Another 12v Volt Drop Query

[Adam Withers]

Yeah - I agree on the remote sensing being a bit overkill.

Regarding the load, I would like options as I may want to look at an irrigation system in future (using a 12v pump in a water butt which is next to the shed). I also would like to have a wireless camera or two up there or security and wildlife watching. If I going to the trouble of putting power up there, I may as well over spec it.

I think the 48v to 12v is the way to go, as long as the PSUs I linked will actually work.

Thanks

Adam.

 

[JohnW2]

Regarding the load, I would like options as I may want to look at an irrigation system in future (using a 12v pump in a water butt which is next to the shed). I also would like to have a wireless camera or two up there or security and wildlife watching. If I going to the trouble of putting power up there, I may as well over spec it. .... I think the 48v to 12v is the way to go, as long as the PSUs I linked will actually work.

Fair enough. However, if I were in your position and wanted plenty of flexibility and 'future-proofing', I think I would (despite what you said about the landscaping) be seriously thinking about possible ways of getting 230V down there. After all, even if its 48V or 12V, you still have to run a cable in some manner that won't be at serious risk of mechanical damage, so it's far from impossible that a way could be thought of that could do likewise with SWA cable (SWA does not necessarily have to be buried).

Kind Regards, John

 

[SimonH2]

However, much as we often see that suggested (sometimes even by myself), I wonder how easy it is to find (I think not in 'consumer/retail' outlets) the 'bare LED element' lamps/bulbs/fittings that such a system requires - I certainly cannot recall often seeing them on offer, even in catalogues/websites of electrical wholesalers (although perhaps that's because I've never looked for them!)

Probably a case of not knowing them when you see them. There are a lot of LED lights that are "350mA" - though mostly in the professional kitchen fitting area for example. When you see a 350mA luminaire, you can be sure that there's just a "bare" LED with at the very most a reverse protection diode added.
Or these days, also 700mA and 1000mA

Fair enough. However, if I were in your position and wanted plenty of flexibility and 'future-proofing', I think I would (despite what you said about the landscaping) be seriously thinking about possible ways of getting 230V down there. After all, even if its 48V or 12V, you still have to run a cable in some manner that won't be at serious risk of mechanical damage, so it's far from impossible that a way could be thought of that could do likewise with SWA cable (SWA does not necessarily have to be buried).

That was my first thought. If he can get a 12V cable down there, then he can get a 230V cable down there - just a matter of choosing a suitable cable (not much you can't do with SWA !)

 

[JohnW2]

Probably a case of not knowing them when you see them. There are a lot of LED lights that are "350mA" - though mostly in the professional kitchen fitting area for example. When you see a 350mA luminaire, you can be sure that there's just a "bare" LED with at the very most a reverse protection diode added.

Well, yes, I obviously know they exist (and that they are quite probably extensively used in commercial and industrial situations) - but, as I've said, I've never 'noticed them' (and I certainly don't frequent "professional kitchen fitting areas"!). Do they (or some of them) come with replacebale lamps/bulbs? If, where would the 'man in the street' get replacements?

That was my first thought. If he can get a 12V cable down there, then he can get a 230V cable down there - just a matter of choosing a suitable cable (not much you can't do with SWA !)

Indeed. As I said, that's certainly the approach I would probably be exploring.

Kind Regards, John

 

[333rocky333]

in the case of runway lights (when loss of all lights could be catastrophic), it seems that rather than put the lamps/bulbs (of whatever type) in series, they use AC and put a whole pile of (wirewound) transformers in series (one for each lamp).

Kind Regards, John

Sorry john how does that stop loss of all lights.
Is it the case that the windings more robust than a lamp

 

[JohnW2]

Sorry john how does that stop loss of all lights. Is it the case that the windings more robust than a lamp

Exactly. Don't forget that we are (or, at least were) talking about incandescent bulbs - and they were/are far, far more likely to fail than a low-power wire-wound transformer (which will probably last 'for ever'). Even an LED would be 'far far' more likely to fail than such a transformer.

Given how safety-conscious they are, I suspect there is additional 'redundancy', "just in case" (just in case a transformer fails o/c) - e.g. by having two strings of series transformers, each string serving 'every other' lamp, or something like that.

Kind Regards, John

 

[SimonH2]

Well I think we can agree that transformers (the wirewound variety) are pretty hardy things. Not only that, but they also tend to fail due to insulation breakdown (shorted turns) which normally causes excess current flow and blown fuses. In the airfield lighting setup, I don't think that would be a problem as it would simply continue passing the same current but at lower voltage, and maybe emitting (some or all) it's input power as heat instead of via the lamp.

 

[JohnW2]

Well I think we can agree that transformers (the wirewound variety) are pretty hardy things.

Indeed - as I said!

Not only that, but they also tend to fail due to insulation breakdown (shorted turns) which normally causes excess current flow and blown fuses. In the airfield lighting setup, I don't think that would be a problem as it would simply continue passing the same current but at lower voltage, and maybe emitting (some or all) it's input power as heat instead of via the lamp.

Again, indeed - and I doubt that shorted turns in one out of dozens or hundreds of transformers in series would hardly 'notice', particularly when driven by a constant-current source. Only an open-circuit failure of the primary would be a problem, and that must be as rare a gnat's teeth in such a device. In fact, if anything killed all the lights in such a 'string', it would presumably be most likely due to a cable/connection failure - and even that could be addressed by the 'redundancy' I mentioned in my last post.

