Page 297 - Offshore Electrical Engineering Manual

284 CHAPTER 1 Installation Practice

will be required for reasons of loading and as a maintenance facility. As the system
will be required to operate during abnormal conditions such as during serious gas
leaks, junction boxes, changeover switches, loudspeakers and isolators should all be
certified for use in zone one hazards, even when sited in areas classified as nonhaz-
ardous during normal conditions.

TELEPHONES

The installation of offshore private automatic branch exchange (PABX) systems is
similar to that for onshore systems but with the addition of the following.

1. In hazardous areas, the equipment including the telephone itself must be certi-
fied for the zone of hazard concerned. Telephone instruments are normally of
the flameproof type in such conditions.
2. In noisy areas such as machinery rooms, the telephone instrument will initiate,
via a relay box, horn and light signals to indicate when the instrument is being
called. The relay box, horn and signal lamp should be mounted near the associ-
ated telephone instrument. In some cases, a relay box may control more than
one horn and lamp arrangement depending on the size and degree of congestion
within the module concerned. Suitable acoustic hoods should be provided.

CABLE SUPPORT SYSTEMS
GENERAL
In the design phase, a great deal of care should be taken to ensure that the correct sizes,
configuration and routes have been shown on the cable and racking arrangement draw-
ings, and if route clashes with pipework or overloading of supports is to be avoided, it
is vital to install both the support systems and cable strictly according to design draw-
ings. However, there is always the possibility that a design error has been made, so the
installer should carefully check the locations as shown on the drawings to determine
if any conflict exists between the new cable route and any other equipment, steelwork,
piping, ducting, etc., and whether, when installed, it will cause a hazard to personnel,
obstruct accessways or prevent the installation or removal of equipment.
On minor routes, not detailed on design drawings, where it is necessary to site
run cable support systems, it is important that separate racks/trays are used for the
following categories to prevent electromagnetic interference between cables:

1. Medium-voltage alternating current (AC) (above 1 kV) with associated control
cables.
2. Low-voltage AC (240–1000 V) with associated control cables. Direct current
power cables, associated control cables and 110 VDC/254 VAC instrument cables.
3. Instrument, signal and alarm cables; telephone and communication cables and
fire and gas cables.