CBUS with Rail Mounted Terminal Fuses

Discussion in 'C-Bus Wired Hardware' started by theboyg, Jan 26, 2006.

  1. theboyg

    theboyg

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    theboyg, Jan 26, 2006
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  2. theboyg

    JohnC

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    You would want to check very carefully the "blow characteristics" of such devices.

    It is my understanding that a fuse has slower / different protection criteria compared to MCBs - in other words they won't offer the Triacs in dimmers any protection if they take too long to blow upon a fault condition.

    Given that, I doubt that Fast-Blow (A/B-Curve) or Slower-blow (C/D-curve) MCB's offer any protection whatsoever against overload of the Triacs either, so I guess it's better to only lose 1/2 a din-module and 1/5 the price per channel, since neither practice will really do anything much anyway :confused:

    ... but I will remain open minded (but unconvinced) on the use of MCB's or Fuses on each output until I see some proper technical study and report on it, rather than the circumstantial evidence & rumours I've read so far :)

    My 2c only, and perhaps misguided... JC
     
    JohnC, Jan 31, 2006
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  3. theboyg

    Josh

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    In your opinion JohnC, what will happen if you connet a 2A (or more) light bulb on one of the 1A dimmer channels?
     
    Josh, Feb 1, 2006
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  4. theboyg

    JohnC

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    MCB on dimmer outputs - do they really protect anyway ?

    Hi Josh - this has always been a controversial issue... but be aware that it is not common practise to bother with MCBs (on Cbus outputs) in countries other than the UK.

    A 1A load @ 240V is 240W, and I doubt anyone will use a higher wattage (single) lamp than that (if you could buy one). Of course there is the possibility that there will be multiple lamps in the circuit, and a home-owner plugs in higher wattage lamps at a later time.

    The Control System should be designed to suit the application, with sufficient "headroom" to allow for future requirements. The first stage of system design is to work out the maximum loads on each "circuit", and then select a Cbus Output Device to suit that load....

    So, if the load is 3 x 50w LV Downlights - including (wire-wound) transformers we have 3x65W = 195W. At 240V that is 0.8A so that can go on a 1A dimmer and still have some headroom. However, if the "circuit" is 4 x 50W LV, we have 260W which requires a 2A dimmer channel.

    Cbus is designed as a hard-wired system, it's not "plug-and-play" on the output side - and since the wiring (of the house) is performed in accordance with those initial plans there is no opportunity to "overload" a circuit since you cannot physically plug in *more* lamps anyway.

    If it was a chandelier with 6 bulbs, then you need to allow for 6x60w = 360W (with E14 or B15 base lamps) as that is the maximum wattage available in those lamps. It is pointless and irresponsible to design a system that is running "right-on-the-edge" of it's capabilities because there is no control over what the homeowner does some time down the track... As someone wrote about Structured Wiring, you have to design for what is required in the FUTURE, not what is (not) there at the moment !

    --------------------

    More interestingly, have you compared the prices for 1A and 2A Cbus Dimmers? In Australia, the Retail prices at at Aug '05 are :

    L5508D1AP (8x1A) = $946.55
    vs
    2 x L5504D2AP (4x2A) @ 451.80 = $903.60

    A similar situation for a unit with P/supply
    L5508D1A = $1029.53
    vs
    1x 5504D2A + 1x 5504D2AP = $1030.30

    So, there is no advantage in using 1A devices anyway - we never use them because it's a LOT easier just to use all 2A and then connect almost any (reasonable sized) load to them. If we need more than 2A (very very rare on any decently designed lighting scheme), then we split the load across 2 channels with the same GA, or if there's a lot of them we use Pro Dimmers.

    With the added complication of individual MCB's on each 1A channel, you end up using a similar amount of Switchboard Modules, and there's all that extra wiring and all that extra cost... I can't see the point, it's heaps easier and cheaper just to use 2A devices !

    --------------------

    Oh, and on the subject of MCBs - let's say that you did use a 1A MCB on that 1A output channel. The response curves of MCBs is such that they do not trip immediately at 1A, but much higher.

    MCBs are primarily designed to protect the wiring of the premises, which is considerably larger diameter than actually needed to carry the loads involved. Ever seen 15A fuse wire - it's a LOT smaller than the 2.5mm sq cable used for the mains it protects !

    Also, the MCBs must have the ability to cope with the inrush current, both from lighting loads and motors like fridges, air-cond, etc. So there is a "delay", which is in proportion to the amount of overload presented to them...

    The most common MCBs are C-curve and D-curve characteristics. At less than about 7x the Rated Tripping Current, these curves have the same characteristics. The tripping times are approximately :
    @ 2 x Current Rating = 30 - 90 seconds
    @ 3 x Rating = 6 - 20 sec
    @ 5 x Rating = 1.5 - 4 sec

    So, in the case of a 2A load being connected to a 1A dimmer protected by a 1A MCB... the question is whether the dimmer's Triac would survive between 30 and 90 seconds of overload before the MCB tripped out ??? :confused:

    I reckon that if the 1A Cbus units truly were as delicate as some UK people think, then a 1A MCB would offer no protection whatsoever against an overload of the magnitude used in this example (2A on a 1A dimmer).

