reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

d) channels available. Channels 1, 2, 5, 10 and

race

will not give rise to the production of third order interrnodulation products. For the power station UHF

lnierference. However it should be noted that if

)ri

(1\cs1 stations operate in the duplex mode, third order in manodulation could arise when two adjacent fixed ,t,itions are transmitting and a handportable on one

tho channels is also transmitting.

TABLE 8.7

Operating channels having no third order intermodulation interference

ct a = 456.075 MHz b = 461.575 MHz = 456.100 MHz

Hien a + b c = 461.550 MHz, which is Channel :I A mobile transmitter frequency.

8.12.2 Half

V, described in Section 8.7.2 of this chapter on r,:ceivers, the first IF of the receiver is 10.7 MHz.

A fixed station operating on Channel 22A will re- ti e the mobile transmit frequency of 461.600 MHz.

For the fixed station there is a choice of local oscil- lator frequency (f 0 ) to mix with the received signal to produce a difference frequency of 10.7 MHz:

fo = 461.6 + 10.7 = 472.3 MHz or = 461.6 - 10.7 = 450.9 MHz

Assume that Channel 22A fixed station receives a

, q_mal from Channel 25A fixed station transmitter at 456.250 MHz.

fo = 472.3 MHz, or 450.9 MHz

Direct wire telephone systems

Channel 25A = 456.25 MHz

Mixer output will produce a difference frequency

= 16.050 MHz or 5.35 MHz

Note that 5.35 MHz has a second harmonic of 10.7 MHz which could also be produced, albeit at a lower level, in the mixer and passed on to the IF stage.

Should the allocation of frequency channels to a power station make this form of interference possible, then the frequency for the local oscillator would be specified as 472.3 MHz.

9 Direct wire telephone systems

9.1 General details

Discrete direct wire telephone systems (DWTS) are provided in power stations where direct and reliable means of speech communication is required between manned control points and the associated plant locations.

The design of each DWTS takes cognisance of the following considerations:

•Absolute reliability to ensure continuance of operation when all station supplies are lost.

•Alternative to the PAX system and other DWTSs were practicable.

•Cabling between equipment elements to be in shortti me fireproof cable to prolong operation during fire conditions in locations through which the cables are routed.

•Operation to be possible when control point or plant location equipment has been mis-operated, i.e., telephone handsets not replaced.

•DWTS common equipment to be suitable for cubicle or control desk mounting.

•The DWTS desk-mounted telephone concentrator panels to have the minimum of equipment mounted on them (Fig 8.58).

•The equipment and control desk panels are based on modules of 10 DWTS extensions, expandable from 10 to 30 extensions. For more than 30 extensions a

second control desk panel would be provided, extending the system to 60 extensions maximum.

For non-nuclear power stations it is the practice of the CEGB to provide a discrete DWTS radiating from each of the following control points:

•Supervisor's desk - CCR.

•Each unit control desk - CCR.

•Coal handling control desk - coal transport and stocking area (applicable to coal-fired stations only).

The additional DWTSs for nuclear stations are described in Section 14 of this chapter.

The elements of the DWTS are:

•Common equipment for cubicle or desk interior mounting.

•Common equipment cubicle (if common equipment is to be cubicle-mounted) (Fig 8.59).

•DWTS telephone concentrator with associated operator's handset at the manned control point (Fig 8.58).

•Telephones in office locations.

•Telephones with associated audible and visual calling units at plant locations.

•Battery and charger.

•Cabling.

•Cable marshalling cubicle.

Block diagrams of the two types of DWTS, i.e., with desk-mounted and cubicle-mounted common equipment are shown on Figs 8.60 and 8.61.

9.2 Common equipment and common equipment accommodation

The discrete common equipment for each DWTS is accommodated, where possible, in a single crate which houses the plug-in modules containing the DWTS equipment printed circuit boards (PCBs).

There are three types of equipment module:

•Module for four DWTS telephone extensions.

•Module for switchboard telephone handset.

•Module for crate alarms and crate power supplies.

A typical DWTS having twenty telephone extensions would require one equipment crate housing:

•Five, 4-DWTS telephone extension modules.

•One switchboard telephone handset module.

•One alarm and power supplies module.

A DWTS having more than thirty telephone extensions would require an additional crate containing the additional telephone extension modules.

9.3 Plant telephones

DWTS telephones in plant areas are robust, heavy duty, weatherproof telephones with an associated audible and visual calling unit. Each DWTS telephone point is electrically connected to the DWTS common equipment by two cable pairs: one pair is used for speech and the second pair for signalling. The use of two pairs permits: