415V immersion on 240V supply

[andrew]

One of the legacy sites I still do a bit of maintenance on has a large sealed thermal store with plate heat exchangers for DHW plus an underfloor loop. It is normally solar powered with a wood burner for high demand.

It has a 3ph delta wired immersion heater provided as back up but, due to c*ck ups in communication there is only a single phase supply.

Obvious solution is to pull the 3ph immersion out and replace with 240V one but nothing has been done about this in 4 years. Spare parts for the wood boiler take about 3 days to source and fit so there is a need for a back up.

I talked with the electrician and pointed out one could supply 240V across two of the immersion terminals and neutral on the third, which would drive ~0.6 of the current and produce 1/3 of the heat of each element in the immersion. So the 9kW 3phase immersion would derate to 2 elements at 1kW each, still better than nothing if the 240V supply is up to it. The suggestion was immediately condemned as unsafe and inappropriate.

I can see that it would be in defensible as it is using equipment in a way for which it isn't designed but unsafe? After all it's using 0.6 the voltage and current it was designed for.

AJH

[Andrew Mawson]

underfloor

demand.

If the six wires that are the two ends of each of the three heating elements are accessable, then they just need wiring so that the elements are in parallel to run off single phase 240v. I did precisely this with a large three phase pottery kiln which was entirely satisfactory (except for the eletricity bill!)

AWEM

[NT]

Surely you could get 2/3 the power into it by connecting the 3 terminals to L,L,N or N,L,L.

Certainly nothing unsafe about it, as long as the single phase supply is all upto it. Clearly not inappropriate either - but when people react like that, any attempt at sense is futile.

NT

[John Rumm]

You would also be reducing the effective voltage, i.e. 240V L to N, rather than 415V L1 to L2 etc.

Too true!

[NT]

yes...

Don't overlook other space heat sources as well. Eg a hob can heat a small or small-medium house temporarily.

NT

[andrew]

This hasn't gone any further, boiler parts haven't arrived and the system is running on the loaned parts.

One other thing crossed my mind, there are 12 flats so I think there is possibly 3ph coming to the site or it would present a bad unbalance to the local grid, or is it common in London to have one phase supply several houses?

At work we have a 100A 3ph supply with several buildings being on separate phases and only the workshop with 3ph. If the flats are the same and given that there may not be enough peak capacity to supply the immersion and all the flats at once; is there a device that would switch off the immersion if the total load on the 3ph supply exceeded 80A and then only switch it back in when it fell below 50A in any one phase?

AJH

[Dave Osborne]

Not really, no. By convention, the solution to your problem is to go to the DNO and ask (pay) for a bigger supply to meet your demand.

Implementing automatic load-shedding on this scale would involve a control panel manufacturer building you a bespoke solution, which would consist of a set of current transformers and an energy analyser to monitor current. The analyser would need to have presetable alarm contacts which would operate a relay when the preset current level was reached in any phase. The relay (along with some front panel override controls, etc.) would operate a contactor which cut out the supply to the heaters.

I would expect a commercial price in the thousands to implement this.

If you had a building management system (BMS) which was monitoring demand, you could leverage this system to load-shed.

[NT]

Or you could get some small scale shop to make one up for several hundred. The technology's simple. It can be a lot simpler than the above.

3 relays with their coils rewound so they switch on at around 50A current (relay winding across a shunt, you dont want all 50A thru the relay). You have one of these in each phase feed. The switch contacts run in series such that you only get power out if all 3 phases are loaded at under say 50A And this power line operates a 3 phase relay, or 3 single ones, to switch the heater feed on and off.

You can adjust switching sensitivity by altering the shunt. Need to do calcs, this scheme may need tweaking.

NT

[Dave Osborne]

There's nothing complicated about the above.

OK, where are you going to get relays with their coils rewound?

so they switch on at around 50A

In what way would "3 single ones" be acceptable?

An interesting solution. However, in broad terms, your solution is no simpler than mine. It has comparable design, construction, installation and commissioning time to my solution and may well (by your own admission) require more tinker time. It would likely save money on parts, I agree. However, if you farmed it out to a contractor, the labour charge would far outweigh the parts costs.

Note that whatever solution you employ, you would need to install a separate consumer unit for the control and the heater, the problem being that you need to monitor demand excluding the heater load, otherwise the system will flap.

[NT]

any electrical wholesaler for the contactors. Enamelled coper wire comes from any electronic parts seller, eg rapidonline

in all ways afaik. Could you be more specific?

4 or 6 relays versus:

It shouldnt require any. Relays typically pull in in the region of half rated voltage/current, and we dont need much accuracy on this system.

yup. With just 4-6 relays its a simple job.

good point. Or we could monitor total current from before the CU, then add a 2nd winding (connected the opposite way) on the sense relay, monitoring the water heater current to cancel its effect out. Either works.

Be better really to not connect the 3 sense relays to each other, then the control for each phase is independant, and the heater is also available more of the time at lower power. The main advantage of this is better reliability, any component failure doesnt affect 2 of the 3 heating elements.

NT

[andrew]

AJH

[terry]

Here on the other side of the Atlantic I have to agree with a previous poster to this thread who wrote ..............

"Many "electricians" are sadly far from fully competent or hide behind the "safety" mantra when in reality there is no risk once the work is completed, tested and covers/enclosures replaced."

Often found during a 40 year career in telecommunications, which often involved power at many voltages and frequencies, that many electricians knew much about 'wiring methods', how many inches of slack, what gauge of wires and how many would fit through something (e.g. conduit) etc. and by 'Rule of thumb' what size of wire or circuit breaker to use etc. Many/most? had little idea of electrical theory, or basic AC/DC principles. They definitely knew more than I did when it came to 'Wiring something'!

But; having been trained in electrical theory, at many frequencies, from audio to microwaves and how to repair and maintain electronic equipment we were a little astounded that there was so little knowledge of 3 phase, or even two phase and the relationships of voltages. And mention mention 400 hertz AC or DC at say 500 amps, or some old time wood/paper pulp mill 25 cycle motors or equipment some of them and it didn't have any impact!

Now many years later my son who is fully trained Instrumentation Technician (Off shore oil/gas with megawatts of power and very high pressures) who has also done some teaching, is finding the same. Many electricians 'may' know rudimentary volts and amps but not much theory! Something missing in the training/technology courses?

Cos we don't see that deficiency here where there are some pretty knowledgeable and practical people ready to advise.

Agree don't see why those three heating elements (control switching aside) can't be paralleled and run at the reduced power. While may need appreciation of whether the unit comes factory wired for 415 volt delta or wye/star, should be do-able safely?

I do recall rigging up some sort of European/UK sourced small potters- craft-maker's kiln (single phase but requiring rewiring for our two hot 120 volt with 240 between them, as used for out hot water heaters and cooking stoves) in some way years ago and apparently it worked for years, safely, with no problems.