Connections, General; Relay Contact Protection And Interference Suppression - ABB 9438 User Manual

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.
• Alarm Relay –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 7.
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 AC applications or diodes for DC 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 AC applications the value of the resistor/capacitor network depends on the load current and inductance that is switched. Initially,
fit a 100R/0.022 µF 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 DC 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.
External
L N
AC Supply
A – AC Applications
NC C NO
Relay Contacts
R
C
Load
Fig. 3.3 Relay Contact Protection
3 ELECTRICAL CONNECTIONS...
NC C NO
External
+ –
DC Supply
B – DC Applications
Relay Contacts
Diode
Load
7