P
AGE
9
2.5.1.2 AC POWER WIRING
Earth Ground
Each unit includes noise suppression components attached to the chassis that require an
earth ground connection. To verify that it is earth ground being attached, make a resistance
check from instrument chassis to the nearest metal water pipe or proven earth ground. This
reading should not exceed 100 ohms.
Neutral (For 115 VAC)
It is good practice to assure that the AC neutral is at or near ground potential. To verify this, a
voltmeter check between neutral and ground should be done. On the AC range, the reading
should not be more than 50 millivolts. If it is greater than this amount, the secondary of this
AC transformer supplying the instrument should be checked by an electrician. A proper
neutral will help ensure maximum performance from the instrument.
2.5.1.3 WIRE ISOLATION
The instrument is designed to promote proper separation of the wiring groups that connect to
the instrument. The AC power wire terminals are located near the top of the instrument
boards. The analog signal terminals are located near the bottom of the intrument boards.
Maintain this separation of the wires to insure the best protection from electrical noise. If the
wires need to be run parallel with any of the other lines, maintain a minimum 6 inch space
between the wires. If wires must cross each other, do so at 90 degrees. This will minimize the
contact with each other and reduces "cross talk". "Cross talk" is due to the EMF (Electro
Magnetic Flux) emitted by a wire as current passes through it. This EMF can be picked up by
other wires running in the same bundle or conduit.
2.5.1.4 USE OF SHIELDED CABLE
Shielded cable helps eliminate pickup of noise the wires may be exposed to. Shielded cable
is a single or multi-pair of insulated wires with each wire or pair of wires surrounded by a wire
mesh or conductive foil and then covered with plastic insulation.
It is recommended that all analog signals be run with shielded cable. Connection lead length
should be kept as short as possible, thus keeping wires protected by the shielding. The shield
should be grounded at one end only, at the transmitter or transducer and the shield should be
stripped back before going inside the unit case .
Wiring Connections 2.6
All wiring connections are typically made to the unit with it installed. Connections should be
made at the terminal blocks, two 12 gauge wires maximum, using copper conductors only.
Terminal blocks are designated TB1 thru TB13. See Figure 2-2 for the terminal block
locations.
FIGURE 2-2
TB3
TB4
TB5
TB1
1 2
1 2 1 2 3 4 5 1 2 3 4 5
1 2 3 4 1 2 3 4 1 2 3 4
TB6
TB7
TB8
TB9
1 2 3 4
RELAY/SSR Driver
Board
TB10 TB11 TB12 TB13
1 2
1 2 1 2 1 2
Current Output Board
Processor Board
TB 2
1 2
Summary of Contents for MRC 7800
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