RKI Instruments RM-5000 Series Indicator/Alarm Unit - Manual (Page 17)

4 How to Use 4-3. Precautions for system designing
- 17 - RM-5000
Lightning cannot be prevented. Cables installed in a metal conduit or under the ground cannot be
completely protected from inductive lightning surge caused by lightning. Although complete elimination of
disasters caused by lightning is impossible, the following protective measures can be taken.
Protection
against lightning
Take appropriate measures in accordance with the importance of the facilities and the
environment.
• Connect the transmission signal route by using optical fiber.
• Provide protection by a lightning arrester (cable arrester).
(Although inductive lightning surge can be transmitted through the cable, it is
prevented by installing a lightning arrester before the field devices and central
processing equipment. For information on how to use a lightning arrester, please
contact the manufacturer.)
Grounding
In addition to lightning, there are more sources of surge noise. To protect units from
these noise sources, the units must be grounded.
* The lightning arrester has a circuit to remove a surge voltage which damages field devices, so that
signals may be attenuated by installing the arrester. Before installing a lightning arrester, verify that it
works properly.
Proper use of alarm contact
The alarm contact of the indicator/alarm unit is used to transmit signals to activate an external buzzer or
alarm lamp. Do not use the indicator/alarm unit for controlling purpose (e.g., controlling the shutdown valve).
The specifications for the alarm contact of the indicator/alarm unit are based on the resistant load conditions.
If inductive load is used at the alarm contact, the following errors will occur easily because counter
electromotive force is generated at the contact.
• Deposition, defective insulation or defective contact at the relay contact
• Damage of any electric parts due to high-voltage generated inside the indicator/alarm unit
• Abnormal operations by an out-of-control CPU
CAUTION
The "b" contact (break contact) under de-energized state may be opened momentarily by a physical
shock, such as external force.
When the "b" contact is selected for the alarm contact, take appropriate actions to prepare for a
momentary activation, for example, add signal delay operation (approximately one second) to the
receiving side of the "b" contact.
CAUTION
• In principle, do not activate inductive load at the alarm contact of the indicator/alarm unit. (In
particular, never use the inductive load to activate a fluorescent lamp or motor.)
• If inductive load is activated, relay it with an external relay (contact amplification). However,
because the coil of an external relay also involves inductive load, select a relay at a lower
voltage (100 VAC or below), and then protect the contact of the indicator/alarm unit with an
appropriate surge absorbing part, such as a CR circuit.