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Extra Low Voltage devices

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Jeff
 Jeff
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@transparent @derek-m @mars

As a general point i think it might be good to split out a few of the threads? 

Some the threads have quickly got into a lot of detail, great for some readers, other readers will glaze over.

As an example the posts around batteries went from asking for recommended installers and asking for comments on quotes from installers to quite a lot of detail on building your own batteries.

Unless there are different threads that cater for different types of customer it gets messy. I can't even put up a curtain rail these days following an injury.

I am all for education and knowing the detail and options is important and the shortfalls and costs and background etc. I really enjoy and value the detail. I wonder about who the target audiences are for the forum. Perhaps the forum will just self select to those that want lots of detail and be very hands on. That is fine also. But it will mean others don't contribute or ask questions. 

I don’t know if this post is useful or not. Just my thoughts. 


   
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Transparent
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Posted by: @robl

Are your ELV applications generally fixed wiring?  There is a fair amount of available 12V automotive plug in equipment, generally using cigarette lighter sockets -phone/laptop chargers, lights.  Would you use those, or are you bespoke / some other preferred plug and socket for use at ELV?

We're moving very fast into another area of discussion within the subject of ELV. I'll address what I've found out concerning connectors here. But please feel free to return to some of the matters already raised above.

Yes @robl - the majority of my ELV use is with fixed devices rather than those which can be plugged in. I'm in favour of both approaches. However, I'm currently renovating/extending a 1930s farmhouse and I'll obviously be running as much cabling for ELV lighting as possible whenever floor-boards are up!

There are many instances where I've run 24v cables for lighting or touch-sensors, but the light-units themselves or the controllers are still missing because I haven't yet found time to design them!

For example, here's an 'individually addressable' LED strip being installed in an aluminium 'corner bead' when I was busy plastering a room. It's not for 'flashy' lighting effects as more commonly offered by controllers bought from the Far East. But I have yet to develop the colour-fading/graduation system which I'd like, and the touch-interface to control it.

LEDfixStripSm

I also have two independent water-purification systems because we don't have mains water. One of these uses an innovative 24v LED-based UV light (265nm) which kills bacteria by denaturing the DNA.

UVlayoutSmLabels

 

ELV plugs and sockets:

Almost all DC connectors are the cylindrical 'barrel' type. The two most commonly found ones look almost identical, differing only by the central pin being 2.1mm or 2.5mm in diameter.

image

Unless otherwise stated, you should assume that these sockets can only deliver 1 or 2 amps. However, you can get 5A versions made by Switchcraft from Rapid Online, and I've used these to make 'wall sockets' in various places around the house. That effectively gives me a nominal 100w socket with a resettable over-current trip beside it.

WallSkt1

The company who make the 'best quality' ELV DC sockets are Lumberg. Their range is extensive, but UK suppliers like Farnell sell the most commonly used connectors. If you're looking for a plug to fit a laptop or an LCD monitor, start by considering their Product Code 1636 05 with a 1.4mm central pin.

Lumberg's range includes a 10A plug and socket 1636 07 & 1614 21. However, to achieve this current rating they take a fair bit of effort to couple and take apart again. The socket would need to be mounted on a substantial faceplate.

image

 

This post was modified 2 years ago 2 times by Transparent

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(@chickenbig)
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Posted by: @transparent

A householder is permitted to run their own cables and sockets for ELV. Employing a qualified electrician is not mandatory.

So if I'm reading this correctly connections from (off grid only?) inverters to batteries (16S LiFePO4) is ELV.

Do you have any favourite reference works for how to correctly wire these ELV systems? How do you feel about the Victron Wiring Unlimited book as a starting point?


   
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Transparent
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The wiring between an inverter and a 48v (nominal) battery does operate within the parameters of Extra Low Voltage, but would not normally be referred to as ELV wiring.

The connections, fuses, trips and isolators required to interconnect inverters and batteries are a special case of ELV. As you rightly point out @chickenbig , that special case is addressed in the Unlimited Wiring guidance document published by Victron.

Personally I think that the UK needs to go beyond the guidance in that document... and that its suggestions fall short of what our British house insurers would expect to see.

For example, I don't agree with the presentation of such a wide range of possible fuses in pages 30-34. This suggests that they are comparable in operation and require selection only according to the maximum voltage they are designed for and the current at which they must operate.

Here in the UK I think the 'standard' should be defined as a Type-T fuse between inverter and battery. These are additionally categorised according to rupture capacity.

TypeT annot

I'm not merely concerned that the fuse should blow if the load current exceeds 250A for example.

If a fault occurs within the inverter which presents a short-circuit, the current drawn from the battery could instantaneously be 25kA or more !

