SUMMARY OF INSTALLATION PRACTICES

Table 10.10 has been prepared as a summary of the material presented in this chapter It is intended to furnish the reader with an easy reference on sound installation practices and problem areas in the installation of reactor instrumentation systems Opinions on sound installation practices are diverse in the nuclear industry, however, the opinions expressed in Table 10 10 are shared by a majority of the people operating and maintaining nuclear power plants

Table 10.10—Installation Practices for Reactor Instrumentation Systems

Do’s

 

Don’ts

 

Don’ts

 

Do’s

 

Подпись: Use triaxial cables with a floating shield operated at a fixed potential Use triaxial cable for low- level signals, such as from neutron detectors and ionization chambers Provide easy access to the back of instruments behind the instrument panels Mount all visually monitored instruments at or near eye level Protect critical controls from accidental actuation Ground all conduits to the main building ground bus Drill drain holes in conduits at low points to allow water condensing inside to escape Use covered wiring trays for power leads, and band all tray sections together Provide adequate tempera ture-sensitive circuitry Be sure that radiation detec tors are not overheated by neutron heating, etc Pull cables by hand when possible to ensure that no problems exist which will cause the cable to stick Terminate high frequency cables properly to avoid end reflections and standing waves Shield all noise-sensitive cir cuits from electrostatic as well as magnetic fields if magnetic fields are a problem Use differential amplifiers to eliminate common mode voltages and ground loop problems Use a sensor with a center tap where possible Use a balanced-line shielded twisted pair of conductors for low level signal transmission Use high-impedance measuring circuits Подпись:Подпись:Подпись:Подпись:Подпись:

Make certain the insulation is thoroughly stripped off wire before crimping on lug

Give installed connector a re­sistance and voltage test to assure proper operation

Reserve one side of terminal blocks for terminating field wiring

Use large-enough conductor to ensure proper grounding

Bond all racks and chassis together and to ground

Support cables and wires at several points

Install coaxial and triaxial signal cables in metal con­duits

Keep switching command cables, such as those for relays etc, isolated from low level signals for de tectors

Carefully inspect coaxial and triaxial cable installations and procedures to ensure work is properly done

Allow extra conductors in cables wherever practical for future expansion (10 to 15%)

Single-end ground spare shielded signal leads in a cable with the shield grounded at the opposite end

Provide terminal blocks with terminals adequately sized to handle the physical as well as the electrical re quirements of both the interior and field wiring

Keep signal lead as short as possible

Use line filters and shielded transformers wherever necessary

Segregate loads on power lines so that motors, welders, and other machin­ery are not on the same line as instrumentation

Consider radiation environ­ment as well as tempera­ture, moisture, etc, when choosing cables

Don’t leave wire strands out of lug when crimping

Don’t use panel structural member as a ground con ductor

Don’t use a common ground return wire for several relays

Don’t support cables and wires by terminations

Don’t install power cables in the same conduit as signal cables

Don’t think that all signal levels in instrumentation are the same and lump all the cables together

Don’t make coaxial cable out of triaxial cable by con necting both shields together

Don’t let cable “fill” in a conduit exceed 40% in mi tial installations

Don’t ground circuits at ran­dom places or allow them to become grounded unless a ground is called for

Don’t forget to mark each wire and cable with appro­priate identification to assist in circuit tracing

Don’t use splices in signal leads

Don’t think interference won’t occur, it will

Don’t neglect to put inter ference suppressors on any sort of device that may generate interference, i e, relays, motors, fluorescent lights, welders, and heaters

Don’t apply higher than rated voltages to coaxial and triaxial connectors

Appendix

ENGINEERING DATA SHEETS ON NEUTRON-MONITORING START-UP-CHANNEL GROUNDING AND SHIELDING PRACTICES AND EXPERIENCE IN SELECTED U. S. NUCLEAR POWER PLANTS*

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