1 Scope
This standard specifies the grounding and shielding requirements of instrumentation and control equipment, and gives some implementation methods for grounding and shielding. These methods may ensure the noise suppression carried out in instrumentation and control equipment loop. This standard also highlights the requirements on the cable laying for instrumentation and control equipment.
This standard is applicable to the grounding design, check and review, maintenance and operation of instrumentation and control equipment at nuclear power plant.
2 Definitions
For the purposes of this standard, the following definitions apply.
2.1 Electrical noise
Non-effective signals appearing in the circuit or component of instrumentation and control equipment, namely, background values.
2.2 Normal mode noise
The noise voltage appears between two signal wires, and forms a cascade state with effective signals (transverse).
2.3 Common mode noise
The noise voltage appear equally between each signal wire and common reference point; common reference point is generally system grounding point or common point (longitudinal).
2.4 low level signal
Signals whose full output value is less than or equal to 100 mV.
2.5 Medium level signal
Signals whose full output value is larger than 100mV but not larger than 30V.
2.6 High level signal
Signal whose full output value is greater than 30V.
2.7 Digital signals
Digital signals may generally be divided into three types:
a) Digital signal: coded message signal outputted by digital computer, D/A converter or other digital transmission terminals.
b) Switching signal: contact closure or off signal caused by alarm relay, process switch and instrument relay，including 125V DC medium level signal;
c) Pulse signal: count impulse signal generated by radioactivity monitoring instrument or electricity meter etc.
2.8 Faraday shield
A layer of static shield placed between the primary and secondary of isolation transformer, it has a low impedance access over the ground.
2.9 Main instrument ground bus
It is a thick bus copper bar which is insulated from the mounting bracket, and the noiseless grounding wires of all the simulation or digital instrument of instrumentation and control system shall be finally connected to this bus copper bar. At a point on the bus copper bar, the main instrument ground bus is connected to the nuclear power plant underground grounding network with a copper-cored cable with insulation coat whose sectional area is not less than 95 mm2. A dismountable connecting piece is installed between main instrument ground bus and cable connector for the use in ground system test.
2.10 Plant ground system
This system includes the steel structure in the plant, the underground ground network, all the protective grounding wires of power equipment, switchgear, enclosure, stander and control panel in plant; this system shall be connected to on-site water well, chemical grounding well or river.
2.11 Noiseless grounding
As for the grounding of the signal zero line or common reference point of instrumentation and control equipment, the noiseless grounding shall be insulated from control panel enclosure and plant ground system before connecting to the underground ground network of nuclear power plant to prevent the electrical interference from the ground.
2.12 Protection grounding
The grounding to prevent electric shock to human when electrical equipment is out of service.
2.13 Grounding at one point
It may only connect the signal zero line or common reference point of any instrumentation and control equipment with the main instrument ground bus by one point.
2.14 Treeing connection
When connecting several instrumentation and control equipment or the signal zero line or common reference point of several instrumentation and control system to the main instrument ground bus, it shall be connected according tree branch mode, take the main instrument ground bus as the trunk, and respectively place different instrumentation and control system on the main branch of the tree, and place the instrumentation and control equipment in each system on the branch of the main branch. Closed ground loop doesn't exist in this connecting mode, therefore, the noise in grounding wire may be minimized. In order to ensure treeing connection, the signal zero line or common reference point of instrumentation and control equipment must be insulated from stander, and the grounding cable must be equipped with insulating coating.
3 Technical Requirements
3.1 Strand requirements of shield
3.1.1 General requirements
a) All the wire of analog signal circuit shall be stranded and shielded in order to minimize static noise;
b) All the cables transmitting low level signal shall be stranded and shielded cables, in order to minimize normal mode noise;
c) The cables transmitting extremely low level signal or cables used in the occasions that require maximum noise suppression shall have double shield, and the outer shield of the double shield shall be connected to the ground bus of local instrumentation, and the inner shield shall be connected with the screening can of the amplifier of instrumentation.
d) As for cable transmitting medium, high level signal shall also be with shield in case of coupling its signals to the nearby other medium, low level signals.
Contents
1 Scope
2 Definitions
3 Technical Requirements
3.1 Strand requirements of shield
3.2 Grounding requirements for instrumentation and control equipment
3.3 Cable laying
4 Examples of grounding and shielding for typical circuit
4.1 Thermocouple
4.2 Resistance temperature probe
4.3 Low level analog signals
4.4 Medium level analog signals
4.5 Milliamp signals
4.6 Digital signals
4.7 Special signals
Appendix A
Appendix B