Kind Regards, John

 

[Adam Withers]

Fair enough. However, if I were in your position and wanted plenty of flexibility and 'future-proofing', I think I would (despite what you said about the landscaping) be seriously thinking about possible ways of getting 230V down there. After all, even if its 48V or 12V, you still have to run a cable in some manner that won't be at serious risk of mechanical damage, so it's far from impossible that a way could be thought of that could do likewise with SWA cable (SWA does not necessarily have to be buried).

Kind Regards, John

I have given a lot of thought (as well as another electrician) as to how we could get SWA cable down there, but to be in the regs, it would mean pulling up paving drilling through planters etc. which I not wanting to do. I have fencing either side so I cannot mount it above ground. With the more flexible cable, there is a route I can take but it will result in a short length at soil level between two planters against the fence. If you wish I could post some images of the garden?

At the end of the day, if I can get a cable down there using the 240vAC-48vDC PSU and one or two 48vDC to 12vDC adapter it would be OK.

Thanks

Adam.

 

[ericmark]

I know the idea, a current transformer produces a voltage normally it feeds a volt meter, but there is no reason why it should not power a bulb, the idea is a series of transformers feed the bulbs so even if a bulb blows the transformers are still in series so current still flows so other bulbs continue to work. This is done with air craft ground lighting, however it seems rather an OTT method to ensure all bulbs have same brilliance.

It would be easier to use a DC voltage above the required voltage and a voltage regulator at every bulb, however this would produce heat and be waste full, however there are 12 volt DC bulbs, which are designed to work with a range of 10 - 30 volt and have a switch mode regulator built in. This would seem a far easier way to get around a volt drop problem.

These people specialise in bulbs for motor vehicles, narrow boats and the like, so a 20 volt supply could have a 50% volt drop and all bulbs be the same, likely a computer power supply would be best bet.

 

[JohnW2]

... With the more flexible cable, there is a route I can take but it will result in a short length at soil level between two planters against the fence.

I'll be interested to hear what others have to say, but I wouldn't personally have a problem with SWA 'at soil level', provided that it was not in a particularly (mechanically) 'vulnerable' position. The greatest dangers arise with shallow burying of cable - if it's visible, it's far 'safer' - and, if the worst comes to the worst, the whole point of having earthed armour on SWA is that, should it ever be penetrated (which is pretty difficult, if it's visible), a protective device should take care of the situation.

If you wish I could post some images of the garden?

That might enable someone to offer some thoughts/ideas.

Kind Regards, John

 

[SimonH2]

I have fencing either side so I cannot mount it above ground.

Why ? It is permissible to mount cables to a fence provided it is mechanically supported and a suitable cable for the external influences it might encounter. Doesn't look too nice, but it's allowable.

With the more flexible cable, there is a route I can take but it will result in a short length at soil level between two planters against the fence.

That too is allowable.

If you wish I could post some images of the garden?

It might help !

EDIT: many years ago I helped someone pull a block wall down in preparation for some building work. We found a T&E cable buried in the cement coping on the top Someone had just run the cable along the top of the wall and then cemented over it to finish off the top of the wall. We had no idea it was there until we pulled part of the wall down and it popped out.

 

[ban-all-sheds]

I have fencing either side so I cannot mount it above ground.

Not sure that the second follows from the first.

I've been looking into "no dig" methods for running cables - one option (depending on how easily you could hide it behind plants) for you might be concrete or GRP cable trough just sitting on the ground at the base of the fence.

 

[Adam Withers]

Regarding mounting to the fence, the electrician was not in favour of it. He gave the example that if there were high winds and the fence blew down into next door garden taking the cable with it, it would leave it possibly unsafe. I guess it is an interpretation of the regs. I would need to get an electrician to certify the installation, this may prove difficult if they think it goes against the regs.

Attached is an image of the garden. At the top right is where the shed now is (see other attachment)
The run will go from left to right against the house - there is a gully along the length of the house (see other attachment) so either put it in there or just above it wall mounted. Then bury it in the pebbles (you can just see them at the bottom right) up to the first bed. It will then go behind the bed, but I would need to make a hole here where the concrete posts are, as the builders have put the wall/mortar up against it. It would then lay on the ground behind the magnolia and again lay behind the second bed up to the shed where it would terminate.

I seem to recall there was a minimum bend radius for SWA which (I assume) would be less of an issue with a flexible cable such as rubber or artic cable? The bends may be an issue at some points for the SWA.

Thanks for your continued help

 

[ban-all-sheds]

Regarding mounting to the fence, the electrician was not in favour of it. He gave the example that if there were high winds and the fence blew down into next door garden taking the cable with it, it would leave it possibly unsafe.

That's the usual objection to fixing to fences - that and the fact that even if they don't blow down they move about, and that can stress joints and connections.

But you have concrete fence posts - as long as they are properly fixed at the base, they would be absolutely fine for fixing cables to. The only problem would be that if you didn't use steel conduit then the cable would sag, as the supports would be too far apart.

I would need to get an electrician to certify the installation, this may prove difficult if they think it goes against the regs.

It may prove difficult anyway - what do you mean by "certify" and "get an electrician"?

The run will go from left to right against the house - there is a gully along the length of the house (see other attachment) so either put it in there or just above it wall mounted. Then bury it in the pebbles (you can just see them at the bottom right) up to the first bed. It will then go behind the bed, but I would need to make a hole here where the concrete posts are, as the builders have put the wall/mortar up against it. It would then lay on the ground behind the magnolia and again lay behind the second bed up to the shed where it would terminate.

Shame that you didn't think about this before you had the turf laid.....

I seem to recall there was a minimum bend radius for SWA which (I assume) would be less of an issue with a flexible cable such as rubber or artic cable? The bends may be an issue at some points for the SWA.

And robustness may be an issue for non-armoured cable.