Page 321 - Offshore Electrical Engineering Manual

308 CHAPTER 4 Hazardous Area Installation

the motor without the risk of explosion. Ex ‘nA’ motors are designed to be used in
zone 2 locations.

NON-SPARKING: EX ‘N’ (NOW OBSOLESCENT)

The design concept for Ex ‘N’ machines is similar to Ex ‘e’, but among other minor
differences lacks several important features. First, the Ex ‘N’ certification does not
include the starting condition. Unlike an Ex ‘e’ machine, the surface temperature of
the casing is allowed to exceed the maximum permitted by the temperature class dur-
ing starting. Secondly, no special overload protection is called for. Because of these
limitations, Ex ‘N’ machines are restricted to use in zone 2 or safe areas only.
Problems have been experienced with large Ex ‘e’ and Ex ‘N’ motors operating at
voltages over 3.3 kV where the motor casing itself has exploded.
The ignition source is associated with partial discharges across the end windings
in machines exposed to heavy salt or other contamination. Several operating condi-
tions have been identified where gas could migrate into the motor casing to provide
an explosive concentration.
First, where a gas compressor or crude oil pump shares a common lube oil
system with its drive motor, gas can become entrained in the lube oil at the process
end, and on reaching the motor bearing where the pressure is lower, leave the lube
oil. Therefore common lube oil systems should not be used. Secondly, minor gas
leaks often occur on gas compressor packages, and the cooling of the motor casing
after a shutdown may draw gas in, so that a hazardous concentration exists within
the motor when it is restarted. Existing motors considered to be an ignition risk are
fitted with a prestart nitrogen purging kit. Type Ex ‘N’ high voltage motors are no
longer specified.

PRESSURISED: EX ‘P’
A pressurised motor relies on maintaining all internal parts of the motor enclosure at
a greater pressure than atmospheric in order to prevent the ingress of any hazardous
gases that may be present in the vicinity of the motor. No special precautions need
to be taken with winding temperatures, but the motor interior, especially the stator
windings, must be designed to allow a purging air flow to clear any pockets of flam-
mable gas, within the required purging time. Be warned that this may not be the case
with the standard industrial version of the machine.
To achieve pressurisation, an external source of dry air must be provided from a
safe area. Although this type of motor is inherently cheaper than an Ex ‘e’ equivalent,
the cost saving may be outweighed by the extra costs for the air supplies, pipework
and control systems necessary. Another possible cause of delay and expense is the
need for special certification to be obtained from an authorised certifying authority
such as BASEEFA, UL or PTB.