6-6 Installation Date Code 20011026
SEL-321/321-1 Instruction Manual
for pin assignments). EIA-232 interfaces are supported by almost all modern relays, meters,
computer, and communications devices. See Figure 6.15 and Figure 6.16.
Figure 6.1: 9-Pin Connector Pin Number Convention
WARNING
!
Do not rely upon pins 5 and 9 for safety grounding, because their
current carrying capacity is less than control power short circuit and
protection levels.
Pins 5 and 9 connect to frame (chassis) ground via the main board ground plane. The
communication circuits are protected internally by low-energy, low-voltage MOVs and passive
RC filters.
You should keep the length of the communication cables as short as possible to minimize
communication circuit interference and also to minimize the magnitude of hazardous ground
potential differences that can develop during abnormal power system conditions. See the next
subsection, Communication Cables, for additional details and restrictions.
Communication Cables
Several standard SEL communication cables are available for use with the SEL-321 Relay. See
below for some representative pinouts. Using an improper cable can cause numerous problems,
so you must be sure to specify the proper cable for the application. To help in this regard, cable
selector software is now available. Please contact SEL for more information or if you have any
questions about cables and cable connections.
Never use standard null-modem cables with the SEL-321 Relay. Using any non-SEL cable can
cause severe power and ground problems on the SEL-321 Relay communication ports.
The following list provides additional rules and practices you should follow to ensure successful
communication using EIA-232 serial communication devices and cables:
• You should keep the length of the communication cables as short as possible to minimize
communication circuit interference and also to minimize the magnitude of hazardous
ground potential differences that can develop during abnormal power system conditions.
• EIA-232 communication cable lengths should never exceed 50 feet, and you should
always use shielded cables for communication circuit lengths greater than 10 feet.
• Modems are required for communications over long distances and to provide isolation
from ground potential differences between device locations.
• Route communication cables well away from power and control circuits. Switching spikes
and surges in power and control circuits can cause noise in the communications circuits if
not adequately separated.
• Lower baud rate communication is less susceptible to interference and will transmit
greater distances over the same medium than with higher baud rates. You should use the
lowest baud rate that provides adequate data transfer speed.
(female chassis connector, as viewed from outside panel)
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