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Appendix 8 Functional Description of Operation
Registers;one register for the phases, one for ground. All current
sensing and pickup data is weighted by the computer accord
ingly.
8.5.5 Watchdog Circuitry
The watchdog circuit checks that the microcomputer is opera
tional. If some transient condition has d isrupted its normal
pattern of operation, the watchdog will momentarily close the
relay disabled output contact, reset the computer, and reinitialize
the program. The computer is then restarted , and the relay
resumes its monitoring function (without human input).
8.6
Outputs
8.6.1 General
Although the output relays are controlled by the microcomputer,
the output interface contains the transistors that energize them.
With the exception of the Relay Disabled contact (described
below), all of the output contacts are of the normally open type
and have targets associated with them.
8.6.2 Relay Disabled Output
The Relay Disabled output contact is of the normally closed (NC)
type. It is held open (by the output relay's energized coil) at all
times after power -up unless a malfunction is detected as noted
below. (This arrangement provides for a fail-safe output signal,
since the contact is able to close an external alarm circuit even
in the absence of power.)
The Relay Disabled output contact will close if:
1 .
The power supply voltage falls below requirements; or
2 .
The upper connecting plug ("paddle") i s removed .
Additionally, the Relay Disabled output contact will momentarily
close if the watchdog circuit detects abnormal operation of the
computer. (Actually, detection of abnormal operation leads to
microcomputer reset in an effort to correct the problem, and the
reset operation causes the contact closure.)
8.6.3 Instantaneous and Command Close Options
One or two instantaneous outputs are available as options. The
trip points for these options are selected by a programming
method described in Section 3, Operation by Front Panel.
When two instantaneous outputs are selected, one of them can
be programmed to operate directly from the system computer
(assuming that the Communications Option is also present) . The
programming information is supplied in the Siemens Power
Monitor™ Display and Monitoring Unit (Manual SG-401 8-01 ) .
(The second output relay, when controlled by the computer,
cannot simultaneously serve as an instantaneous output .)
Optionally, this extra output relay can be programmed as a
dedicated ground trip contact.
8.7
Target Indicators
Targets are provided for each function (TIME, INST 1 , and INST
2/CLOSE or GROUND), and for each phase and ground that is
monitored on multi phase relays. Each target indicator is visible
on the front panel of the relay with the cover in place. When
operated, the disk in the target changes from black to orange,
and is magnetically latched in this position .
The FUNCTION targets only operate when a minimum of 0.2 A
flows through the associated relay's output circuit. This is to
provide confirmation that the contact actually did close and that
a signal was delivered . The ELEMENT targets, i.e., phase and
ground are internally operated .
To monitor output current, the coil of a special reed relay is
placed in series with the output contacts. The reed relay signals
the target. (The series impedance of the reed relay coil is less
than 0.1 ohm.)
To reset the target(s) after an abnormal system condition has
been cleared, manually raise the target reset switch on the front
of the relay, or its extension bar that protrudes through the
bottom of the cover. (The relay must be under power to reset a
target.)
NOTE
Resetting the targets in this manner will not reset the
alarm messages provided on the communication link.
8.8
Communications Option
The SCOR relay includes provision for the future addition of the
optional communication board as standard . If the communica
tions option 2-C is selected initially, the relay is provided with an
RS-232 port on the front panel, and an RS-485 port on the back
of the case.
Page 39
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