Page 337 - Offshore Electrical Engineering Manual

324 CHAPTER 4 Hazardous Area Installation

The American standards allow defined, signposted area zoning of IEC standard
areas, and this is more workable on a shore process plant, where say a new process
plant certified to European standards is being built in the middle of an existing site.
However, this is NOT recommended offshore and hopefully the Client Company and
the Certification Authority involved would not accept it anyway.

EQUIPMENT FOR USE IN POLAR REGIONS

The very low temperatures involved may cause materials to behave very differently,
and this must be taken into account when specifying, for example, cable insulation.
The certification for hazardous area Ex equipment, particularly Ex ‘d’ has a lower
temperature limit, which must be observed. This is because if the explosive gas mix-
ture is present, its density will be greater at low temperatures and hence the gas
present in the Ex ‘d’ enclosure will have a higher calorific value, leading to a more
powerful explosion. The ‘explosion-proof’ nature of the enclosure will then be com-
promised. Canadian Coast Guard standards are helpful with regard to this issue (See
Canadian Marine Safety Guidelines Transport Canada).

INTRINSICALLY SAFE BARRIERS – GALVANIC OR OPTICAL
ISOLATION?
In most situations, opto-isolation is preferred, although usually more expensive and
this will add up if there are a large number of control loops.
Using opto-isolators may have the following benefits:

1. Prevents issues with earths (grounds) at different potential producing circulation
currents.
The MTL website states - ‘The current flowing though the bonding conductor
generates a potential difference between the IS earth point at the barrier and
the neutral star point. The outside of the field-mounted instrument is bonded to
the neutral star point, and the internal circuits of the instrument are connected
to the barrier busbar. The potential difference between IS earth point at the bar-
riers and the neutral star point is therefore transferred to the hazardous area. It
is normally safe because these internal circuits are isolated from the instrument
housing, but this potential difference should be minimised so that if there is a
local insulation failure no danger can arise. The installation conditions of bar-
riers, screened transformers etc. nearly all call for a return path impedance less
than 1 ohm. A figure of 0.1 ohm is normally achievable and is more desirable.
It is important to remember that the resistance involved is that of the return con-
ductor between IS earth points at the barriers and the neutral star point and the
resistance of the earth mat is not important for this purpose. These principals
are normally applied within the UK’.
2. Opto-isolators may be more reliable giving the control loop a higher SIL Level
(see PART 9 Chapter 2).