Should a thermal resistor be equipped with a surge protector?
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For the projects I’m working on at the moment, there are an increasing number of ones related to lightning protection. Aside from grounding work, the most common task is installing surge protectors for various instruments; for the standard ones, it’s sufficient to provide whatever is required. However, there are some somewhat ambiguous aspects regarding which protections to use, and I would like to ask everyone for their advice. 1. First of all, many regulations require that if on-site instruments do not have built-in surge protection, external protection must be installed. If the transmitter does not have a spare port for installation, a tee fitting can be used; however, in practice, installing a tee fitting is quite troublesome and the wiring process is also difficult. Our current approach is to use clip-type surge protectors in the wiring box, but it’s not clear whether this will be acceptable during on-site inspections or by safety authorities. 2. Many technical specifications mention AI and AO signals for surge protection – does AI include thermistors in this context? (We generally don’t provide surge protection for the thermal resistors on the field side; to put it simply, thermal resistors are inexpensive, while surge protection costs more, so it’s sufficient to add surge protection in the control room.) ) Regarding whether RTD is an AI signal, I’ve seen many different opinions – is there any standard definition for this? Thank you
Yes, AI includes thermal resistors. In theory, surge protection should also be installed for the thermal resistors on the field side, but due to cost considerations, it can be installed only in the control room. Whether to configure it specifically still depends on the project’s technical requirements and the actual conditions on site. .
With a thermal resistor, it can’t be connected over very long distances, right? Without using a transmitter. I think for thermal resistors, no power supply is needed, right? So why is it necessary to protect them from thunderstorms?
It can be seen that section 7.4.4 of the \"SH∕T 3164-2021 Code for Lightning Protection Design of Instrumentation Systems in Petrochemical Industries\" specifies that surge protectors should be installed on the surface thermoresistors in above-ground tank areas.
SH∕T 3164 7.4.4 The following field instruments in above-ground tank areas that meet the conditions of 7.4.2 shall be equipped with surge protectors: a) Electric and electronic measuring instruments that convert signals into electrical signals, such as transmitters; b) Gas detectors and analyzers; c) Electric converters, electric valve positioners, solenoid valves, electric actuators, and other electric/signal actuating devices; d) Thermal resistors; e) Electronic switches; f) Relay coils; g) Network and communication equipment installed in fixed locations; h) Other instruments that are sensitive to lightning surges or cannot withstand such surges. 7.4.5 Instruments for which surge protectors are not required: a) Thermocouples ； b) Mechanical contact switches, buttons: c) Non-electric or signal-independent instruments: d) Other instruments capable of withstanding lightning surges.