INM 937x-FB2-Px-SS Rev 1
8
Note:
It is important to prevent this connection from loosening and also to protect
it from corrosion.
Where the trunk and spur cables are protected by steel wire armour, this should
be treated as part of the “protective” ground. Normal practice is to bond the
armour to the plant structure at both ends of each cable. Where armour-protected
cables enter the Fieldbus Barrier enclosure, the armour must be connected to the
enclosure’s internal grounding plate using suitable cable glands.
4 .2 .2 Cable shield ground
The cable shield is normally electrically isolated from the protective earth ground
although in some cases the two may be deliberately interconnected, as explained
below. In both cases the trunk cable shields and the spur cable shields should be
connected to their appropriate terminals (‘S’) in the enclosure.
The enclosures can then be configured by the user to either of two grounding
methods. The user should adopt the one that conforms to their system’s normal
grounding method.
IMPORTANT:
Whichever method is used, the screen/shield should only be grounded
AT ONE END of the cable. Grounding at both ends creates “ground loops”, which will
be troublesome and may even be dangerous.
The following options are available to the user.
WARNING !
The following may involve changes to wiring in the Trunk Terminal
Assembly. No part of the Trunk Terminal Assembly may be worked while
the enclosure is powered, unless the environment is known to be non-
hazardous.
4 .2 .2 .1
Single point of grounding at host; trunk & spur shields interconnected
Figure 4 .1
- Option 1
In many installations, the preferred method of grounding is to ground the fieldbus
cable shields at one point only, normally at the fieldbus power supply. In this case,
the trunk and spur cable shields are connected to each other at the Fieldbus Barrier
and are not connected to ground in the field. For this arrangement, insert the link as
shown in Figure 4.1.
Note: this is the default grounding method for factory-supplied
937x-FB2 enclosures.
Host
ground
Ground
stud
trunk shield
LK2
spur shields
LINK