What's important is that the fuse link must blow sufficiently fast and with enough (chemical) reaction to prevent arcing. Unless it does so, then the inverter itself could catch fire.

 

When I'm using the term ELV wiring here in this forum, I'm referring to loads operating throughout the house which are deriving their electricity from the DC battery without it passing back through the inverter.

Yes, there is a need for guidance, both in regard to safety and best practice. We can all contribute to this.

The discussions here may resolve themselves into a future ELV Installation Guidance document. The same guidance applies whether a householder attempts such wiring themselves or chooses to employ a qualified electrician. We don't need the Government to do everything for us!

This post was modified 2 years ago 6 times by Transparent

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Transparent
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Let's return to the possibilities for using ELV devices within the home:

 

This post was modified 2 years ago by Transparent

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You may find this of interest.

https://eclipsepower.co.uk/project/cowstead/


   
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Transparent
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That story about Eclipse Power's installation of 49MW storage is a carbon copy of the situation I raised in a post here about North Tawton on 21st April.

In the case of the Cowstead site, this is funded by overseas investment. Sembcorp Utilities PTE is based in Singapore. Here's the relevant entry from Companies House, posted 18 months ago:

SembcorpPTE

The UK is sleepwalking into our energy pricing and availability being foreign owned and controlled.

As you can see from what I posted about North Tawton, the great majority of the grid infrastructure upgrades are funded by us UK consumers via the daily standing charge. The overseas investor gets a price-matched subsidy from us... which is why they keep building 'centralised' battery storage facilities here.

Let's not be misled by claims of 'grid balancing' and 'frequency stability'. There is no underlying documentary evidence that our grid is becoming inherrantly unstable in the areas where these storage facilities are being located. Neither National Grid nor the DNOs are asking for another Battery Storage bank.

Nor is there any formula by which the requirement for 49.99MWh has been calculated. That's merely the upper limit for G99 approval by the DNO, which they may not unreasonably withhold. 50MWh and above constitutes a major generating station for which different rules apply.

Companies are doing this because they can make enormous long-term profits from the population of the UK.

Let me also point out that the Cowstead storage facility is connected to the 132kV line. The electricity has yet to pass through three transformers on the Distribution Grid before it arrives at a house.

It is the grid constraints which are the problem in this country. My argument is that the energy storage should be biassed towards the end-user, requiring no further transport across the grid when it is required.

Why are we discussing this in a topic about ELV devices? 🤔 

This post was modified 2 years ago 4 times by Transparent

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Majordennisbloodnok
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Posted by: @transparent

...

These are devices in the home which can be run directly from the battery rather than via the 240v AC output of the off-grid inverter.

Why should we want to do that?

  • ...
  • ...
  • It is more efficient to connect devices this way

Each time we pass the stored electricity through a conversion unit to change its voltage/frequency, we lose some of the energy in the process. It makes little sense to take the 48v DC from a Storage Battery, pass it through the Inverter to create 240v AC, and then use a 'transformer plug' to connect a landline phone at 12v DC.

I have a question that, given my general lack of electrical expertise, is probably both basic and dumb. I've been watching the thread for a little while now in the hope someone else would accidentally answer my issue whilst making one of their points, but no dice. As a result, I've no choice now but to risk revealing one or more fundamental misunderstandings in physics, but if I don't ask.....

Basically, I'm wanting to question the assumption I've highlighted above that it is more efficient to connect home devices directly to the battery rather than the standard 240V socket setup. You'll have to bear with me whilst I wander through my reasoning, and I'm sure I've just missed a step somewhere, but if my conclusion is faulty I'd like to find out where I made the mistake.

My reasoning is as follows:

  • It's well known that power is lost as heat (and potentially noise too) as a result of a wire's resistance which, in turn, is related to the wire's length.
  • The relationship with power loss and resistance is based around the square of the current, hence why transforming to high voltage (and relatively low current) is employed by the DNOs to deliver power over any significant distance, then stepping back down to lower voltages for domestic use.
  • The run of electrical cable between my house's distribution board (conveniently just the other side of a wall from the battery) to the furthest socket on the same floor (ours is a bungalow, so not a large house by the standards of many on this forum) is around 20M.
  • I've looked up the efficiency of my inverter (98.7%) and of a typical transformer (98.76% for the power pack for one of my laptops) and calculated the losses due to both the wire and going through inverter/transformer for a 200W input from the battery or solar panels via 240V transmission, and the losses due to the wire and DC/DC transformer for the same input wattage transmitted at either 48V or 12V.
  • I have assumed the DC/DC transformer is right next to the consuming device although with you mentioning elsewhere that you'd have various 48V/12V staging points I suspect that assumption isn't entirely safe and that I'd have to up the estimated power loss for the distance the electricity would have to travel at 12V.

