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708 and 721

alanblaby

This came up on another forum yesterday, caravan supply cables.

32 amp caravan supply cable that was 4mm flex on a caravan park which supplies an outlet to each pitch.

708, fig. 708 gives a drawing of the supply, Note 2, page 270 says "..for a higher current, the CSA must be chosen so that secure tripping of the over current protective device is achieved..."  But the note above says refer to 721.55.2.6

So, from that, a 4mm 3 core flex of 25 metres would be acceptable - Page 470, Table 4F3A, albeit, volt drop and R1 + R2 will need to be taken into account, but we are assuming the lead is short enough to satisfy those.(ICBA working that out now, but a rule of thumb says 25m will satisfy all requirements)

Then we have 721.55.2.6 ,and  Table 721, Minimum CSA of flexible cables for caravan connection, which, for 32 amp, says 6mm is required.

Why the confusion on this, between 708 and 721?  Surely they would be better as one chapter on their own, rather than separate chapters which need reference to each other?

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    alanblaby said:
    Why the confusion on this, between 708 and 721?  Surely they would be better as one chapter on their own, rather than separate chapters which need reference to each other?

    There is no confusion (that is, if you know the wording in which standards are written - separate discussion, but BS 7671 has to be written using a specific style to help address things like this). The text in 708 is part of "Note 2" t Fig 708 and is therefore informative text - it contains no requirements and is there purely for information.
     I agree, though, that the text could have been indented so it's clear it's under Note 2 to Fig 708. The other "giveaway" that in 708 the cord set is not a requirement, is the use of the word "should" rather than "shall".

    In Section 721, however, we have  a completely different set of wording, with its own Regulation number:

    721.55.2.6 The means of connection to the caravan pitch socket-outlet shall be supplied with the caravan and shall comprise the following (see Figure 708):
    (i) A plug complying with BS EN 60309-2, and
    (ii) a flexible cable of continuous length 25 m (±2 m), having a harmonized code designation of H05RN-F or H07RN-F (BS EN 50525-2-21) or equivalent, incorporating a protective conductor, with conductors to be identified according to Table 51 and of a cross-sectional area in accordance with Table 721, and
    (iii) a connector, if any, compatible with the appliance inlet installed under Regulation 721.55.1.

    Here, the word shall is used (highlighted above), and therefore this is a requirement. So, the only requirement for the caravan connecting cable set is in Regulation 721.55.2.6, and that is what is to be used. Which makes perfect sense because the connecting means also has to be provided with the caravan (rather than as part of the pitch).

    I will forward on the formatting comment regarding indentation of the text that is part of NOTE 2.

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    alanblaby said:
    Why the confusion on this, between 708 and 721?  Surely they would be better as one chapter on their own, rather than separate chapters which need reference to each other?

    There is no confusion (that is, if you know the wording in which standards are written - separate discussion, but BS 7671 has to be written using a specific style to help address things like this). The text in 708 is part of "Note 2" t Fig 708 and is therefore informative text - it contains no requirements and is there purely for information.
     I agree, though, that the text could have been indented so it's clear it's under Note 2 to Fig 708. The other "giveaway" that in 708 the cord set is not a requirement, is the use of the word "should" rather than "shall".

    In Section 721, however, we have  a completely different set of wording, with its own Regulation number:

    721.55.2.6 The means of connection to the caravan pitch socket-outlet shall be supplied with the caravan and shall comprise the following (see Figure 708):
    (i) A plug complying with BS EN 60309-2, and
    (ii) a flexible cable of continuous length 25 m (±2 m), having a harmonized code designation of H05RN-F or H07RN-F (BS EN 50525-2-21) or equivalent, incorporating a protective conductor, with conductors to be identified according to Table 51 and of a cross-sectional area in accordance with Table 721, and
    (iii) a connector, if any, compatible with the appliance inlet installed under Regulation 721.55.1.

    Here, the word shall is used (highlighted above), and therefore this is a requirement. So, the only requirement for the caravan connecting cable set is in Regulation 721.55.2.6, and that is what is to be used. Which makes perfect sense because the connecting means also has to be provided with the caravan (rather than as part of the pitch).

    I will forward on the formatting comment regarding indentation of the text that is part of NOTE 2.

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