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Thread: Am I going a bit OTT?

  1. #1
    Automated Home Guru Nad's Avatar
    Join Date
    Jul 2008

    Default Am I going a bit OTT?

    Hi all,

    First off I'd like to point out that I'm a total control freak so I need to know how, what, where and why for everything. I tend to have detailed plans for everything that needs to be done so as you would expect I've been figuring out how many modules I'm going to need and how many CU housing I'll need to put it all in. I have to admit, I didn't think much of the final figure while i was doing the working out but it came to around 3 QRHs (All power sockets), 20 QRIs (Mix of lights, and circuits requiring a on/off type of control) and 10 QLDs (All lights).

    The way I've designed the layout has them all requiring 14 22 way MK consumer unit enclosures which did bring about a WTF! kind of response from my builder. The following is some of the reasoning i had in my head when i was doing the design:

    • Each enclosure is to have a master switch to power it down.
    • Multiple enclosures will be fed by outputs from the main CU in groups which would allow 1/2 of the lights to remain operational in the event of a main MCB tripping out.
    • Each lighting circuit is to have a 3A MCB between the load and the Idratek module (housed in the CU) so I don't have to mess about wit the little 4A fuses in the module and I can power each circuit off individually if required.
    • Each QLD output is limited to 4 LED bulbs (Philips 5W Master) each (3A MCB in CU on load side).
    • All QRH feeds to have a 13A MCB between Idratek module and load (housed in the CU).
    • Using QRIs to control power to the radiator valves, curtain switches, window openers, etc

    So now I'm wondering if I got a little too carried away and if there are more economical ways in order to do this. So please feel free to let me know your thoughts


  2. #2
    Automated Home Legend Karam's Avatar
    Join Date
    Mar 2005


    I understand what you mean about the pesky little fuses, but my opinion is that it probably is overkill to have an additional 3A MCB between each load and output. In the past we overcautiously used to use fast fuses but now have moved to time lag ones and these are better able to handle events like incandescent bulb failures which tended to be the main 'regular' fuse breaking events. What this means is that fuse replacement is likely to be much rarer. Of course you do need MCBs to protect the overall rings/feeds and I'd suggest RCBO's rather than just MCBs. Another note here is that although our over cautiousness has led us to rate QRH's at 13A/ch simply because we wanted to give a margin to 16A rated relays and saw applications being at most a socket's worth. In fact nothing else internally (such as tracking) prevents the use up to the actual relay rating of 16A. I suppose if you wanted to be able to get up to the standard ring 32A aggregate output from any one QRH fthen you will need MCBs on each output, but if you could live with 16A aggregate then only one MCB per QRH feed.

    Another thing you should consider is the use of QTIs in place of some QRIs if you are switching light AC loads and in particular for things like CFL or LED lamps. The QTIs are lower current rated but have zero crossing triac switching meaning they are less noisy (both electrical and mechanical) and will not suffer contact spot welding if loads have a high inrush current characteristic (CFL, LED, SMPS).

    One downside of QTI's (and QLDs) is they generate a bit more heat than QRIs/QRHs if working near the current limits across all channels simultaneously, so would need to avoid packing loads into a badly ventilated tight enclosure. Where people go for the mass DIN style configuration they tend to use larger metal cabinet type enclosures which may ultimately look neater from the outside and easier to wire on the inside if less segregated. I think domestic electricians in the UK are probably not so used to this kind of structure as they might be on the continent. Another possibility is a distributed structure of smaller enclosures or collections of CUs, e.g per floor.

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