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

Steve93

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Trainee
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Eicr today had a 4mm T+E connected into the bottom of a main switch which went to a separate dB with 1 20amp rcbo. Then swa to a hot tub. That part is fine. What I’m unsure about is the reg I can say that the cables are not ok in the bottom of the main switch. The other dB is only about a ft away from the main one. What’s people’s thoughts on this? I know it’s rough as you like!!
 
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Steve93

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Trainee
  • Thread Starter Thread Starter
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What would you code it? If at all? It’s rough as but can’t find anything on it
 
D

Deleted member 9648

If the 4mm is securely terminated along with the busbar/neutral link then I cant see a non-compliance. You need to use your judgement on whether the termination is sound. The different size conductors in the terminal may not result in a sound connection
 
I assume the OP is referring to a standard 100a MS in a DB with a busbar and N link in the bottom
If that is the case you must question whether this is a means of connection which is condoned by the consumer unit manufacturer. Are the conductors suitably sized to withstand fault and short circuit current.
 

GBDamo

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Supporter
If that is the case you must question whether this is a means of connection which is condoned by the consumer unit manufacturer. Are the conductors suitably sized to withstand fault and short circuit current.
Not using a type tested assembly as intended by the manufacturer.

C2, as it could cause poor connection and possible CU fire. Bit of a judgement call.
 
[QUOTE - So the 4mm is protected by the cut out fuse?]

For fault current yep but depends on what type supply

[QUOTE So by the cut out fuse then?
What other ocpd is there?]

What over current can the 4mm suffer when it can pull no more than 20 amp?
 

Ian1981

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Mentor
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[QUOTE - So the 4mm is protected by the cut out fuse?]

For fault current yep but depends on what type supply

[QUOTE So by the cut out fuse then?
What other ocpd is there?]

What over current can the 4mm suffer when it can pull no more than 20 amp?
Overload protection is not the issue,How about short circuit protection?
 

gazdkw82

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Trainee
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
 

gazdkw82

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Trainee
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.
 

Richard Burns

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Arms
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A very poor installation but could be compliant if the connections are sound and the cable route to the 20A RCBO is suitable, then the RCBO can provide sufficient protection to be acceptable.
However there are a lot of caveats to the compliance so not generally a good idea.
PD after point of protection.jpg
 

Charlie_

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Arms
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?
 

Lucien Nunes

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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:

Wilko

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Arms
Esteemed
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) .
 
D

Deleted member 26818

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.
 
I've Harmonised that for you.
My pet hate.....Colour Harmonisation!
3 phase meter change...all grey apart from L3 load which was brown....it was grey when it was finished!
Strange thing is on CT metering all the CT wires and the sense voltage wires are red!
 
D

Deleted member 9648

My pet hate.....Colour Harmonisation!
3 phase meter change...all grey apart from L3 load which was brown....it was grey when it was finished!
Strange thing is on CT metering all the CT wires and the sense voltage wires are red!
Couldn't agree more. Done a TPN board change a couple of weeks ago, half the blacks neutral, half phase......same with the blues. Despite the passage of time I still struggle to see how a panel of (allegedly) intelligent techies spent weeks debating it and came to the conclusion that was a good idea.
 

gazdkw82

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Trainee
The circuit originates from the OCPD (main switch), not the 20a rcbo.

It's a little frustrating because it's one of those grey areas like multiple spurs from 1 single point and not confirming r1+r2 on minor works etc. There is no real documentation clearly outlining it's not right but we all know it's a Badgers arse
 

Wilko

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Arms
Esteemed
We could come up with a few of our own classifications, like they’ve got for films - RABA in this case :) .
 

gazdkw82

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Trainee
Sounds like you are inferring that a main switch and OCPD are one and the same?
No I'm saying the circuit originates from the main switch which IMO is inadequate protection. When designing circuits you choose a protective device to protect the entire circuit.
 
D

Deleted member 9648

No I'm saying the circuit originates from the main switch which IMO is inadequate protection. When designing circuits you choose a protective device to protect the entire circuit.
Not necessarily. The regulations permit omission of devices for both fault current and overload current under certain conditions, such as short tails to a DB at the intake. An OCPD on the load side of those tails effectively limits the current on the supply side. The same principle allows an unfused spur from a 32a RFC when effectively the CCC of the 2.5mm is around 26a. The fact that only one twin socket is permitted means only 2x13a=26a can be drawn.
 
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