I was 'assuming' or 'presuming' that all devices are rated to allow for the fusing factor of the likely protective device.
1. Lighting circuit safe area, does it need RCD? 2. RCD maximum current smaller than combined rings
- Thread starter TofuSpaceship
- Start date
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Yes, I realise that you were, but that assuming/presuming is again getting very close to applying common sense to something (a regulation) which doesn't necessarily correspond with common sense!
In this case, I'm not even sure that your assumptions/presumptions are necessarily correct, because it's not quite the same as 'cable protection', when what happens is that the cable gradually warms up. We are discussing a situation in which something (like the switch breaking a current flow) could happen 'very suddenly'. What answer do you think you would get if you asked the manufacturer whether a "100A Main Switch" could safely break a current of 200A (even though the fuse would prevent such a current flowing for moire than about 1 hour)?
Kind Regards, John
Very unlikely, yes, but again we are talking about a regulation (and it wasn't me who started this!), not common sense.
Kind Regards, John
I would say it should - or must - just like MCBs do, otherwise the 100A switch will have to be protected by a 50A fuse. Such situations would quickly become ridiculous.
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Where low voltage is mains 240v not your 8v doorbell which is extra low voltage?
Where is this requirement you are postulating for switches (or anything else other than a cable) to be protected by an OPD (of any rating)?
As I said in my last post, protection of cables by an OPD (for which there is a 'requirement') is a different, and specific (to cable), matter
Kind Regards, John
I thought that was what Flameport posted.
One wouldn't have a light switch on a cooker circuit.
One wouldn't have a light switch on a cooker circuit.
You're right - it was such a new concept to me that the possible implications didn't sink in!
So, if I understand that reg correctly (and I have to say that I find it quite hard to read and understand), the so-often-cited principle that "the OPD is only there to protect the cable" goes out of the window, and we are presumably back to having to tell people that they mustn't connect 20A switches to 32A circuits etc. ?
Kind Regards, John
The question I have is why they make 40A rcds for the 2 way CUs.
We know the answer. The wires thru the torrod can be much smaller. And therefore cheaper and easier to make
We know the answer. The wires thru the torrod can be much smaller. And therefore cheaper and easier to make
I must say that I didn't know that they did.
As EFLI said, I would imagine that the difference in manufacturing costs would be so small that they might as well make them all 100A.
Kind Regards, John
Well, not really. Perhaps circuit rather than just cable, although that is how circuit design is done, isn't it? Load, appropriate OPD, appropriate cable.
Not just switches, but junction boxes and other connectors - and - what else? Must be something else.
I don't see why.
It's surely only the load current through the switch - otherwise you can't have sockets.
Yes, but that reg appears to be talking about OPD protection for a switch.
Well, for a start, there are 'fault conditions' (which we obviously do have to take into consideration when an OPD is protecting a cable) - in which "the current through the switch" could be excessive?
I have to say that my greatest surprise is that I have never 'noticed' that reg before!
Kind Regards, John
I am getting confused.
If the switch has a current rating ≥ the cable then it will protect the switch, will it not?
Edit - I know 20A is less than 32. I said I was confused but it's the same as a spur cable isn't it? ≥ to its load.
...but all switches are subject to that.
If they get cannot cope and get destroyed or welded shut then so be it.
Neither have I - but is it not, apart from the odd wording, just common sense.
We are back to a 6A switch for a cooker. No one with any knowledge would do that.
Is it not like JBR's dimmer with one blown lamp?
No one has said he should have had a 1kA (or more) switch.
Edit - typo corrected - 1kA
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It reminds me of this post...... SO what he right ? 
https://www.diynot.com/diy/threads/demand.526820/#post-4419715
https://www.diynot.com/diy/threads/demand.526820/#post-4419715
Yes, it would provided that the 'over-current tolerance' of the switch were the same as is deemed to be the case for a cable (e.g. able to happily carry 1.45 times its Iz for an hour). However, as you go on to say, we're actually talking about "a switch which has a current rating < the cable" . As you say ..
Are you talking about a 'spur cable' with downstream OPD protection (as in plug fuses etc.)?
Not if they were treated like cables - i.e. were required to have over-current protection which would prevent a high current flowing for long enough to cause damage to them.
In many ways, it is (common sense). As you know (from some of my talk about 1A BS1362 fuses
) what I have thought deviates a bit from common sense is the concept/belief that over-current protection "is only there to protect cables" - why not everything else in the circuit (which I suppose is probably what this reg, previously unfamiliar to both of us, is trying to say)?Kind Regards, John
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