Earthing Flashcards
What earthing systems aren’t suitable for use in potentially explosive atmospheres?
TN-C and TN-CS protective multiple earthing (PME). This is because the neutral is also the earth conductor.
Connection of this conductor to true earth is at the generator/transformer. Thus any neutral current would introduce voltage drop in this conductor, which could raise the voltage at the main earthing terminal (MET) above true earth.
Note that as the MET is not normally connected to true earth at the consumers installation, if it is connected to a true earth and incendive spark could occur, possibly initiating an explosion.
What earthing systems are suitable for use in potentially explosive atmospheres?
TN-S, TT and IT earthing systems, but where earthing electrodes are to be used, consultation with the appropriate superior technical authority is advised.
Correct earthing is essential to safety of personnel and equipment.
What are the correct earthing conductor sizes?
Phase conductor - Earth Conductor
<= 16mm2 - Same as phase but not less than 2.5mm2 if protected otherwise 4mm2.
16-35mm2 - 16mm2
Over 35mm2 - 0.5 x phase conductor.
Discuss equipotential bonding.
Equipotential bonding conductors are used to maintain extraneous metallic parts at substantially the same potential (usually earth). This conductor does not replace the need for an earth continuity conductor.
How are earth paths created?
The integrity of the earth is maintained throughout the system via screwed conduit joints, joints made through glands and through the use of special bonding cables.
What should be done with all unused cores in a multicore instrument cable?
All unused cores should be terminated and earthed as indicated in the documentation.
How are screens earthed?
Where used to preserve signal integrity, the screen is normally earthed at one end only, preferably, but not always, at the safe area earth terminating point. There are exceptions. Refer to documentation.
Comment on screens and intermediate junction boxes.
Each and every screen should be through-connected without any earthing of the screens within the box.
Comment on general earth tests.
If there is a requirement for earth loop impedance and other earthing tests to be carried out, these tests should be defined by the appropriate superior technical authority.
