8
3.2
Connections, General
Information.
•
Earthing (grounding) – stud terminals are fitted to the transmitter case for bus-bar earth (ground) connection – see
Fig. 3.1 or 3.2.
•
Cable lengths – The cable length between the flowcell and the electronics unit is provided as ordered, and suitably
terminated at both ends.
•
Cable routing – always route the signal cable and mains-carrying/relay cables separately, ideally in earthed metal
conduit.
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
7 mm 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 3.2.1 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 8.
Since the retransmission output is isolated the –ve terminal must be connected to earth (ground) if connecting to the
isolated input of another device.
3.2.1
Relay Contact Protection and Interference Suppression – Fig. 3.3
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 malfunction 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. 3.3.
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. 3.3A. If the instrument malfunctions (incorrect
readings) or resets (display shows 88888) the value of the RC network is too low for suppression – 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. 3.3B. For general applications use an IN5406 type ( 600 V peak inverse voltage
at 3 A – part no. B7363)
✶
Note. For reliable switching the minimum voltage must be greater than 12 V and the minimum current greater than
100 mA.
…3
ELECTRICAL CONNECTIONS
Fig. 3.3 Relay Contact Protection
R
C
Load
L
N
External
a.c. supply
Relay Contacts
NC C NO
Load
+
–
External
d.c. supply
Relay Contacts
A – a.c.Applications
B – d.c. Applications
NC C NO
Diode
