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How to supply a 20A Distribution cct?

Farmboy
Sorry if this is too simplistic a question but I'm wondering what you think, given I've had conflicting views from a trade assoc tech dept and Hager tech dept.


A 65m, SP+N, 20A (estimated, but could be subject to some increase) distribution cct, fed from within a commercial setting (an osteopathy/acupuncture service provided in a converted ground floor flat), is to supply a shed/summerhouse with a couple of sockets (for a kettle and TV) and a couple of LED bulkheads (via cb's in the shed CU), sited at the end of the garden. How would you supply the distr. cct at the meter position?

- From a henley block in the existing tails:

              - Its own modular enclosure containing DIN rail mounted main switch and 20A fuse carrier.

              - A rotary handled 20A fused switch disconnector.

- From a 20A cb in the existing CU.


From the trade assoc., one said from the CU, another said it's got to have its own main sw, another said no problem with the modular encl and fuse carrier. While Hager said its got to be the rotary type, not the fuse carrier.


F
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    It's not a lighting-only circuit, so the larger volt drop figure applies.



    But v.d. requirement is from the origin of the installation to the light, not just the final circuit. So to achieve 3% overall and allow say 0.5% for the final lighting circuit, you'd only have 2.5% to play with for the distribution circuit - rather than 5%. (Or specify in your design that only luminaries with a wide voltage tolerance are to be fitted ?)


    As for discrimination between MCBs and a 20A fuse - it's the MCB manufacturer's data you'd need for that (and depends on the maximum fault level involved) - but I would suspect 20A would be too low to be useful - e.g. this MEM data (https://www.tradingdepot.co.uk/info/wp-content/uploads/2013/02/the-guide-cat865-circuit-protection-control.pdf) would suggest that a 20A fuse will only discriminate with an MCB up to 6A and then only for fault currents up to 160A. For a 16A MCB and a decent fault level you'd be looking at something closer to a 63A fuse to ensure discrimination.


    I don't think there's much mileage in using a C type MCB for the submain rather than a B-type - C types aren't slower as such - they simply don't respond (at all) to smaller fault currents (below 5-10x rather than 3-5x) if the fault level is above the MCB's (magnetic) threshold then it'll practically trip just a quickly as a B type - and if your Zs values are within limits then the fault currents will be high enough to trip it. D types just might work where you can use a 5s disconnection time - as unlike B or C types, they'll trip within 5s on the thermal mechanism for a small range of fault currents - but it would mean some very careful arrangement of circuit impedances (so a ransom to DNO changes for instance).


    I guess it's really down to how much discrimination (selectivity) the customer really requires - if none at all, then you could just run a 20A circuit for the sockets and run the lights off that with a 5A fuse in a FCU (or just run a 16A circuit and forget the FCU). If full discrimination is really needed then either a largish fuse on the submain (perhaps relying on the downstream MCBs for overload protection), or simply run two separate final circuits instead of one submain (could even be a single 4-core SWA for a truly minimal solution).


       - Andy.
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  • 0

    It's not a lighting-only circuit, so the larger volt drop figure applies.



    But v.d. requirement is from the origin of the installation to the light, not just the final circuit. So to achieve 3% overall and allow say 0.5% for the final lighting circuit, you'd only have 2.5% to play with for the distribution circuit - rather than 5%. (Or specify in your design that only luminaries with a wide voltage tolerance are to be fitted ?)


    As for discrimination between MCBs and a 20A fuse - it's the MCB manufacturer's data you'd need for that (and depends on the maximum fault level involved) - but I would suspect 20A would be too low to be useful - e.g. this MEM data (https://www.tradingdepot.co.uk/info/wp-content/uploads/2013/02/the-guide-cat865-circuit-protection-control.pdf) would suggest that a 20A fuse will only discriminate with an MCB up to 6A and then only for fault currents up to 160A. For a 16A MCB and a decent fault level you'd be looking at something closer to a 63A fuse to ensure discrimination.


    I don't think there's much mileage in using a C type MCB for the submain rather than a B-type - C types aren't slower as such - they simply don't respond (at all) to smaller fault currents (below 5-10x rather than 3-5x) if the fault level is above the MCB's (magnetic) threshold then it'll practically trip just a quickly as a B type - and if your Zs values are within limits then the fault currents will be high enough to trip it. D types just might work where you can use a 5s disconnection time - as unlike B or C types, they'll trip within 5s on the thermal mechanism for a small range of fault currents - but it would mean some very careful arrangement of circuit impedances (so a ransom to DNO changes for instance).


    I guess it's really down to how much discrimination (selectivity) the customer really requires - if none at all, then you could just run a 20A circuit for the sockets and run the lights off that with a 5A fuse in a FCU (or just run a 16A circuit and forget the FCU). If full discrimination is really needed then either a largish fuse on the submain (perhaps relying on the downstream MCBs for overload protection), or simply run two separate final circuits instead of one submain (could even be a single 4-core SWA for a truly minimal solution).


       - Andy.
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