 

Given all that, it seems to me that at its most generous swapping to 48V DC power would lose one ninth of a percent less than the 240V AC setup, but that sending 12V DC over that 20M run would lose 19% of the 200W input. That seems to me a very small power saving for the expense of a second loom of wiring and associated transformers/sockets/peripherals.

Now's the kicker. My calculations (which I'm more than happy to share) say one thing but I can't imagine for a moment that with your self evident experience, @transparent, you'd have missed something like that. I realise there's a value in being able to run off-grid in the event of a power cut (and in the rebalancing effects on the grid you mentioned elsewhere), but in purely financial terms I'm struggling to see a benefit in running off DC unless I've got a major flaw in my understanding.

What am I missing?

105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and inverter
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; suus solum profundum variat"


   
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Transparent
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Wow... thanks for that curve-ball @majordennisbloodnok

I'm going to drastically simplify the discussion for a moment. Others will arrive here later who may not understand our level of technical dialogue.

Within the home we commonly supply electricity to ELV devices using a 'transformer' which looks like these:

Xformer

In the 2020s neither of these is likely to be an actual transformer, but rather an electronic circuit called a Switched Mode Power Supply unit (SMPSU). They take 240v AC in, and provide a DC output at a specified voltage.

Efficiency varies enormously - anything from 85-98% is possible. It isn't mandatory to specify the efficiency, and I tend to distrust the claims for those which are reported in the specifications. After all, most of these transformer plugs/bricks are bought-in by a manufacturer who really trying to sell you something else. The power-supply unit just needs to be 'cheap'.

Let's imagine that we start with a 48v Storage Battery, and then have a chain of 'transformer' units eventually running a 12v WiFi router. To keep the maths easy we will assume that each of these units operates at 93% efficiency:

Efficiency

This simple example isn't unrealistic. I faced this precise situation last month when I decided to run a 48v DC cable from my LiFePO4 storage battery to the server rack. The cable distance is 8m, and then another 1.3m from the 12v DC-DC Converter to the WiFi router, which sits on top of the metal cabinet. I used 10mm2 copper wire.

The SPF5000ES Inverter literature gives the 'line mode' efficiency as 95%, and peak-efficiency of 93%.

The transformer plug which was supplied with the WiFi router from TalkTalk 'ran hot', so I'd previously replaced it with one at 92%. I would've achieved greater efficiency buying a transformer 'block' but that's more expensive.

 

The additional factor which @majordennisbloodnok introduces above is the voltage drop along cables. It's a valid point.

When using DC cabling I will try to retain the higher voltage (48v) for as long as possible, and only convert to the required 12v, 19v, or 24v at the last moment.

Larger (higher current) DC-DC converters are considerably more efficient, but also more expensive than using a small unit for each separate load-device. So there are compromises to be made.

Since the 48v battery is storing cheap electricity, I care less about the power lost in the final LED lighting circuit (for example) than if I was using full-price electricity from the grid.

This post was modified 2 years ago 3 times by Transparent

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Majordennisbloodnok
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It certainly wasn’t meant to be a curve ball, @transparent. It’s just I am in the habit of questioning assertions made without accompanying evidence. Most of the time, I find the evidence is out there and others less ignorant than me would accept the assertion as self evident fact.

In this case, you say I’ve made a valid point, so I’m pleasantly surprised not to be as naive as I expected. However, if that’s so and the difference in efficiency between the 240V approach and the 48V one is small then I’m still wondering what the practical benefit we’re trying to realise by installing a second electrical circuit in a house other than immunity from power cuts - although that is certainly valuable in itself.

105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and inverter
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; suus solum profundum variat"


   
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(@derek-m)
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@majordennisbloodnok

I share your concern that having spent time, effort and expense installing all the necessary equipment, there would actually be little if any actual energy saving. Having looked at the specification for several DC to DC Converters, they would appear to have efficiencies within the 80% to 90% range, so unless you can utilise the 48v DC without any form of conversion, then any possible energy reduction would be minimal.


   
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Majordennisbloodnok
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@derek-m, thank you.

What @transparent is outlining is one of those solutions I really want to be convinced by; it ticks a lot of boxes for me, lessened reliance on grid availability not least of all. The issue of power loss over distance at low voltages, however, gets me thinking it’s a solution that isn’t “one size fits all” and that it would take some careful tinkering, detailed maths and accurate assessment of the performance of components used to ensure gains are realised.

I’m still a novice, though, so still very open to learn from all sides of this discussion.

105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and inverter
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; suus solum profundum variat"


   
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