Fsu and number of sockets/flex outlets

[stredwizard]

Quick background, decided to mount our tv on a stud wall, no suitable sockets nearby so I added a 3 way junction box into the ring main in the loft and ran a spur in accordance with the regs inside the partition vertically to a switched Fcu with a flex outlet for the tv, all worked good.

Then we decided to buy an Apple TV of course it needs to be powered behind the tv. I have at the moment, changed the switched fcu to one with A neon, added the original fcu for the tv off of that and added a socket into the terminals of the tv fcu to power the Apple TV, all using 2.5mm t+ e

Is this allowed and the correct way of doing it, I'm 99% sure it is in regards to spurs off of spurs etc, also considering if we move I will remove all cables I installed and just leave the junction box.

Thanks Scott[/b]

 

[ericmark]

I know it's called a fused spur but really it's a radial once a fuse is added and there is no limit to number of sockets that can be added. Except of course they must also abide by volt drop rules and earth loop impedance rules.

Clearly any new socket must have RCD protection unless marked for a special use and then it would need the special cable to feed it. The same applies to a Fused Connection Unit used as a cable outlet although it does not required RCD protection for the outlet as supplying a device which will never change the buried in wall rules still apply.

In real terms most DIY people can't measure the loop impedance and when supplying a switch mode power supply volt drop is hardly a problem but you are asking about rules.

 

[Risteard]

when supplying a switch mode power supply volt drop is hardly a problem but you are asking about rules.

But rules and guidance should not be mixed up.

 

[EFLImpudence]

As you now have a socket for the Apple, why did you not just replace the original FCU with a double socket?

 

[stredwizard]

Becuase there is limited depth behind the wall mounted tv, the Apple TV plug is very slim compared to other plugs I have around the house, further more i may wish to add a sound bar at a later date as well.

Thanks for responses from everyone

 

[JohnW2]

when supplying a switch mode power supply volt drop is hardly a problem but you are asking about rules.

But rules and guidance should not be mixed up.

Indeed so. All the 'rules' (BS7671 itself) say about voltage drop is that it should not be such as to impair the safe functioning of equipment - which, as eric says, is not going to be a problem with an SMPS (or, indeed, most loads!). The oft-cited 3% and 5% figures (in an 'informative' Appendix of BS7671) are just 'deemed to satisfy' provisions ('guidelines', if you wish), for those who do not want to think for themselves.

Kind Regards, John

 

[plugwash]

It's unlikely to be a problem in this case but don't let the wide input voltage range of modern SMPPSUs fool you into thinking they will work correctly with a supply that involves massive voltage drop when fully loaded by the SMPSUs.

If you graph the current at a given voltage for a resistive load you will see current increases linearly with voltage. If you graph the current for a PSU with a linear regulator or a string of LEDs with a series resistor you will see current increases nonlinearly but still monotonically with voltage.

If you then graph the output characteristics of your source you will normally find that voltage decreases monotonically with increasing current.

Put the two lines on one graph and you will typically find the lines cross at exactly one point. That is the point where the system will settle.

However if you graph voltage vs current for a SMPSU you will see a very different pattern. At low voltages the current increases with voltage, then reaches a peak as the switching circuit starts working before dropping again.

If the impedance of the power source is too high then the graphs will cross at three points. The middle crossing is unstable so the system won't settle there but the upper and lower crossings are both stable and potential places for the system to settle. If the system settles at the lower crossing (as it very likely will at initial switch-on) then it won't operate correctly and may overheat the SMPSU or trip the supply breaker.

To properly specify a power circuit for powering a SMPSU you really need a voltage/current graph for the SMPSU covering all voltages from 0V up to the open circuit voltage of your supply. In the absence of such information I would be very wary about using the wide input voltage range of modern "universal" SMPSUs as an excuse to install high volt drop circuits.

 

[JohnW2]

It's unlikely to be a problem in this case but don't let the wide input voltage range of modern SMPPSUs fool you into thinking they will work correctly with a supply that involves massive voltage drop when fully loaded by the SMPSUs. ....... To properly specify a power circuit for powering a SMPSU you really need a voltage/current graph for the SMPSU covering all voltages from 0V up to the open circuit voltage of your supply. In the absence of such information I would be very wary about using the wide input voltage range of modern "universal" SMPSUs as an excuse to install high volt drop circuits.

Everything you say is perfectly true, but I don't think that any sane person would consider designing a circuit which involved "massive voltage drops" - indeed, one might well run into Zs problems even if one tried. I was merely making the point that the 'deemed-to-satisfy' 3% and 5% VD figures are very conservative in relation to most loads.

Any sort of 'regulated' power supply (including SMPSUs) obviously is, by its nature, more tolerant of supply voltage variation than many other types of load, but I think the point is that even totally 'unregulated' loads (let alone those with some regulation) will rarely be intolerant of voltage drops a bit over the 'magic' 3% or 5% figures (particularly given that they have to accept variations in permitted supply voltage appreciably larger than that).

I was merely suggesting that people should be prepared to do a little thinking/research, rather than taking the lazy approach of accepting the 'deemed-to-satisfy' figures and thereby 'condemning' a design which involved (only at maximum theoretical load, which would very often never happen, anyway) VDs just a little over those 3% or 5% figures.

Kind Regards, John.