domestic solar dc lighting

[Matt]

Hi Folks,

I am considering re-using existing ac mains lighting circuits to distribute 24v dc to led lights, supplied by a stand alone solar battery system. Each light has its own SELV rated driver. I have considered the voltage drop issue already and the need for dc rated switches. What I am not sure about is whether to bond the dc negative supply terminal to the ac mains MET and whether it would be accepted as safe to have ELV dc in wiring that is rated for ac (particularly JB connections) and could be mistaken at a later date as ordinary mains cables- even with warning notices at the CCU. Also does anyone know if it possible to get single phase type B (smooth dc sensitive) rcds?

Cheers

Matt

 

[binky]

if working with 24V, why would you need RCD? Think car technology - simple fuse and big battery, no RCDs required.

 

[Matt]

Although there shouldn't be any connection between the ELV dc and the mains ac circuits, under fault conditions, particularly if the negative of the battery is bonded to ac earth, dc may find its way onto the ac circuits and cause malfunction of ac protection devices. I am also interested for higher voltage dc side solar systems, which require Type B rcds if used with output transformer-less grid connect inverters (712.411.3.2.1.2 (!)). I can only find ones which work on 3phase.

Cheers

Matt

 

[kme]

I wouldn`t be "bonding" the DC-ve to ANYTHING on the AC side, TBH.

KME

 

[binky]

segragation of circuits springs to mind - the DC is obvioulsy ELV and needs to be kept 50mm from LV or protected by conduit/ trunking/ a.n.other form of insulation. Ergo if this is complied with, the chances of shorting to ac side is extremely low/ negligble

 

[Larnacaman]

How much money is this idea of yours going to initially cost you?? You should know that, before you even consider going ahead with your scheme.. Only then, will you have any idea of how long, ...time wise, will the payback start...

You say that you have already considered using the existing 1mm cables and AC rated light switches. So what did your calculations give you, in regards to volt drops around the lighting circuits?? Personally, i don't think 24 volts is going to get very far, but then i don't know the size of the property and the length of your lighting circuits, or the size and amount of LED lamps your intending/proposing to use....

By the way, there is no reason for you to connect your systems DC negative to ground. You will not need RCD or MCB protection either, just correctly sized and positioned HRC fuses, and they don't care if it's AC or DC circuits there protecting.... Don't forget that a system such as your proposing, is not a fit and forget proposition, it will need maintaining/checking at fairly regular intervals....

 

[Matt]

Thanks for the replies. Not referencing the dc supply to earth seems to be a debatable issue. The Regs for ELV dc are mainly aimed at caravans (Annex A), the best they offer is to warn against mutual, detrimental influence- which does point to the issue of ELV dc getting into the ac system and preventing the ac protection operating correctly. Then there are regs for ac/dc systems on boats and Regs for telecoms both of which bond ac and dc earthing. The DTI guide to PV installation recommends floating dc but does not prohibit earth bonding the dc negative , in fact some module technologies require it (these guidelines are written essentially for inverter, grid connect systems though). In the latest Wiring matters there is debate as to whether solar modules can really be considered class 2 devices due to the risk of double faults to the frame leading to fire risk. Using ELV as a protective measure is classified as FELV, PELV or SELV sytems in the Regs. Both FELV and PELV are earth referenced whilst SELV is isolated from earth, but the requirements are too stringent to meet for a distribution system such as I am proposing. So this leads me to think that bonding the negative of the dc supply at its origin to the MET is required. NB A mains energised back up battery charger may also form part of the system.

Cheers

Matt

 

[Sidewinder]

Matt,

You really need to understand FULLY the requirements for, & differences between SELV, FELV & PELV in SERIOUS detail before you go forward with these works.

 

[Matt]

Your reply is a bit too cryptic for me, if you think I've missed the point or something please tell me, but I am studying the requirements closely.

Cheers

Matt

 

[Andy™]

I wouldn`t be "bonding" the DC-ve to ANYTHING on the AC side, TBH.KME

just to add another view, my van has its chassis connected to negative....

