Control, signal and alarm wiring often has to follow the same route as power cables. SS 638 clause 528.1 sets out when the two may share a conduit, trunking, tray or cable. In short: not by default, but yes where one of six permitted arrangements is used, and other requirements (SELV/PELV separation, safety services, telecommunications and interference) may still govern the installation. This article summarises the clause, referring to SS 638, and what it means for your choice of cable.
Band I and Band II
Clause 528 is written in terms of voltage bands, as defined in SS 638 (which follows IEC 60449). In broad terms:
| Band | In broad terms | Typical circuits |
|---|---|---|
| Band I | The extra-low voltage range (up to 50 V AC or 120 V ripple-free DC), used where the low voltage itself gives protection against electric shock, or is limited for operational reasons | Control, signalling, bells, intercom, sensors, 24 V DC field wiring |
| Band II | Low voltage above Band I, up to 1000 V AC | 230/400 V power, lighting and socket circuits |
Check the definitions in SS 638 itself where a circuit is borderline.
What clause 528.1.1 requires
Referring to SS 638 clause 528.1.1, a Band I circuit and a Band II circuit should not be contained in the same wiring system (the same conduit, trunking, tray or multicore cable), and neither band should share a wiring system with a circuit above low voltage, unless one of the following methods is used.
The six ways to comply
| Method | In practice |
|---|---|
| 1. Insulate every cable for the highest voltage present | Run the ELV circuits in cable whose insulation is suitable for the highest voltage in the shared containment |
| 2. Insulate every core of a multicore cable for the highest voltage in it | One multicore cable carrying both mains and ELV cores, every core insulated for the mains voltage |
| 3. Use a separate compartment | Each circuit insulated for its own voltage, in its own compartment of a multi-compartment trunking or duct |
| 4. Partition the cable tray | A physical divider on the tray between the ELV and power runs |
| 5. Use separate containment | A separate conduit, trunking or duct for the ELV wiring |
| 6. Screen the cores inside a multicore cable | Band I cores separated from Band II cores by an earthed metal screen able to carry as much current as the largest Band II core |
SELV and PELV circuits
If a 24 V circuit is intended to be SELV or PELV, its protective separation from other circuits is governed by SS 638 clause 414.4. A suitable cable voltage rating on its own may not be enough to share containment with power cables, so check the 414.4 requirements first.
Choosing cable for shared containment
Method 1 may allow an ELV control cable to share containment with power cables where the control cable's insulation is suitable for the highest voltage present. For example, on a 230/400 V system, a 300/500 V control cable may satisfy this requirement, subject to the cable's applicable standard, construction and installation conditions (see what U0/U means).
- YSLY control cable is rated 300/500 V, so it may be suitable under method 1 alongside 230/400 V power cables, subject to those conditions.
- LiYCY is rated 250/250 V, which does not cover the 400 V between phases of a 230/400 V supply, and UL2464 is also a lower-rated signal and data cable. Run them in a separate compartment, behind a tray partition or in separate containment (methods 3 to 5).
Always check the rating and construction on the datasheet against the highest voltage present in the shared containment.
Safe is not the same as interference-free
Clause 528.1 is about electrical safety: stopping mains voltage from reaching ELV wiring. It does not stop power cables inducing noise into signal circuits. For analogue signals, encoders and data lines, keep distance from power cables where you can, and use a screened cable such as LiYCY, with its tinned copper braid earthed as the equipment maker specifies.
Circuits that need their own segregation
Referring to SS 638 clause 528.1.2, some circuits follow other codes, whatever their voltage:
- Fire alarm and emergency lighting must be segregated from all other cables and from each other, in accordance with SS CP 10 and the SS 563 series.
- Telecommunication circuits are segregated in accordance with the Code of Practice for Info-communication Facilities in Buildings.
This article is a summary. Refer to SS 638 itself for the full requirements, and have a Licensed Electrical Worker (LEW) confirm the arrangement for your installation.
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