When wiring an Ex'd' instrument which requires a 24V DC power supply, does the earth terminal inside the enclosure have to be wired up to an earth conductor in the power supply cable? Or can the external earth on the outside of the enclosure be used instead?

Benm

Can you sure it is continious.
The internal terminal may only be for terminating a conductor if it was included, i would consult the manufacturer,as if it required and found out later say at commissioning, it could be an expensive return visit.

The best way for this one is if you do not know then get someone who does.

Look at the regulations for earthing in hazardous areas

IS it a steel wire amoured cable

How big is the eexd box, what gas group are you in, what gland do you intend to use, how do you intend to certify the work. Then look at the cable is it SWA and combine this with earth practices in the hazardous area.

an internal CPC within the supply cable is the best method for solving this issue. An Exd enclosure is designed to contain any internal explosions and therefore prevent the internal explosion from igniting the external atmosphere. The instrument should be given its own CPC as to comply with the relevant instructions and the external earth connection is merely an equipotential bonding point. (remember to use the correct glands i.e. E1FW Glands etc).