Process monitoring instrumentation refers to those systems outside the neutron-monitoring group which indi­cate, record, and control all operational systems within a nuclear reactor facility A channel includes the primary sensor, the interconnecting conductors, the measuring circuit, and displays The primary sensors are located at a point in the process system The measuring-circuit instru­mentation (amplifier, recorder, power supply, etc ) is located in the main control room or in an auxiliary control area

Grounding and shielding of instrumentation systems accomplish two major purposes (1) proper operation of the instrumentation by reducing or eliminating erroneous signals and (2) personnel safety with respect to hazardous operating parameters

(a) Personnel Safety. Equipment grounding provides safety for personnel who might come in contact with the equipment As far as instrumentation is concerned, two mam hazards exist for which protection must be provided electrical shock and heat All external surfaces should be at ground potential and at a temperature less than 120°F Where this is not practical for the component itself, protection for personnel safety should be provided by completely surrounding the instrument or installation The National Electrical Code gives rules for grounding that have wide commercial application These same rules, with more details added by individual manufacturers or by special applications, can be used to ensure proper personnel protection for most types of installations and equipment

(b) System Problems (Involving Erroneous Signals) In

instrument systems, good stable grounds are needed to provide a measurement reference, a solid base for the rejection of common-mode signals, and effective shielding for low-level circuits

A stable reference for measurement can be provided positively in only one way by referencing all measurements to a single-point ground However, this is not possible except through the use of a floating system (1 e, all system components completely isolated) A floating system also provides a firm base for the rejection of common-mode signals Compared to Single-point grounding, the instrumen­tation reference ground is inferior, however, it is more generally used The instrumentation reference ground uses a grid or bus that is maintained, as nearly as possible, at a consistent fixed electrical potential Circuits and systems using this type of a ground bus system must be grounded at one point only Grounding the system at more than one point will create ground loops, which will cause erroneous signals to be introduced into the equipment, either directly through one of the signal leads or induced through the shielding

Since in most instances the primary element is located some distance from the measuring circuit, there most certainly will be a difference in ground potential or reference Also, owing to the distances involved, sizeable differences m conductor impedance to ground could exist These conditions cause two of the main problems in low-level measuring circuits, ground loops and common­mode signals These two problems can be dealt with either by eliminating the conditions that cause the problem or by rejecting the erroneous signals that are produced as a result

Some elimination remedies follow. These remedies are usually difficult to implement [see Fig 10 23(a)]

1 Interrupt the continuity of the ground loop while preserving the path for the sensor signal [l e, increase the ground impedance (Zg) ideally to infinity]

2 Reduce the resistance of the ground conductor (Rg) to zero [This effectively shorts the total ground potential

(VAB) ]

3 Break the ground-loop current path (Ig) by floating the system (l e, isolate the sensor or the amplifier and power supply grounding at a single point only) Note If the amplifier is used to feed signals to a recorder, analog-to — digital (A/D) converter, display, or other data handling device, the path of the ground-loop current may be reestablished through these devices

Generally, some method of rejecting the erroneous signals is more practical than the elimination remedies discussed above One of these rejection remedies is to interrupt both the signal and ground-loop currents and then transmit the signal while blocking the ground-loop current Two common methods to achieve this are

1 A transformer and a modulator—demodulator can be used [see Fig 10 23(b)] Some of the advantages of this method are (1) a wide voltage range of both signal to amplifier ground and signal to common mode can be handled and (2) error rejection is independent of closed — loop gam Some of the disadvantages are (1) the bandwidth is limited by the modulating frequency, (2) output errors are induced by mtermodulation, and (3) error-reducing feedback cannot be used without reestablishing the ground-loop path

2 A switched capacitor can be used [see Fig 10 23(c)] This method has the same advantages and disadvantages as the transformer method with the added disadvantage of poor frequency response

A popular method of curing ground-loop problems is by using a differential amplifier Identical fractions of the ground-loop voltage are applied to the inverting and nonmverting inputs of the differential amplifier This causes the ground-loop voltages to be seen as a common mode voltage, and, as such, they are rejected (to a degree depending on the particular amplifier used) This method also becomes directly applicable to differential transducer signals that are imposed on relatively high levels of common-mode voltage Here using the differential amplifier allows extraction of these low-level signals from the high-level common-mode voltages

