240V AC Mains Contactor Rating

[martinwinlow]

Hi,

I'm trying to source suitable contractors for a pair of fairly chunky immersion heaters (IH) for my thermal store (TS). Initially I wanted one 6kW and another 9kW IH - these were supplied with the TS. The 9kW IH has 3 separately wirable elements for a 3 phase/415V supply but (according to the manufacturer) can be wired with all 3 elements in parallel to work with 240VAC single phase.

I want to time these 2 IHs to come on only during Eco7 hours and as their thermostats are rated at 16A I intend to control the 2 IHs with a suitably rated contactor and timer all in a DIN rail box - the TS is about 4m away from the main consumer unit.

As it turns out I cannot find a suitable source of the 2m of 10mm 180 deg heat tolerant flex I need to connect to the 9kW element (I am in a bit of a rush) so I am going to use just the 2 elements in parallel to end up with 2 x 6kW IHs. I am going to use 2 runs of 10mm cable from consumer unit to the TS DIN rail box each with its own MCB (one at 45A, the other at 30A) and use similarly rated contactors so that eventually (if indeed required) I can use the 9kW IH at its full capacity. I can obtain 6mm 180 deg heat tolerant flex suitable for the 2 x 6kW IH connections from contactor to element head.

The thermostat/timer/contactor coil signalling cable will also be 180 deg (silicone) flex at 1.5mm.

So, questions:- 1/ Can anyone see any issue with this setup and 2/ can someone please explain the rating info on contactors so I can try to source suitable contactors more cheaply than my electrician can?

My main issue is that the more common (and therefore cheaper) ones are multi-pole - usually 3 pole. If they are rated say at 11kW, is this per pole or for the 3 phases all working together? If the latter then I guess each pole will have a much lower power limit than 11kW? I gather my 6kW IH needs 6k / 240 = 25A (say 30A) rated contacts and the 9kW, 38A (say 45A) ones. Can I connect the poles together in parallel to get an 11kW 3pole/phase contactor to work with 11kW 1 phase load?

Thanks in advance, MW.

 

[Adam_151]

What size supply do you have, this is a big load to be putting on a domestic supply.

I wouldn't bet much on you being able to terminate a 10mm flex into a device thats probably expecting 5core 1.5mm. Are the neutrals able to be separated or are they permantly connected together (one would hope if this is the case, then they have sized the link suitabily if they are saying its suitable for single phase). The ideal solution if separata-ble would be 3 separate circuits.

I wouldn't put such a big constant load on a domestic consumer unit either, I'd split the tails and install a commerial SPN board for the heating loads, RCDs are not likely to be necessary.

Is this an unvented system?, if so then it needs to be fail safe and incapable of going into boil over mode should a contactor stick closed for example

It looks like you are going to need a commericial/industrial electrician for this. And possibly consider how much water heating load you actually need...

 

[martinwinlow]

Adam_151, Thanks for taking the time to reply.

This is a pic of the head of the IH...

Untitled

• martinwinlow
• 23 Jun 2012
• 1

As you can see, all the L's & N's are separate (although the N's are all linked together currently).

I have an 80A main fuse and although I am getting a bit high in my amps with both IH's running, they are designed for a worst case scenario and only on between 0000 and 0700.

I will discuss the idea of splitting the tails with the electrician as that sounds a bit more efficient in cable (and installation) terms.

The TS is vented with expansion tank and I will be adding a fail-safe, non-resetting, over temp cutout in the contactor coil circuit controlled by a Danfoss ITC-100 Immersion Control Thermostat. The cylinder also has an overflow to drain fitted.

Although it appears over-powered, the idea is that as well as providing the bulk of day-to-day hot water needs, the TS will also provide sufficient output during the Eco7 period to heat the ground floor UFH thus relieving the oil boiler of the need to do this, saving ever-more expensive oil.

The TS also has a coil for connection to the boiler tho I may change this to an air source heat pump in due course. It also has a direct solid fuel connection. So, should the world as we know it come to an end I can put a wood burning stove on the ground floor (got one upstairs already - it's an up-side down house!) and use it to heat the TS alone!

