Page 326 - Offshore Electrical Engineering Manual

Loop Calculations for Galvanic Barriers 313

plastic materials, as well as partitioning walls and the mounting of the apparatus are
specified in this standard. According to EN 1127-1, temperatures shall not exceed
80% of the limit temperature of the temperature class.
Junction boxes and switches, however, may be treated as T6 (85°C) at a maxi-
mum ambient temperature of 40°C because, by their nature, they do not contain
heat-dissipating components.
A wide variety of ‘Feed Through’ and ‘Disconnect’ terminals can be fitted in
simple apparatus enclosures. Disconnect terminals that do not require the conductors
to be removed from the terminals for test and calibration purposes are particularly
useful during operational conditions.
It is important that the external terminal connections maintain 3-mm clearance
between bare metal parts of the same IS circuit and 6 mm between bare metal parts of
different IS circuits. Some users prefer an Ex ‘ia’ certified enclosures for IS circuits.
Simple apparatus such as level switches and other safety switches should have
gold-plated contacts due to the low voltages present in IS circuits, otherwise the
change from closed to open circuit will not register.
The essential requirements of an intrinsically safe system are that:

• The system must work.
• The apparatus in the system must be ‘certified’ or ‘simple’.
• The compatibility of the apparatus must be established.
• The required level of protection of the system is known.
• The required temperature classification and ambient temperature rating of each
piece of apparatus is known.
• The parameters of the cable are known.

LOOP CALCULATIONS FOR GALVANIC BARRIERS
Every intrinsically safe loop will be made up of one or more field devices, at least
one cable and the zenor barrier. The field devices and the cable types used will have
known values of capacitance and inductance (capable of storing energy). In order
to ensure that the barrier operates reliably, a loop calculation must be performed
for each type of IS circuit, i.e., where the field components and the barrier type are
identical, and the cable type is identical (see Fig. 5.4.3). The calculation will then
be valid for those components, and for a maximum length of the cable type utilised.
Where the field device is a certified item (i.e., not simple apparatus, e.g., a tem-
perature transmitter or a solenoid valve), extra checks are necessary. The certificate
of the field device will include its maximum input parameters which will specify one
or more of the values Ui, Li and Pi. Compatibility must be checked by ensuring that
the maximum input figures of the field device are not exceeded by the maximum
output values of the chosen barrier. If the system includes more than one item of cer-
tified apparatus, then compatibility with the barrier must be checked separately. The