Is this acceptable under 433.3.1?

bsr

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Current situation is the top diagram below. Using 433.3.1 (ii) is the second diagram OK as the characteristics of the load (2x luminaires each rated at 40W max) mean the conductor is not likely to carry overload current and is adequately protected against fault by the B32? Do I need to check using an adiabatic equation and what do I use as I? If I use a typical PFC of 3kA and t=0.4s then I get A=252mm2 which is clearly wrong!

If it's not OK then to avoid further boxes I was thinking of adding an inline BS1362 5A fuse holder inside the outdoor 2G socket like this:
https://cpc.farnell.com/bulgin/fx28...PING&s_kwcid=AL!5616!3!490819648758!!!network}!559570449253!&gclid=EAIaIQobChMIypDy6LKg-AIVBYBQBh0ipwNmEAQYCCABEgJxyvD_BwE

Any thoughts/suggestions?

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If I use a typical PFC of 3kA ...
In what situations is a PFC of 3 kA "typical"? That implies a Zs of only 0.077 Ω, and that includes the R1+R2 of the circuit as well as the Ze. In domestic installations, I've personally never seen a PFC even as high as 1 kA - but that doesn't prove much :)
... and t=0.4s then I get A=252mm2 which is clearly wrong!
Disconnection time at a current of 3 kA would only be a small number of milliseconds, if that, not 0.4 secs. In any event, you need to look at the tabulations or graphs of I²t provided by the MCB manufacturer, rather than attempting to work itself for yourself ... and, furthermore, as EFLI has said, the fuse would be providing fault protection.
Any thoughts/suggestions?
As has been said, both your diagrams would be fine - although in the second one you don't need 2.5 mm² cable downstream of the FCU - indeed, given Amendment 3, it could be 1.0 mm².

I would add that, before Amd3, there might have been debate about whether the bit of 1 mm² cable in your first diagram was acceptable - which would only have been the case if one argued that the FCU created a 'lighting circuit' (since the minimum for anything other than a lighting circuit was then 1.5 mm²).

Kind Regards, John
 
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I suppose one could put a BS1363 socket on that lighting circuit, without there being the usual moaning? :unsure:
I wouldn't bank on that,, since plugging a vacuum cleaner into the socked might blow the 5A fuse and hence plunge the two LEDs into darkness :)

Kind Regards, John
 
The second diagram does not need the FCU. The plugs that go into the spurred 2 gang socket have fuses.
I agree, as drawn. However if, as I suggested would now be allowed, the second socket were wired with 1.0 mm² cable, then the FCU would (at least, strictly speaking) be required.
Edit: typo corrected - "10 mm" corrected to "1.0 mm² "
 
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The second diagram does not need the FCU. The plugs that go into the spurred 2 gang socket have fuses.
It's not clear that the two sockets will be used for the lights or whether the light cable is connected to the spur.

If plugs to be used then there was no point mentioning the lights; they could be anything.

If an extension to the spur is to be used then, as it is drawn, the FCU, according to the regulations, is needed.
 
It's not clear that the two sockets will be used for the lights or whether the light cable is connected to the spur.

If an extension to the spur is to be used then, as it is drawn, the FCU, according to the regulations, is needed.

The light cable is to be connected to the spur after the sockets, effectively a mini radial. So as I have more than one "thing" on the spur then I thought the FCU is required. In any event it's already there so I just change the current 5A fuse to a 13A one.
 
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Yes, you are correct - even though two 5W lights would actually make no difference to the circuit.
 
@JohnW2 that's really useful thank you. So in my ignorance, I was being overly conservative. As I don't have an earth loop tester I was using the 3kA break rating of the MCBs and the maximum permissible disconnection time of 0.4s.

If I use the actual Zs at the FCU of 0.2 ohm and the quoted disconnection time of 0.1s then I get a much more sensible PFC of 1200A which gives me a minimum CSA of 0.75mm. So ok.

Does that sound better?
 
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You will not be relying on the B32 for fault protection because the fuse is there.

But in reality which would go first? In my very limited experience I would expect the MCB to trip before the fuse blows. Is there any discrimination for faults?
 

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