Emerson IP221 appendix Ii Head Amplifier Unit, Iii Control unit, Relay Ratings

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	Relay		Relay contact	Maximum fuse rating
Unit			rating (each)	at 120V	at 240V
MEP3R/*D	DPCO		45VA	400mA	200mA
MEP3R/*S	SPCO		500VA	4A	2A
MEP3L/*D	DPCO		500VA	4A	2A
		Table 2.1 - Relay Ratings

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