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ii) Head Amplifier Unit
The head amplifier terminals A+ and B- should be connected to the cores of the control unit cable. In
the case of the dual marine unit, lower sensor connections A+ and B- are connected to the two cores
of one cable, and upper sensor connections, C- and D+ to the other. The screen should not be
connected in any head amplifier unit, as this could cause an earth loop. The marine head amplifier
has cable grips which should be used. Care should be taken not to overtighten these, as this could
result in damage to the circuit board. The cable is then taken out through the cable entry in the head
amplifier.
When an adjacent head amplifier (AI or XI) is used, the sensor cable pins 1 and 2 are connected to
terminals 1 and 2 respectively in the sensor head amplifier. The green earth wires are connected to
the ‘E’ terminals, as shown below.
Red wire with
No. 2 tag to 2
green wire to E
Red wire with
No 1 tag to 1
green wire to E
To sensor
Figure 2.5
iii) Control unit
The system is designed to be noise immune; however, it is good practice to separate the
instrumentation cables from power cables. Figure 2.6 shows the terminal arrangements. The head
amplifier cable cores are connected to the + and – terminals on the control unit, ensuring that +A and
+D is connected to the + terminal, and –B or –C respectively is connected to the – terminal. The
screen is connected to the “SCN” terminal marked sensor on the rack-mounted unit.
The main connections are made to the L and N terminals of the control unit. The control unit must be
earthed, this is mandatory on intrinsically safe units.
The free standing control unit has one relay, which is a double pole changeover (DPCO) relay. This
relay is normally energised and will release if an alarm or fault condition occurs. The rack-mounting
unit has a single pole changeover (SPCO) fault relay, and an SPCO or DPCO alarm relay. The alarm
relay is normally energised and releases if an alarm or fault condition occurs. The fault relay
energises if a fault occurs. Thus in ‘normal’ mode with the green light on, the alarm relay is
energised, and the fault relay is not, as shown in Figure 2.4.
An HBC fuse should be included in the relay circuit as specified in Table 2.1.
All fuses are 20mm x 5mm
Control
Unit
MEP3R/*D
MEP3R/*S
MEP3L/*D
Relay
DPCO
SPCO
DPCO
Relay contact
rating (each)
45VA
500VA
500VA
at 120V
400mA
4A
4A
at 240V
200mA
2A
2A
Maximum fuse rating
Table 2.1 - Relay Ratings
.