Although differential amplifiers in general have ex­tremely good common-mode voltage rejection, they are not perfect Certain system parameters affect the level of these common-mode signals and can be manipulated to help reduce the problem Ideally, the common-mode signals can be eliminated by either of two methods [Fig 10 23(a)] reducing the conductor resistances of both the signal and ground (Rs and Rg) to zero or increasing the impedance to ground of both the signal and ground conductors (Zsg and Zgg) to infinity Practically, this same result can be attained by making the ground and signal conductor resistances (Rg and Rs) equal and the signal and ground impedances to ground (Zsg and Zgg) equal This can be accomplished by observing the following rules

1 Use a “balanced line” between the sensor and the amplifier (l e, equal resistance and impedance in both the signal and ground conductor between the sensor and amplifier and the conductor and ground) This can be done most easily by using a shielded twisted pair of conductors for transmission of the signal from the sensor to the amplifier

2 Keep signal cables as short as possible

3 Use a source with a center tap if possible

Both the signal source and the cable impedance have a shunting effect on the input signal to the amplifier. Having equal impedances at each end (sensor and amplifier) idealizes maximum power transfer; however, in transferring voltage signals, this is detrimental owing to line losses caused by system impedance. Therefore, if low-impedance sensors and high-input-impedance amplifiers are selected, the amount of signal current can be kept at a minimum, thus minimizing system error due to voltage drop. For systems where higher sensor impedance is required, corre­spondingly higher amplifier input impedance should be used. Good practice dictates that the input impedance of the amplifier should be at least 10 times the output impedance of the sensor.

The following general rules should be observed in the installation of low-level signal systems:

1. Avoid ground loops.

2. Provide a stable signal ground and a good signal — shield ground.

3. Ground the signal circuit and the signal shield at one common point.

4. Never use a signal-cable shield as a signal conductor.

5. Ensure that the minimum signal interconnection is a uniformly twisted pair of wires with all return current paths confined to the same signal cable.

(c) Primary Elements (Sensors and Transducers). Ob­servance of the following basic rules with respect to
primary elements will eliminate or alleviate many of the problems associated with grounding and shielding of low — level-signal transmission systems

1 Use low signal-source impedance devices whenever possible This not only reduces system noise but also minimizes the shunting effect at the input of the measuring circuit

2 Use a center tap on the sensor output whenever possible This permits the signal-cable shield to be firmly fixed and operated at a minimum potential with respect to the signal pair, thus providing the most effective shielding

3 Use special configurations, such as the noninductive strain gage, to reduce or eliminate interference problems from electromagnetic fields, magnetic fields, and other types of induced noise

4 Ensure proper isolation from all mounting hardware for isolated sensors

(d) Interconnection. Observance of the following basic rules with respect to interconnection will eliminate or alleviate many of the problems associated with grounding and shielding of low-level-signal transmission systems

1 Use a “balanced line” between the sensor and the amplifier (i e, equal resistance and impedance m both the signal and ground conductor between the sensor and amplifier and the conductor and ground) Use twisted, shielded pair

2 Keep signal cables as short as possible

3 Never use splices in signal leads

4 When using connectors (multipin) (1) use adjacent pins for signal pairs, (2) carry shield through pins adjacent to signal pins, (3)use spare pins as a shield around signal pair by grounding them together and then to the signal shield

5 Separate low-level-signal cables and power cables by the maximum practical distance and cross them, where necessary, at right angles

6 Isolate signal cables with conductive conduits and wireways

7. Ensure that spare shielded conductors in signal cables are single-end grounded, with the shields grounded at the opposite end

(e) Measuring Circuit. Observance of the following basic rules with respect to the measuring circuit will eliminate or alleviate many of the problems associated with grounding and shielding of low-level-signal transmission systems

1 Ensure that the measuring circuit has (1) high common-mode signal-rejection ratio, (2) high input im­pedance, (3) good d-c stability, and (4) wide bandwidth

2 In terminating the signal cable to the measuring device, use twisted leads exposed for as short a distance as possible from the shielded cable

10-5.7 Radiation-Monitoring Instrumentation

Most radiation-monitoring equipment requires the use of remotely located detectors connected to the monitor and control section by a multiconductor cable Generally, these systems use a common ground for signal reference, power, and chassis A separate conductor as well as the shield for the signal leads should be used between the control unit and the detector unit The manufacturer’s recommendation for grounding and shielding should be followed explicitly, and care should be taken to ensure that the mounting and assembly of components is proper Most of the problems and their solutions discussed earlier m this chapter are directly applicable to radiation-monitoring instrumentation