Switched FCU For Lighting Circuit

[spark arrester]

Although this is my first post I'd like to say how valuable I have found the information posted on this forum, so a big thanks to all the contributors. I've come across an issue filling out an EIC for a consumer unit I've just fitted to a domestic property. A conservatory was added to the property about 10 years ago & a radial 2.5mm socket circuit with a 20A MCB was installed by the electrician. The lighting & fan are supplied via a switched FCU off this 20A radial using 1mm T+E (13A fuse is currently fitted..... possibly by the owner not the original electrician). The question is how should this arrangement be correctly recorded on an EIC ? Where should the R1+R2 value be taken (or R2) & at what point should Zs be measured ? I'm sure this example is covered in the regs but I've been unable to find a definative answer. Thanks in advance...
Spark Arrester

 

[Farmelectrics]

note its circut number numberof points served your r1 r2 and your zs should be recorded at the furthest point on that circut ie light or fan

 

[spark arrester]

note its circut number numberof points served your r1 r2 and your zs should be recorded at the furthest point on that circut ie light or fan

In other words do you mean that there's no requirement to record the different conductor sizes & the fuse rating of the FCU & just treat the whole circuit as a 20A radial ?

 

[telectrix]

yes, but make a note on the cert, and i would label the FCU " 5A fuse"

 

[Farmelectrics]

yes its one circut as tel said make note on test cert and make sure theres a 3 0r 5amp fuse in spur

 

[spark arrester]

I've changed the fuse for a 5A but I take your point on labelling the FCU. I'm still struggling with this issue as I've been asked to add some additonal lighting to his conservatory including a couple of wall mounted outdoor lights. If the circuit is merely treated as a 20A radial then the max permissible Zs under BS7671 would be 2.3. If the FCU is fitted with a 1361 5A fuse BS7671 would allow a max Zs of 10.45 (for the lighting downstream of the FCU). Treating the whole circuit as a 20A radial gives no Zs increase advantage to the 5A 1361 that would negate the higher resistance of the 1mm cable and allow a longer cable run. In reality, as far as this job is concerned I have a Zs at the fan of only 0.58 so I'm well within the max permissible for 0.4 disconnection but I'd like to be sure of the regs when I come across the same arrangemnent that may return higher Zs values.

 

[Farmelectrics]

refer to gn3 page 111 20a c type max zs=0.93

 

[spark arrester]

Thanks Farmelectrics, it's a B type 20A so the 80% Zs value (OSG) is 1.85.

 

[truckster]

Just to be devils advocate..if the lights are protected by a 5A fuse would that not make it a seperate circuit and the 20A then becomes a distribution circuit?

Discuss

 

[spark arrester]

Hi Truckster, that was my initial thought. The 20A is a final circuit for the sockets & a distribution circuit for the FCU making the lighting circuit a final circuit. The EIC forms don't appear to take this into account because there is only one box to tick.... distribution or final. I would argue that in this scenario the 20A cable is both

 

[truckster]

Thats the shortfall of these forms it can be difficult to make every situation fit. For the EIC I would take the Zs at the furthest socket as the spur is only the equivalent of something plugged in and you wouldn't take the Zs at a kettle for instance.

For the additional lights use a Minor Works Form and use the 5A spur as the protective device for the modified circuit.

 

[spark arrester]

That seems to cover the issue & neatly allows for the greater permissible Zs value provided by the 5A 1361. It's slightly different to the kettle example that you use in that in this scenario the FCU is almost being used as a mini CU so I imagine that any upstream items need to be treated the same as any other circuit.

 

[electric-ian]

Yep. Agree with the other posts re. the certs but note that it's a 1362 in FCU

 

[Des 56]

It could be listed underneath the main schedule of tests on the Eic as a seperate sub circuit

The 20 amp radial labelled as feeding sockets and conservatory supply spur

The conservatory supply can have the Zs as a sort of Zdb written above this line
The light/fan circuit can then have its own device type,rated overcurrent and max Zs all listed seperately to the main list

It seems to be more relevant to me when its included with the main schedule than a seperate certificate or even an add on schedule of tests ( whatever that extra certificate type may be)

 

[huntos33]

Interesting thread, sounds stupid but would this be the same as say a hi lo spur in a kitchen feeding under counter sockets?