Any info on the contactors? MW

 

[martinwinlow]

OK, couldn't wait for a reply so asked Chint (lots of their stuff on eBay)...

The power (or currant) rating specified is per pole so for example the CHINT NC-0901 (9A 4kW on ebay for under £10) can take 9A on each of its 3 poles. Further, Chint say that if you parallel three poles up for use with a single phase supply you can double the rating (but not triple). So on a single phase the example contactor could take 18A or 8kW across the 3 poles. MW

 

[bernardgreen]

You need to be carefull about the effect of breaking the current when the contactor opens.

The 3 contacts do not all open at exactly the same milli-second which means the last to open will be breaking the whole 18 amps which is twice its rated current.

The life time of a contactor wired with two 9 amp contact in parallel can be considerably shorter than one with a single contact rated at 18 amps and therefor also capable of breaking 18 amps.

 

[flyingsparks]

Also, although the items may be rated sufficent, it definitly doesnt pay to buy cheap brands like chint for loads like this. Go for something like schneider electric, may cost more, but will almost certainly last longer and less likely for heat build up, and fire..

 

[Spark123]

instead of trying to combine the cable into 1, keep the three supplies separate from the contactor onwards then it doesn't matter if the contacts don't break simultaneously.
Still need to look into safety aspect of using a contactor as adam said above.

 

[333rocky333]

Is this an unvented system?, if so then it needs to be fail safe and incapable of going into boil over mode should a contactor stick closed for example
:

Good point, i went to a job last year where the heaters used merlin Gerin CT contactors, when the element had failed it had welded the contactors shut.
The plumber not realising this had replaced the 3 phase 18kw element and a while later after hed gone, the vent pipe was spraying out scalding hot water and they were all baffled why.

 

[SimonH2]

My thoughts reading this would be to split the elements, use the three poles of the contactor (one per element), and feed them with three separate breakers rather than one large one.

That way :
If an element fails you can switch off it's supply and leave the others working
The overcurrent protection is more closely sized to the individual loads.

I'm thinking that if you paralleled up the elements, then potentially you could be pulling about 38A between them - so would need at least a 45A breaker. If one element fails short-circuit to earth at it's (say) mid point, then it would take twice it's normal current and you'd be up to about 50A total with 25A going into the faulty element. A 45A MCB will go a considerable time without tripping at 50A, and a fuse will go indefinitely without blowing. So you could have certain types of fault which wouldn't trip your protection but which would cause a hazardous situation.
Using three separate breakers, you'd probably use 16A MCBs and the individual breaker would trip when loaded to 25A - leaving the other two elements working.

That does leave the potential for a breaker to trip and you not notice a 33% reduction in heating capacity.


One last thought, if you use contactors for both IHs, and you want to avoid the possibility of running both at once (60 to 65A is getting rather close to your main fuse rating) then you can easily wire the coil for one contactor (probably the day one) through the NC contact on the other (night) one. So when the night IH switches on, the day one will switch off (if it was on).
There are also mechanical interlocks, but that's probably overkill for your application.

 

[martinwinlow]

Hi and thanks to you all for all your comments.

I checked my main supply fuse and it is, in fact, 100A so I should not have any issues there.

Had I not bought my 3 core, 6mm, hi temp flex already, using a separate circuit (and contactor pole) for each element would have been a very sensible option as this would remove the 'contactor poles not opening at the same time' issue and spread the separate IH element loads as they should be.

I assume the only disadvantages of using a larger contactor than strictly required are: extra size (probably not), more power wasted on a bigger coil (though minimal), extra cost? If so it would probably pay to use over-rated contactors and leave a note on the control box to change them regularly - every 5 years or so.

On the stuck contactor issue, I will use the contactor's aux contacts to power a 'Contactor Closed' light to warn about that eventuality and use an immersion thermostat for an audible alarm for an over heating scenario. The main consumer unit is a safe distance from the TS so I can isolate the TS IHs and contractors safely if needed.

Again thanks for all the interesting and useful input.

Regards, MW.