Innotech Device Network Cabling - Manual (Page 13)

Innotech Device Network Cabling Installation Instructions
© Mass Electronics Pty Ltd 2007 Chapter 1 – Common Requirements Page 13
1-2 Definition of Earth Types
There are three types of earth referred to in this document:
• Bonded, hard or clean
• Soft
• Floating
These definitions are listed below in relation to the Innotech networks.
Type Description
Bonded,
Hard or
Clean
A bonded, hard or clean earth is defined as a low impedance earth point with little
or no chance of conducted noise either already present or likely to be created when
bonded to a circuit. For Innotech’s reference we also explicitly mean there is no
potential difference to true earth.
Soft
A soft earth is where there is a path to earth via a circuit. This provides a tie to earth so
potential differences are minimised. This path has higher impedance than a bonded
earth. This does force strong earth currents, i.e. leakage currents from large AC motors,
to remain on the proper heavy gauge earth wires, while leaving the soft earth paths
unaffected. So whilst having multiple earth connections we have not created any
effective earth loops.
Soft earth is often used in 24 V supplied devices. The ‘tie to earth’ is in fact a ‘tie to 0 V’
because there is no dedicated earth terminal available. It is therefore expected that
the 0 V is earthed near the supply transformer.
Floating
Floating simply means that there is no path to an Earth point at all provided by the
device, not even a soft earth path. A floating device has no earth.
Table 1-1: Earth Types
1-3 Earth Points
It is imperative that the screen is earthed at one point only and this is defined as any point along a
network that is the best earth point along its length. Where ‘best’ earth point, is the point with least
amount of earth potential fluctuations. i.e. A controller in the basement of a high rise building is
closest to a good earth point than a controller in the plant room on top of the roof.
While there are internal jumpers on some devices to link the ‘S’ terminal internally to earth, there are
variants in hardware types and revisions, simply connect a bonded earth as shown externally. This
will suit all installations, regardless of age. If a repeater is installed each side of the repeater is to be
treated as a separate standalone network and each must have an earth connected.
When there is more than one connection point to earth the screen becomes a conductor and no longer performs
correctly. Care should be taken to ensure only one bonded earth point is ever connected on a primary network.
Refer to Chapter 2-6 for details on use of a Repeater in a primary network.
NOTE