I don't think any of the above answers are completely correct.
Single compression and double compression glands are used on
the basis of area classification.those who are affiliated
with oil and gas sector they will easily understand about
area classification.in zone 0 where the presence of
hydrocarbon is obvious (IIC)double compression gland is used
because the flame path in case of double compression gland
is much more than in case of single compression gland. the
logic behind this is that if there is any explosion inside
the terminal box of the motor no flame should be able to
come out through the cable gland in order to prevent fire
hazards but where there is no presence of hydrocarbons i.e
no danger of fire hazards (IIA/ IIB)single compression glads
are used. it has nothing to do with mechanical strength.
even in case of lighting fixtures used in IIC zone double
compression glands are used.
Although # 4 answer is on right track. But apart from area
classification, mech grip is also equally imp. to Sum up:
Ex'e'(single compression) equal to IP54.(IEC 60079-0,7)
Ex'd' (double compression) equivalent IP66. (IEC 60079-1).
Double is better always... but may not be necessary for all
IN MY INDUSTRIES 125 HP MOTOR USED FOR THAT ONLOAD AMPS UP
SO CAN I USE ONE LOWER KVA DG SET WITH STEP UP TRANSFORMERS
THAT MEANS WITH LOW POWER INPUT TO GET CONSTANT HIGH POWER
EG:125KVA DG SET = 250 KVA STEP UP TRANSFORMERS