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Installation of Single Core double insulated cables on wire basket

Calum Doherty

Hi all, 

Does anyone know why there are no current-carrying capacities listed for cables below 25mm² for Reference method F in Table 4E1A?

I can only assume it is related to mechanical protection, however cannot find a regulation stipulating this.

The closest I can find is reg 522.6.1 - ' Wiring systems shall be selected and erected so as to minimize the damage arising from mechanical stress, e.g. by impact, abrasion, penetration, tension or compression during installation, use or maintenance' or Reg 522.8.4 - ' Where conductors or cables are not supported continuously due to the method of installation, they shall be supported by suitable means at appropriate intervals in such a manner that the conductors or cables do not suffer damage by their own weight.' however none of these specify a minimum cable size.

The reason I'm interested is I'm designing -48Vdc systems where the prospective fault level of the batteries can be relatively large, typically 6-8kA at source but for some applications up to 18kA.  The charger systems are feeding telecoms racks which occasionally have dual fed PDUs, however the maximum rated short circuit capacity of the circuit breakers is 1kA which can easily be exceeded in these systems.  One solution would be to reduce the cable CSA below 25mm² hence reducing the prospective fault level below 1kA at the load end.  However, I'm unsure if this would be acceptable as no CCC is provided.  I have suggested using the CCC for Reference method C, as the airflow and thermal dissipation around the cables installed in basket would theoretically increase the CCC above the Ref. C value. 

The installation is within a controlled environment where only trained personnel are permitted un supervised; hence the risk of mechanical damage is minimal.  I've suggested that the reduced cable size is the pragmatic solution based on the trade off between risk of mechanical damage vs. failure of the MCBs in the event of  said damage.

Any insights or recommendations would be appreciated. 

Thanks, 

 

Calum Doherty

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    I have suggested using the CCC for Reference method C, as the airflow and thermal dissipation around the cables installed in basket would theoretically increase the CCC above the Ref. C value

    Is the sort of approach that is often required - it is not reasonable that the (informative only ; not normative) annex at the back of the regs perfectly describes all possible configurations of cable type mounting and grouping, and in any case - especially for things like ladder or tray, it is also affected by choice of vertical or horizontal direction, which is not usually mentioned, and the obstruction or pre-heating of the airflow by things above or below, which is hard to assess or control. 

    Given that the ratings are all the results of hours of instrumented tests under very specific conditions, and perhaps a bit of interpolation, and then some safety factors, it is expensive to create extra tables and may not necessarily add much to the overall understanding.

    Keep your wits about you and realize that the heat per unit length for a given cable size goes more or less linearly with resistance, so rounding to the next size up if in doubt, usually means things run very cool. Also that it also goes as the square of current, so a similar small increase in current is more serious. (a 10% rise in current needs almost a 20% rise in cross-section or reduction in duty cycle to compensate.)

    The approach however is correct, and generally safe.

    Mike.

      

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