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Requirements for screens of cables for interference
Posted: Sat Jul 03, 2021 10:48 pm
by JamieP
Just curious around what rules to apply for screens, such as those in screened instruments cables or other similar arrangements when the screen is there for preventing interference
Would such be considered FEs? (Functional earths) They are not used for any protective purposes
The main things I'm curious of is around insulation and colour identification requirements
We often use just green heatshrink of a very small size for the drain/screen wire and I'm just trying to work out obligations in terms of compliance
Re: Requirements for screens of cables for interference
Posted: Mon Jul 05, 2021 9:13 am
by AlecK
Many such cables are not within scope of Wiring Rules; and therefore the requirements for ID of conductor function do not apply.
Where Wiring Rules do apply, then yes any earthing for other than safety purposes is functional earthing.
Definition 1.4.66 applies; but in my view doesn't make the necessary point.
The above description comes from IEC.
Protective earthing conductors and equipotential bonding conductors are required to be identified as green / yellow,
and this colour combination is not permitted for conductor functions active or neutral.
Plain green & plain yellow are also restricted to non-active / non-neutral
No particular colour is set for functional earthing conductors, and the above restrictions [3.8.1] do not exclude use of any of these colours for FECs.
However since an earthing conductor can be serving more than one earthing purpose; care is required of opting to use any colour other than G/Y
Re: Requirements for screens of cables for interference
Posted: Mon Jul 05, 2021 9:53 am
by JamieP
When you say outside the scope of the wiring rules is that due to the fact you wouldn't consider it installation wiring?
Re: Requirements for screens of cables for interference
Posted: Mon Jul 05, 2021 4:54 pm
by AlecK
more that it's not part of the installation in many (most (?) cases.
("installation wiring" being only part of what is within Scope; there's also 'equipment wiring" that can be part of the installation.)
Instrumentation, like fire alarm, burglar alarm, TV, telephone, & data cabling generally aren't part of an "electrical installation" as they deal with communications in various forms rather than carrying energy.
However these days we're getting more grey areas; eg using the CCTV co-ax not only for signal but also to provide power to the camera, including for adjusting aim & zoom and powering an IR illumination source. Similarly power-over ethernet.
The clear distinctions that used to exist are becoming increasingly blurred.
It may well be in future that for some applications two sets of rules apply.