9
NC C NO
External
D.C. Supply
+
–
Relay Contacts
Load
Diode
NC C NO
External
A.C. Supply
L
N
Relay Contacts
C
R
Load
4.2
Connections, General
Information.
•
Earthing (grounding) –
stud terminal(s) is (are) fitted to the transmitter case for bus-bar earth (ground) connection – see
Fig. 4.1 or 4.5.
•
Cable lengths
– the integral cable may be extended using a suitable junction box but the total cable length must not
exceed 164 ft. (50m) for cells with a constant of <0.05 (as is the case for cell model 2278/305).
•
Cable routing
– always route signal output/conductivity cell cable leads and mains-carrying/relay cables separately,
ideally in earthed metal conduit. Employ twisted pair output leads or use screened cable with the screen connected to the
case earth stud.
Ensure that the cables enter the transmitter through the glands nearest the appropriate screw terminals and are short and
direct. Do not tuck excess cable into the terminal compartment.
•
Cable glands & conduit fittings
– ensure a moisture-tight fit when using cable glands, conduit fittings and blanking plugs/
bungs (M20 holes). The M16 glands ready-fitted to wall-mounted instruments accept cable of between 4 and 7mm
diameter.
•
Relays
–the relay contacts are voltage-free and must be appropriately connected in series with the power supply and the
alarm/control device which they are to actuate. Ensure that the contact rating is not exceeded. Refer also to Section 4.2.1
(below) for relay contact protection details when the relays are to be used for switching loads.
•
Retransmission output
– Do not exceed the maximum load specification for the selected current retransmission range
– see Section 10,
SPECIFICATION
.
The retransmission output is isolated therefore the –ve terminal
must
be connected to earth (ground) if connecting to the
isolated input of another device.
4.2.1
Relay Contact Protection and Interference Suppression – Fig. 4.3
If the relays are used to switch loads on and off, the relay contacts can become eroded due to arcing. Arcing also generates radio
frequency interference (RFI) which can result in instrument malfunctions and incorrect readings. To minimize the effects of RFI,
arc suppression components are required; resistor/capacitor networks for a.c. applications or diodes for d.c. applications. These
components can be connected either across the load or directly across the relay contacts. On 4600 Series instruments, the RFI
components must be fitted to the relay terminal block along with the supply and load wires – see Fig 4.3.
For
a.c. applications
the value of the resistor/capacitor network depends on the load current and inductance that is switched.
Initially, fit a 100R/0.022
µ
F RC suppressor unit (part no. B9303) as shown in Fig. 4.3A. If the instrument malfunctions (locks up,
display goes blank, resets etc.) the value of the RC network is too low for suppression and an alternative value must be used. If
the correct value cannot be obtained, contact the manufacturer of the switched device for details on the RC unit required.
For
d.c. applications
fit a diode as shown in Fig. 4.3B. For general applications use an IN5406 type ( 600V peak inverse voltage
at 3A – part no. B7363).
✶
Note.
For reliable switching the minimum voltage must be greater than 12V and the minimum current greater than
100mA.
4
ELECTRICAL CONNECTIONS…
Fig. 4.3 Relay Contact Protection
A – A.C. Applications
B – D.C. Applications
Summary of Contents for 4623
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