Intelligent era of industrial level instrumentation system automation is increasingly high, the detection of sensors throughout the process system corners, so the stability and reliability of the system is particularly important, but the instrumentation system generally has a low insulation strength, over-voltage and over-current tolerance capacity is poor, sensitive to electromagnetic interference and other weaknesses, and the impact of lightning is more significant, for the field instrumentation equipment, how to better lightning protection, we combined with the "petrochemical instrumentation Lightning Protection System Design Code" for a brief introduction.

Instrumentation system lightning protection mainly using external lightning protection and internal lightning protection measures for integrated protection. External lightning protection measures include flashover, lead wire, grounding devices, etc., internal lightning protection measures are signal line protection and power supply line protection should include wire and cable shielding, cabinet shielding, equipotential connection and grounding, reasonable wiring, equipped with lightning surge protector (SPD) and the use of instrumentation systems with a high degree of anti-interference measures.

Instrumentation system lightning protection of the basic methods are:

    a) Equipotential connection and grounding;

    b) Shielding and grounding of signal cables;

    c) Shielding and grounding of instrumentation equipment;

    d)Reasonable wiring;

    e) Setting surge protectors.

Lightning protection precautions for field instruments:

1. Field instrumentation protection

(1), the surge protection of field instruments should be shielded, grounded and installed surge protector method;

(2), the metal shell of the field instrumentation, metal protective box should be fully enclosed;

(3), the need for lightning protection of non-metallic shell level measurement instrumentation should be installed in the protective box made of steel;

(4), the field level measurement instrumentation should be avoided to become a flash equipment;

(5), when the installation location of the field instrument may become a flash object, but can not be moved, the instrument should be installed in the steel plate material protection box.

2. Grounding of field instruments

(1) The metal casing of the field instrument, instrument protection box, junction box and cabinet should be grounded nearby or connected to a grounded metal body.

(2), the field instrument metal casing can be naturally grounded through the metal mounting bracket or metal equipment.

(3) Field instruments on metal equipment, vessels, towers and operating platforms should be equipotential connected to the equipment and operating platform.

(4) Instruments and metal brackets located in Zone 1 explosion hazardous locations should be prevented from having connection gaps to avoid sparks caused by lightning currents.

(5), non-metallic equipment installed on the top of the instrument, should be close to the ground.

(6), the field flow instrumentation should be in accordance with the technical specifications for grounding connections

3、Cable laying

(1) The wiring of the field instrument should be laid through the steel pipe or cable groove, and should not be insulated material pipe. Between the steel pipe and the instrument, between the steel pipe, the instrument cable trough should be closed steel plate structure. The trough and all metal parts should be electrically continuous throughout.

(2), cable laying path: cable and lightning lead cross-laying spacing should be greater than 2m; parallel laying spacing should be greater than 3m. When the laying spacing can not be met, the cable should be shielded through the steel pipe, shielding steel pipe should be grounded at both ends.

(3), when the cable through the steel pipe or laying in a closed metal cable groove, the above distance can be halved.

4、Cable shielding

1), cable shielding:

a) outdoor laying of cables (including signal cables, communication cables and power cables), should be used for shielding cables throughout the steel pipe or closed metal cable groove laying;

b) When using metal armour shielded cable or double shielded cable with mutual insulation, can not be used to wear a steel pipe or closed metal cable groove way of laying.

2), cable outer shielding layer:

a) Cable laying of metal grooves, protective steel pipes and other protective layers;

b) The outer shielding layer of double-layer total shielded cable;

c) The armour layer of metal armoured shielded cable;

d) The total shielding layer of the sub-shielded plus total shielded cable.

3) The inner shielding layer may be utilised:

a) The shielding layer of shielded cables that are fully enclosed through steel pipes;

b) The inner shielding layer of a double-layer total shielded cable;

c) The inner shielding layer of metal armoured shielded cables;

d) The inner shield of the sub-shielded plus total shielded cable.

3), cable shielding grounding

a) The outer shield should be grounded at least at both ends, and the inner shield should be grounded at one end.

b) Signal shielding cable inner shielding layer grounding should be a single point of grounding, should be based on the signal source and receiving flow instrumentation of different situations using different methods. When the signal source is grounded, the shield of the signal shielding cable should be grounded at the signal source end, otherwise, the shield of the signal shielding cable should be grounded at the signal receiving instrument side.

5、Grounding continuity

1）Cable trough and protective steel pipe should be electrically connected throughout.

2), Galvanised cable machine can be directly crimped, cable trough connecting plate shall be crimped and fixed by at least two sets of iron zinc steel bolts with anti-loosening gasket.

3), If the cable path is renewed or branched through a metal junction box, the outer shield or armour layer shall be connected in the junction box and connected to the box of the junction box.

(4), the metal junction box box should be close to the ground.

 (5), non-metallic junction box, the outer shielding layer should be connected in the junction box and should be close to the ground.

(6) The inner shield of the cable should be connected in the junction box and insulated from other conductors.

(7), the cable's internal and external shielding layer in the junction box should be connected using the terminal connection. Different shielding layers should be connected separately and not mixed.

6, the grounding of the outer shield

(1), laying cable protection steel pipe, cable trough ends should be close to the ground, or with the grounding of metal equipment, structures, frames for electrical connections;

(2), instrumentation signal cable groove and threaded protective steel pipe into the building at the outdoor should be connected with the grounding facilities of the electrical profession, into the interior should also be set up with a separate grounding row connection.

(3）When armoured shielded cable is used, the armoured metal of the armoured cable should be grounded at the site end, and it should be connected with the separately set grounding row when entering the room, and the grounding connection should be made by armoured grounding connector.

(4）When double shielded cable is used, the outer shield of the cable shall be grounded at the site end, and shall be connected with the separately set grounding row when entering the room.