4 How to Use
4-3. Precautions
for system designing
- 13 -
Protection against
lightning
Take appropriate measures in accordance with the importance of the facilities and the
environment.
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. Before installing a lightning arrester, verify that it works properly.
Proper use of alarm contact
The alarm contact of the detector is used to transmit signals to activate an external buzzer, alarm lamp or
rotating lamp. Do not use the detector for controlling purpose (e.g., controlling the shutdown valve).
The specifications for the alarm contact of the detector are based on the resistance 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 detector
Abnormal operations by an out-of-control CPU
If load is to be activated, appropriate measures must be taken to stabilize the operation of the detector and
protect the alarm contact referring to the following information.
Relay it with an external relay at a lower voltage of 100 VAC or below (contact amplification). At the
same time, the surge absorbing part SK1 suitable for the specifications must be attached to the
external relay.
In addition, the surge absorbing part SK2 must be attached to the loaded side of the external relay if
necessary.
It may be recommended that the surge absorbing part should be attached to the contact for certain
load conditions. It must be attached to an appropriate position by checking how the load is
activated.
CAUTION
In principle, do not activate inductive load at the alarm contact of the detector. (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 detector with an appropriate
surge absorbing part, such as a CR circuit.
Summary of Contents for PT2-177
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