Screen and Drain Earthing
Find where to earth a screen, drain wire or armour.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
Where is a cable screen earthed?
AS/NZS 3000:2018 Clause 5.5.3.2(b) earths a screen, armour or sheath at the end next to the switchboard or accessory at which the cable originates. That is one earthing point. The standard names no both-ends arrangement, so the tool gives none.
Does earthing at one end stop a hum loop?
Yes. With one earthing point there is no closed path through the screen, so no current circulates in it and no hum loop forms. Earthing a screen at two points creates that path.
When does a screen not need earthing?
Clause 5.5.3.2(b) gives two exceptions. The first covers a screen or braid held apart from the live conductors by double insulation, under a non-conductive covering, with joints and terminations kept clear of live and exposed conductive parts. The second covers any case Clause 5.4.1 leaves out.
Can the screen be the earthing conductor?
Clause 5.3.2.2(ii) allows a conductive sheath, armour or screen as a protective earthing conductor. Clause 5.3.2.3(b) adds the condition that the equipment is supplied only by live conductors in that cable. Clause 5.5.4.3 calls for continuity and the impedance of a copper earthing conductor.
Where does the screen construction advice come from?
From HELUKABEL and LAPP catalogues. That is manufacturer data, not a standard, and the tool labels it as such. A braid acts mainly against low-frequency interference and a foil against high-frequency interference. The catalogues cover termination hardware. Neither they nor AS/NZS 3000 back a both-ends arrangement.