User guide

Projecting Guidelines
System Description, AutroSafe Integrated Fire & Gas Detection System, 116-P-ASAFE-IFG/XE Rev. B, 2005-10-27,
Autronica Fire and Security AS
Page 45
5.6 System Shielding and Earthing
5.6.1 Introduction
The AutroSafe Integrated Fire and Gas System (IFG) has been
designed to supply power and communications to advanced flame
and gas detectors. The system is able to deliver up to 100W of power
and uses a digital communications system, which modulates the
PowerLoop at approximately 130kHz.
The PowerLoop system also allows hundreds of metres of cable to be
used between the loop driver and the field equipment.
The PowerLoop concept provides the user with a potentially powerful
radio transmitter and an excellent antenna. It is therefore vitally
important that adequate precautions are taken to screen the signals
from such an arrangement to avoid crosstalk between similar
PowerLoops and also to keep the system operating within the
prescribed Electromagnetic Compatability (EMC) limits.
This chapter deals with rules and limitations for PowerLoop
installations.
5.6.2 Definitions
Local Frame Earth The electrical connection to the framework at the
described physical position, such as the frame or chassis of a cabinet,
the power cable outlet etc.
Shield Conductive structure encapsulating the wire in a harness or
cable, normally in the form of a mesh or foil forming a Faraday cage.
Armour Mechanical protection to avoid physically damaging electrical
cables or circuits.
Instrument Earth (IE) An earth reference that is normally used to
reference measurements of electrical signals. It may be the same as
the Protective Earth (single earthed systems).
Protective Earth (PE) An earth reference that is normally used as a
coupling path for unwanted electrical signals, like transients and over-
voltage. The chassis or framework of the installation is normally
considered to be the local Protective Earth. As the name implies, it is
intended to provide a safe electrical potential for human safety.
Ground Synonym for Earth, in this handbook Earth is used.