"it is that pesky SY braided stuff again; so what's wrong with it"

Question

More a discussion come question perhaps, but are there any really good reasons why a choice of SY would be made for a fixed wire low voltage power circuit over something else ? 
 
 My understanding of the braiding, is that it has nothing to do with armouring and is not acceptable as armouring; but is it still an exposed conductive part to be earthed and if it is not [earthed], how much of a risk ? 
 
 from reading Eland (who have a pdf on the SY stuff) to mention one manufacturer, is that it is a somewhat standards tested cable ( https://www.elandcables.com/media/13rlt2rk/ec-statement-on-the-use-of-sy-cy-yy-cables-rina-18th-ed-with-summary.pdf ) 
 
 In the case observed being a 3phs circuit, it was 5core (one as cpc) from a metal db (through plastic stuffing gland) to a plastic interlock socket, 16a 3pole mcb and it appeared the braiding was unterminated. it was not on a run likely to be disturbed. Nothing noted on the EIC about its use either.

regards 
 Habs

gkenyon · Answer

SY, CY and YY cable is used extensively in Machinery as defined in the Supply of Machinery (Safety) Regulations, and also for interconnecting parts of machinery and its control systems. This is the cable's intended use, and with CY and SY types, the braid is there to help with EMC issues (e.g. VSDs or control circuits sensitive to EMI from associated power cables). CY and SY provide a means of extending the electromagnetic shielding of a metal enclosure, through the braiding, to another metallic enclosure, and this is achieved using 360-degree bonding glands specifically made for these cable types. 
 Where the cable is part of Machinery as defined, BS EN IEC 60204-1 applies, and this is outside the scope of BS 7671, see Regulation 110.2 (xi). SY, CY and YY cables are generally safe to use provided that: 
 
 The cable is installed in an application and environment for which it was intended. 
 
 Not usually suitable for outdoor applications 
 Braid of CY and SY should NOT be considered to provided additional mechanical protection as required by BS 7671 for various circumstances (coverage of SY braid may not be suitable to act as cpc for mechanical protection against penetration by fixings etc.) 
 A number of types of SY, CY and YY cables are auxiliary cables (control cables) - these must not be used for power applications. 
 Flexible conductors terminated suitably (i.e. if connected via terminals marked 'sol' or 'r', appropriate ferrules used).

The cable does not exceed voltage and current ratings in the installation conditions. 
 A copper (CY) or steel (SY) braid is not used as a CPC on its own (internal CPC muse be used), but also the braid should be appropriately connected to a means of earthing or bonding at least at the supply end via an appropriate gland. 
 The cable has a constructional specification provided by the manufacturer, preferably verified by an independent test body (such as BASEC). 
 Where conformity for the installation to BS 7671 is claimed:
 
 information required by Regulation 511.2 is recorded on the EIC; and 
 it is recommended evidence is kept on file, demonstrating 'at least the same degree of safety as that afforded by compliance with BS 7671' as required by Regulation 511.2

Guidance along these lines has been provided in the On-Site Guide for many years (see Section 7.9, Page 100, of the 2022 Edition). 
 Usually, there is a suitable alternative for circuits within the scope of BS 7671, so I'm not sure why the discussion keeps going round and round to be honest.

"it is that pesky SY braided stuff again; so what's wrong with it"

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