Does this comply.... thinker of the day!

[Darkwood]

Im bored so I thought Id get your heads thinking and take note who's posting this so Im not after a straight answer yes or no! Back your comments up please with regulations.

I have a small house and they complain when a lamp blows the bs3036 5amp wire blows and they have to replace it (nuisance tripping/blowing), the lighting is on 1 circuit and covers 5 fittings - kitchen, room, stairwell, bathroom and bedroom.. they are adamant to keep using their stock of filament lamps so for the sake of this debate we are not replacing the style of lighting nor upgrading the board or fuses...

Can i fit a 15amp or 30amp fuse wire in the lighting fuse carrier noting that under the circumstances the Zs is still in compliance and still be within BS7671 and not breaching any regs.

Remember alternative solution are off the table this is theoretical to test your understanding of the regs.

 

[Richard Burns]

What cu is it? Could you try a 10a mcb?

If we could use MCBs then I would be aiming for a different solution that would meet more of the regulations, but would be no more compliant than the suggested modification.

 

[NickD]

For fault current on a 15A BS3036 I calculate the csa for adiabatic compliance would be ~0.5mm² or less, so that would be covered (543.1.3)

Based on what fault current have you calculated that, please? Needs to be the PFC at the origin doesn't it (neglecting any resistance within the CU)? To handle worst-case of a screw going through the cable within a foot of the board? And we don't know the PFC.

30A does not protect the cable from overload, nor does 15A unless entire run is reference method C, so we would be reliant on downstream overcurrent protection of which there is none unless you count the lamps, and there is nothing to stop someone adding a ton more light fittings, so it doesn't comply....?

<Edit: question is only about whether existing installation strictly complies with regs so scratch the bit about extra fittings....mea culpa...>

 

[Darkwood]

Various points to look at here....

Mr Burns saw the fact that id specified incandescent lamps and your typical lamp in domestic of this nature will be subject to regulation 559.6.1.6 so a 30amp wire is out of the window and won't comply.

Thus we drop down to 15amp with the same question ...

Note rating of switches etc are only subject to the load they are switching and not the fuse that covers them or we would have a lot of non compliant installs out there.

Install is your basic drilled through joists buried in plaster and insulation in loft - just to clarify although it changes little.

Nick your last sentence is key to part of it...

and there is nothing to stop someone adding a ton more light fittings, so it doesn't comply....?

 

[Richard Burns]

Based on what fault current have you calculated that, please? Needs to be the PFC at the origin doesn't it (neglecting any resistance within the CU)? To handle worst-case of a screw going through the cable within a foot of the board? And we don't know the PFC.

...>

I did the pathetic calculation of 15A BS3036 will blow in 0.1s with a fault current of 180A from the time current graphs. Based on that the minimum csa is 0.49mm². However even if the fault current is higher, as one would hope it would be, the time decreases (but is off those graphs) and should make it a lower csa, if I have it right!

 

[nickblake]

it could boil down to junction boxes if the fuses are BS3036 then you could well have an abundance of 5 amp junction boxes up in the loft so the first one could have the complete load on it

 

[Richard Burns]

The complete load is only 2.17 A assuming 100W max for each fitting though with 150W lamps you could get up to 3.26A.

 

[Redvanman]

How much stock have got

 

[spark 68]

Various points to look at here....

Mr Burns saw the fact that id specified incandescent lamps and your typical lamp in domestic of this nature will be subject to regulation 559.6.1.6 so a 30amp wire is out of the window and won't comply.

Thus we drop down to 15amp with the same question ...

Note rating of switches etc are only subject to the load they are switching and not the fuse that covers them or we would have a lot of non compliant installs out there.

Install is your basic drilled through joists buried in plaster and insulation in loft - just to clarify although it changes little.

Nick your last sentence is key to part of it...

and there is nothing to stop someone adding a ton more light fittings, so it doesn't comply....?

I thought you said we are not changing the type or number of luminaires ?

 

[Darkwood]

Apologise posts coming quicker than i can respond .... yes your PFC is a factor that could affect the answer and another point awarded for bring it up as a query but as you'll find I've carefully chosen

Small house
5 lamps
1mm cable installed in standard methods

So PFC affecting this scenario is a very unlikely situe even with a very poor PFC but good thinking!

 

[Darkwood]

I thought you said we are not changing the type or number of luminaires ?

I didn't say he was right he just that its a key point now we are not adding to the circuit or changing lamps do we have to account for other peoples future work if we changed this wire to 15amp?

 

[telectrix]

if you are considering th ecircuit as a fixed load, then the OCPD may need to provide short circuit protection only.

 

[spark 68]

I didn't say he was right he just that its a key point now we are not adding to the circuit or changing lamps do we have to account for other peoples future work if we changed this wire to 15amp?

The simple answer to that is no, if someone wanted to add to/alter that circuit at a future date then they should be assessing that it is suitable for the intended modification.

 

[nickblake]

1mm its almost the same size as the fuse wire ,oh how often has that been used instead

 

[Richard Burns]

You now have the consideration that one of the lights is in a bathroom and there has been no mention of 30mA RCD protection (701.411.3.3).

 

[D Skelton]

Yes, I would say that it would comply, so long as certain conditions are met. I ain't gonna beat around the bush.

If this work was not carried out in an installation with significant fire risks present, the circuit's Zs falls below the maximum corrected value for the new fuse, the circuit cannot be overloaded by nature of the very load itself, the cpc sizes have been calculated as adequate and the requirements for thermal constraints under fault conditions have been met, then you have confirmed that the circuit cannot be overloaded, the conductors cannot damaged under fault conditions and that disconnection times can still be met.

Would I do it??? NO!

Anyone answering no I would be pointing them in the direction of regulations 433.3.1.ii and 434.5.2