Discuss 4mm cable into the bottom of main switch. in the Electrical Forum area at ElectriciansForums.net

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The problem is, you can't rely on the 20a rcbo to protect the 4mm. What I mean by this is, future alterations, who's to say someone won't split that 4mm and feed something another db etc. There is nothing to protect that 4mm from overload.

It's the same as having a 1.5mm t+e for a lighting circuit protected by a 45a but at fused down to 6a at the first luminaire.
 
Assuming the main switch is a 100a/60a main switch then the 4mm is not suitable protected for over current and fault protection.

It's a c3 for me
It's a Bolls up for me
 
Under fault conditions what is the maximum current/time a 4mm t+e can withstand?
What size fuse is protecting the 4mm cable from L to N & L to E faults?
 
you can't rely on the 20a rcbo to protect the 4mm. What I mean by this is, future alterations,

It is not necessary or possible to protect an installation against incorrect future modifications. All systems can be modified so that they become dangerous.

It's the same as having a 1.5mm t+e for a lighting circuit protected by a 45a but at fused down to 6a at the first luminaire.

Not quite the same. Top of head without checking the numbers, think the 1.0mm² CPC would not be protected against fault current by a 45A fuse, whereas I think the 4mm² is protected by the cutout as would be any other tails if the CPC is 4mm² too, but if it's T+E, would have to look it up, sounds doubtful as mentioned above.

But the crux is 433.2.2 which allows OPD to be located somewhere other than at the source of the circuit, so long as certain criteria are met to reduce the probability of a fault on the circuit up to where the OPD is. It may be the case there that those criteria are met, whereas I don't think they would be on your hypothetical lighting circuit.

E2A:
What do you mean by in the bottom of the main switch.
what goes into the top?

Agreed, we should refer to supply side and load side. Those of us who spend most of our time with TP+N DB's are used to putting the supply into the botttom of the switch.
 
Last edited:
Hi - in order to make use of 433.2.2 as they’ve done I think they need to install the cable with specific protection against impact (say in metal conduit). Which I’m guessing won’t have been done. Reg 433.2.2 refers us to 434 and then 434.2.1 (ii) .
 
If the enclosure it supplies is metal the cpc must be able to withstand fault current.
 
Hi - in order to make use of 433.2.2 as they’ve done I think they need to install the cable with specific protection against impact (say in metal conduit). Which I’m guessing won’t have been done. Reg 433.2.2 refers us to 434 and then 434.2.1 (ii) .
That is not the case.
Generally tails are not contained in conduit, the mechanical protection is provided by the sheath.
Hence why we no longer allow unsheathed tails.
 

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