User guide

Projecting Guidelines
5) Maximum cable length
The PowerLoop Calculator will define the maximum length of each
cable segment, this tool however determines length only from the
power loss of the specified cable. The cable or wire capacitance will
not affect the cable length as much as the resistive loss will restrict the
power, not the communication.
5.6.4 Dual Earth Systems – Power Loop
Some installations use both Instrument Earth (IE) and Protective Earth
(PE) as two separate earthing paths. In this case, the shield (inner
layer of cable) must be continuous and earthed at one end only. The
outer braid, shield or armour, is then connected to the Protective Earth
connection. This will normally be the local connection point to the PE,
the chassis of the cabinet or a chassis connection close to the field
equipment. The two earth systems must be kept isolated.
BSD-340
IE
PE
PE
IE 8
PLA+ 6
PLA- 7
PLB+ 9
PLB- 10
PE will make earth loops as they are terminated at multiple points,
however the loop area will be narrow or closed as the armour will be
close to the frame or earth. It is essential that the armour is kept on
the same equipotential around to avoid EMC problems or large
current flow in the PE. PE may be connected through glands to the
junction boxes to form a continued protective armour or shield. Still
the internal shield and IE must be kept isolated from the PE.
System Description, AutroSafe Integrated Fire & Gas Detection System, 116-P-ASAFE-IFG/XE Rev. B, 2005-10-27,
Autronica Fire and Security AS
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