed differently. If communications
in one direction is down, one terminal will trip during an internal fault. The second terminal will trip sequentially if
the "Loss-Of-Load" feature is enabled. If both lines go down because of a common-mode failure, the relay can
only function on overcurrent.
29.8 MODEM SECURITY
The RFL 93B CC module is programmed with the squelch rules that govern whether messages will be ac-
cepted. With the exception of a DTT input, incoming messages can not cause a trip unless the RFL 9300 locally
detects a fault or system disturbance. Fault detector supervision prevents false trips due to random noise. The
fault itself could cause common-mode noise; the squelch rules in the RFL 93B CC module filter and guard
against accepting false messages in an impulse noise environment. Figures 28-2 and 28-3 contain the graphs
for theoretical security.
29.9 SQUELCH RULES
The RFL 93B CC's microcontroller counts the number of good and bad words. It uses this information to decide
if the channel is good or bad. The word segments are binary, and the errors that are counted yield a bit error
rate (BER) which is a measure of communication quality. The noise on a channel may be long-term average
noise, or the noise may come in pulses for a short duration.
The quality of the analog signal received by the modem is expressed as a signal-to-noise (S/N) ratio. If the line
coming into the modem is 3002 conditioned, the spectrum is not flat and equalization is required. The link be-
tween S/N and BER (Bit Error Rate) changes slightly with added equalization (2 dB). If the S/N is better than 16
dB, the dependability of the relay is 100 percent with a 3002 conditioned line.
RFL 9300 RFL Electronics Inc.
May 5, 2002 29 - 3 (973) 334-3100
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