Several types of products, commonly found in homes and industry, are fitted with integrally moulded plugs at the end of a length of flexible electric cable, referred to as “cabtyre”. These products typically include:
- Cord Sets (Plug and cord)
- Interconnection Cord Set (Plug, cord and connector)
- Cord Extension Sets (Plug, cord and socket outlet)
- Extension reels. (Plug, cord on a reel assembly and socket outlet).
What are the Legal requirements:
South Africa has Compulsory Specifications, issued by the National Regulator for Compulsory Specifications (NRCS), for many electrical products which specify the technical and administrative requirements in order for these (compulsory listed) products to be sold legally in our market. The purpose of these regulations is aimed at the protection of consumers, ordinary people, in their homes and places of work.
In the case of the products listed above, the specification is:
VC8029 Compulsory specification for cord sets, interconnection cord sets, and cord extension sets. (Gazetted: 3 Feb 2012 Nr 34983 – R 69 – In terms of the NRCS Act 5 of 2008)
Which lists the following standards to be complied with:
- SANS 60799: Cord sets and interconnection cord sets.
- SANS 1661: Cord extension sets (which includes extension reels)
These standards also list the applicable standards for components i.e.:
|SANS 164-0: Plug and socket-outlet systems for household and similar purposes for use in South Africa|
|SANS 1574-2: Electric flexible cables with solid extruded dielectric insulation – Part 2: PVC insulated flexible cables for domestic, office and similar environments (cords).|
|SANS 60320: Appliance couplers for household and similar general purposes.|
What do the specifications say about the cables?
In all the above specifications, they mention that the cable standard shall be either SANS 1574-2 or its IEC equivalent SANS(IEC) 60277 and specifically the composition of the cores (current carrying thin wires making up each core) shall be annealed copper, for class 5 types of cables, which are deemed “flexible”
Why is this important?
The desireable characteristics of the cabtyre is twofold:
- It must have low resistance and
- It must withstand flexing operations.
Cable resistance is stated in the resistivity of the conductors where:
Copper 1.0 mm2 section is 19.5 W/m, whereas Aluminium is around 40 W/m (depending on the Aluminium alloy)
Which means that at the rated current of 10A the heating effect on the Aluminium cabtyre will be 2.5 times higher.
Quick and easy test for Aluminium conductors:
The proper method of determining the material composition of conductors would require an appropriate metallurgical and chemical analysis, however there is a “quick-and-easy” method that immediately produces a clue as to whether or not the material is copper.
Each core of the 2 or 3 core cabtyre is stripped of its PVC covering in order to expose roughly 20mm of the bare metal strands.
The strands are fanned out and a flame (match or gas lighter) is brought under the fanned strands.
COPPER will glow bright red but remain in their fanned position
ALUMINIUM strands will quickly collapse under the flame.
Flexing test (not quick or easy!)
Since Aluminium has a Tensile Strength 38% that of Copper, the flexural performance of cabtyre made up with Aluminium strands will inevitably be less robust than a cabtyre made up with Copper strands. This is recognized in all standards as for example IEC 60228: Conductors of insulated cables; Under Class 5: Flexible conductors; Clause 6.1 states:
- a) Flexible conductors (class 5 and 6) shall consist of plain or metal-coated annealed copper.
Cable flex testing is described under SANS 60884-1; Clause 23.4 where a sample of a non-rewireable plug connected to a PVC cable is tested in an apparatus as follows:
|The flexing apparatus clamps the plug in such a way that the cable is flexed through an arc of 90o whilst the free end of the cable is loaded with a mass of 20N (2 kg) for a cable size ³ 0.75 mm2.
The plug circuit is connected to a low voltage supply and a small current is passed through all the conductors in order to measure continuity during the flexing test. If there is a break in continuity the test is stopped, and the sample deemed to have FAILED
The test is conducted for 10,000 oscillation at the rate of 60 flexings per minute.
Copper 1.0 mm2 section – withstands this without any difficulty
Aluminium – has a high failure rate where the core strands shear off, either in part or in full.
Which means that the aluminium cabtyre can either open circuit or progressively heat up to the point of burning.
There is no doubt that the use Aluminium conductors in cabtyre is dangerous and should be avoided at all costs. The notion that this is a “cost-saving” initiative is invalid as it must be offset against a significantly reduced safety levels of the electrical products mentioned.