    And, if the system was designed using the cheaper 2A devices in the first place... well, there'd be no need to worry about it anyway :eek:

    I hope that explains my point of view - I have not found anything written by CIS that touches on this subject, and I've seen nothing in any documentation from CIS stating that MCBs are recommended on the outputs. So, here in Aus almost everyone doesn't use them and it doesn't seem to make any difference at all. All the failed dimmers I've had (3 off) have had faults in the control side of things, not on the outputs.

    Cheers, John

    PS: I was on site last week doing some re-programming and found that an electrician had connected 2x200w Halogen Uplights onto a 1A dimmer channel. It had been working fine (I have no idea how long, at least 6 months) although I issued instructions for them to be moved to a spare 2A channel just in case.
     
    JohnC, Feb 1, 2006
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  5. theboyg

    Josh

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    Thanks for your response, I understand your point of view, and I would agree that in your case it does not make sense to use 1A dimmers. But in our case the price difference between 1x L5508D1A and 1x 5504D2A is about AU$ 90.00 and if you combine that with limited din rail space, you are looking at a 1A dimmer as an option.
     
    Last edited by a moderator: Feb 2, 2006
    Josh, Feb 2, 2006
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  6. theboyg

    JohnC

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    LOL, Josh !

    That AU$90 difference in cost means that Clipsal is definitely "fiddling the prices" on one of our markets... hmmm, the question is WHICH ONE? :cool:

    Regarding limited DIN rail space - an 8ch dimmer plus 8xMCBs takes 20 modules, while 2x8ch takes 24... so 4 poles is not really that much, but I guess it does depend a lot on what shape boards are being used. For example 4x12 is ideal for the std modules, while some wider boards are 3x18 which would work well with the MCBs alongside the dimmer.

    But never the less - I still am not convinced that the MCBs actually *DO* anything to protect the dimmers, so if DIN space is truly an issue just delete those and save yourselves a heap of space, cost and wiring time :rolleyes:

    Anyway, it's up to you guys how you choose to install them (and the use of individual MCBs is definitely handy to allow safe disconnection of a channel - that dimmer trickle current still gives a scary jolt)

    Cheers, John
     
    JohnC, Feb 2, 2006
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  7. theboyg

    mikegriff

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    You'e joking
    in the UK List price
    <TABLE class=productListing cellSpacing=0 cellPadding=2 width="100%" border=0><TBODY><TR class=productListing-even><TD class=productListing-data>4-Channel Dimmer, 220V AC, 2A, with power supply </TD><TD class=productListing-data align=right>?387.78 (?455.64 inc VAT)</TD></TR></TBODY></TABLE>
    <TABLE class=productListing cellSpacing=0 cellPadding=2 width="100%" border=0><TBODY><TR class=productListing-even><TD class=productListing-data>8-Channel Dimmer, 220V AC, 1A, with power supply </TD><TD class=productListing-data align=right>?376.02 (?441.82 inc VAT)</TD></TR></TBODY></TABLE>Mike
    :eek:
     
    mikegriff, Feb 2, 2006
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  8. theboyg

    richms

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    Its those UK prices that made me initially write cbus off as being way to expensive, it wasnt till I saw the mention of cbussales on the uk home automation mailing list that I realised that it was actually affordable for me to put it in.

    I feel sorry for the brits since they cant even get it from cbussales over there, and if I had put as much out as them for the dimmer I would be putting fuses, breakers, and personal bodyguards to stop them getting blowed up since the investment is so high
     
    richms, Feb 6, 2006
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  9. theboyg

    JohnC

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    Well, as at today's rates, that's approx AU$890 - which is double the Trade price here in Australia ($420.73) :eek:

    LOL - any one of the Aussies here could make a very nice business out of buying 2A Dimmers locally, and then parallel exporting it to the UK.

    - Ebay anyone? :cool:
     
    JohnC, Mar 6, 2006
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  10. theboyg

    richms

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    I also seem to recall some mention on a mailing list some time ago about some funny fusing requirements for lighting circuits in the UK that meant they had to have a 6A max on there lighting circuits in some cases, if thats the situation then you would need to put the breakers downstream of the din module unless you wanted to be limited to 6A total across all 8 channels.

    The only dimmers I have ever had blow (normal hpm wall ones) have all being driving GU10 spots, so the solution is obvious to _that_ problem.
     
    richms, Mar 7, 2006
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  11. theboyg

    JohnC

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    That may be the case, and I see no issue with downstream fusing to meet Regulatory requirements. The purpose of the fuse / MCB is to protect the wiring within the house, and the use of a 6A MCB on each channel makes good sense in that situation.

    However, using a MCB as a means of protecting a C-Bus Dimmer from the possibility that something might on the off-chance perhaps happen sometime in the future, even though nobody has ever experienced or reported a failure, nor does the manufacturer recommend it - in my opinion, that is madness... :)
     
    JohnC, Mar 7, 2006
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