What qualifications do you need to sign off mains wiring

Hi All

I've been making a Central Heating monitor which monitors temperatures. Works great, but I'd alos like to monitor the state of pumps, motorised valves etc.

The electronics for this is very simple and to isolate the low voltage computer from the mains supply for the pump, I'd use a simple 240v AC relay.

However, I assume it would be illegal for me to connect it up as I would need to be a 'competent person'.

What qualification would I need to legally connect a relay to my CH wiring?

Cheers

Steve

Dunno, but I suspect it has nothing to do with being a 'competent person' as in part P or member of a certified scheme. Even they only install components that are to a certain certified standard.

I assume (but might be wrong) when you say

The electronics for this is very simple and to isolate the low voltage computer from the mains supply for the pump, I'd use a simple 240v AC relay.

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that the relay coil is 240 volt and you are switching a low voltage signal to your 'low voltage computer'. Correct? if so and it's in your own home then I don't see any 'legal' issues, but you may void the warranty on your boiler, and you will be feeling a bit silly if you burn your house down.

Don't take the above the wrong way, it's just not good practice to say 'good luck, let us know how you got on' as it might encourage others.

Have you investigated CT's (current transformers) instead of relays as a monitoring device? Completely isolated and you would also be able to monitor how much power your pump is using, not just if it is on or off.

Whereabouts are the boiler/pump/valves?

Hi both

Thanks for your replies. At the moment I'm developing this for myself, but with one eye on the fact it might be saleable in the future. What I want to do is monitor very aspect of the system and then graph it.

Do you know how you get a design certified?

Yes, you have the idea correct. The 240v coil would, for example be wired in parallel with a pump and the contacts would ONLY connect to a separate 5v power supply/resistor/computer etc.

I haven't taken your comments the wrong way as I'm on the forum for advice from others and will take that advice in good spirit - unless you tell me to take a jump in the lake. Mind you it is quite hot

I did chew over a current transformer, as it's non-intrusive apart from the fact that someone would have to disconnect a single wire to pass through the coil. This would be much more reliable than a relay. I could rectify the output, smooth it and then measure the voltage. This is how a clamp meter works, isn't it?

Hm you've made me think again now.... I'm not a great fan of relays, being mechanical and all, but a solid state device would be good.

I've got some fine enamelled wire, I could have a play winding some coils and see how it works.


Hi ban-all-sheds

The boiler/pumps/valves are mostly in the airing cupboard, with an accelerator pump in the loft and a motorised valve by the woodburner.

Please don't ban my shed. It's my home!

Cheers

Steve

Stick with the relays or look at optical isolation devices, don't go near current transformers!! You can end up with nasty high voltages on them!

Hi Spark123

Good point. A likely measurement would be for a pump which I think are around 100 watts, or just under half an amp.

I guess the worst case scenario would be a manis short circuit, which would then induce a much higher voltage into the coil until the fuse/circuit breaker went.

This could be clamped by adding a couple of zener diodes on the output. These diodes do nothing below their rated voltage, but present a load when the volatge goes higher than their rating.

How about this?

View media item 2383
I used to know the formula for working out induced voltage/number of turns etc, but that was 3 decades ago. Anyone help me out?


Cheers

Steve

picitup said:

Hi both

Thanks for your replies. At the moment I'm developing this for myself, but with one eye on the fact it might be saleable in the future.

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To households?

What's it going to do for them except interest those more interested in technology for its own sake?

For commercial/industrial you should probably search out what's already available in terms of monitoring & controls.

Do you know how you get a design certified?

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Dunno, but if you're talking about the certifications for getting an actual product onto the market they are probably onerous and costly. If you believe you have got a novel and viable product design you might be better off seeking a partner with manufacturing skills.

The boiler/pumps/valves are mostly in the airing cupboard, with an accelerator pump in the loft and a motorised valve by the woodburner.

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Bear in mind that installing even ELV control/signalling cables in a bathroom is notifiable...

If you're considering a marketable design you ought to look into wireless connections - (a) to avoid cable installation issues, and (b) because people might want to use a wirelessly connected computer.

Please don't ban my shed. It's my home!

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Ah - I mean this:



not this:

Yes, to households. It's been an invaluable tool for me in optimising my woddburner/heat store/gas boiler combination. I've diagnosed a couple of faults with the system just using temperature data and it would be even better with state data ie pumps on/off etc.

I had a problem with the neutraliser reverse thermosyphoning up the gas boiler. By holding the pipes, I thought the flow was forward, but the temperature data showed me it was opposite to this as the gas boiler return heated up before the return, but then the trend reversed.

I won't bang on about it, but it's been a great tool so far.

I'd still like to find out how much and how to get something certified. You're probably right though, too onerous and too expensive!

I'm not a great fan of wireless as it's very prone to interference. This system uses a "1-wire" network so you just run a single cable and connect devices where you like.

So I have a single CAT5 UTP network cable running up with the stove flow, into the loft, across the loft and down into the airing cupboard with sensors dropping off at interesting points.

Food for thought....

Steve

How about using a pilot lamp and a light dependant resistor?
Or a small circuit to drop the mains to TTL level driving the source side of an opto isolator? You should be able to make one pretty easily using capacitors, resistors, diodes and zener diodes etc. Like those really really cheap power supplies - an opto isolator only needs somewhere in the region of 20mA. The other side of the isolator is what you use to connect to the PC, therefore if something should go wrong on the mains side you aren't going to be feeding mains into your PC! Will blow up your 50p opto isolator tho

edit
Pinched the figures for this from a PIC controller, but should do the trick:

The figure for the opto isolator source resistor will depend on each case.

picitup said:

I'm not a great fan of wireless as it's very prone to interference.

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OK, so you build in a small embedded system, and run Linux, or one of the specialised small footprint embedded OSs. That talks TCP/IP over 802.11 to your wireless router or direct to the monitoring PC. Should be resilient enough to avoid and/or cope with interference. Make sure you get the security provisions right though - you won't want the spotty lad across the street to hack in and start controlling your heating system.

Get the right gateways on the PC and you could do really cool things like monitor & control your heating via a PDA or iPhone etc anywhere in the world. SMS alerts when the woodburner needs more wood. All kinds of stuff.

ban-all-sheds said:

SMS alerts when the woodburner needs more wood.

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Hi Sorry it's been so long in reply - I've just got a proper job in London so I've been tied up with that.

It may be I get to do my Part P with the company so that will help.

Thanks to all for your replies.

Steve