...its also connected to the earth from the EHU & genny

and i would imagine its the same in motorhomes

 

[Sidewinder]

EHU?

Genny is LV thus "metalwork" must be bonded to earth.

 

[Andy™]

EHU?

electric hook up

 

[Sidewinder]

Ahh, same applies then.

---------- AUTO MERGE Post added at 21:11 ---------- Previous post was at 21:06 ----------

Matt,

Nothing wrong per se with what you propose, however, you MUST ensure that you can justify your design in light of the requirements of BS7671.

What you are proposing IMHO does not immediately spring to mind as a "standard" circuit, thus you must be able to justify your design decisions and calculations.

These will probably NOT be covered by 7671.

Not that you can't do it, but YOU are the designer and thus YOU MUST "carry the can" for the design and the full ramifications of the install, regardless, end of.

 

[Matt]

Sidewinder,

Thanks for the reply. You are correct of course. I was hoping someone might have had some experience of ac/dc systems that would be applicable.

Cheers

Matt

 

[Larnacaman]

Is there any particular reason why you want to go DC for your properties lighting requirement Matt??

I'd still be interested to know how your design calc's went, for distributing the 24v supply on the existing 1mm lighting circuits??....

 

[Matt]

Larnacaman,

I am interested in dc as many low power appliances actually run on ELV dc, having to convert from mains ac. Solar PV generates dc, so if we can harvest sufficient solar energy for our low energy devices we can eliminate the need for an inverter and keep voltages to ELV. Electronic dc/dc buck converters can have efficiencies of over 90%. If manufacturers took onboard this notion we could have our laptops, mobile phones, lights etc run or charged in this way. In fact it is already possible to get car chargers for 12v systems.

As far as sums:

An example of 4 rooms and a hall way. 10m from battery to hall way light, then, 5m between the other 4 lights. No lighting current in switch wires, the 24v/12v dc/dc buck converters just enabled/disabled by a pulse from the replacement switch. Lamps are 12watt led about 60-70watt equivalent. 5 x 12w = 60watt total. 60/24= 2.5A total.1mm cable @36.2mohm/m. Vdrops = 0.905,0.362,0.2715,0.181,0.0905v to each lamp in succession. Vdrop total = 1.81v. 1.81/24 x 100%= 7.5% at the last lamp and 3.77% at first lamp- So within the realms of practicality. Plus the fact that some or possibly all the wiring could be in 1.5mm in some situations, the results would be better. I think 48 volts is a better option as used in telecoms, and may be the way I would go finally. Also bare in mind LED efficacies are improving on a monthly basis.

Hope this is helpful.

Cheers

Matt

 

[Larnacaman]

Matt,

Yep, fully agree with you, that LED is definitely the future for lighting....

Maybe you have a good point too, in the idea of having lower power DC outlets in the home, for our low power electronic appliances. Not so sure about the economics of such a system at this point in time though!!

Which brings me back to one of my original questions to you. ie, the cost of the complete system your proposing to install. Seems that anything that has a solar power label attached to it, doubles the cost of the item ...lol!!!

Yes your load/Vd sums seem quite solid, and will certainly be a good deal better if you go with the 1.5mm and a 48v system. Are you presently thinking to using 1 x 24v battery or 2 x 12v batteries? For the 48v system you will probably have to go the 2 x 24v batteries....

Keep us informed of your progress please, all sounds very interesting, though still dubious about the overall cost, that may be involved to set a system like this up....

 

[steptoe]

after reading the first few posts on this my simple answer would be the OP needs to understand the matter of simple separation at the very least before anything else.

then he needs to grasp the principle of having 2 different systems in one property,

which is a whole different kettle of cod, and I dont think a general discussion is the place for it.

 

[Andy™]

steps - motorhomes & caravans have a 12 & often 230V system in use at the same time without a problem. why should a proplem then suddenly appear just because its a similar setup, but in a house instead

 

[steptoe]

to start with it would be a minimum to have all cables properly identified, not that easy when using the existing T&E.

how is OP going to comply with 7671?

or are we going down the deviations route?