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Switching a 3KW (240V) immersion heater: Solid state or mechanical relays?
Posted by
mianchen
on 28 Sep, 2013 11:21
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I'd like to switch and 3KW immersion heater (and other home appliance, but a heater is the most powerful above all) on/off remotely via internet. So far I've managed to control a few LED (retrofit mains powered) lights using normal PCB relays without problem. But when it comes to 3000W load, I think I should ask the more experience engineers here. I don't want to kill myself or set my house to fire if I done something wrong.
Basically my question is for a high current high voltage load, which is better? a solid state relay/triac? or mechanical latching or non-latching relays?
Thanks
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#1 Reply
Posted by
Psi
on 28 Sep, 2013 11:44
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I'd probably use a zero crossing solid state relay, just because they're easy to use, sealed/isolated and switch cleanly during the waveform crossover.
The disadvantage is they're more expensive.
However, a good brand 20A 230V relay will work ok for a 3kW 230V resistive load, like a heater.
Just be careful that the connections are good and tight and have protection on the 230v side to stop anyone touching it (including you while you're working on it)
If your load was 3kW and heavily inductive i wouldn't recommend a mechanical relay. Switching something like that really should be done at zero crossing.
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#2 Reply
Posted by
Kjelt
on 28 Sep, 2013 11:51
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When the heating element is cold it is very low ohm, I wonder if a SSR will like that rush in current peak over time ?
Anyway from personal experience if you go for a SSR don't get them from China from Ebay cause a lot of them are fakes and can not handle a full load.
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#3 Reply
Posted by
Psi
on 28 Sep, 2013 12:01
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Yes, whenever your dealing with high voltage and/or high energy don't go with cheap china parts.
Get it from a reputable company and double the current spec so you have some safety margin.
If you're worried about the low ohm cold current measure it.
It's the best way to find out
Then you can spec a switching device that can handle it.
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#4 Reply
Posted by
SeanB
on 28 Sep, 2013 13:33
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You are forgetting that the relay has to handle a closure into a short circuit and survive long enough for a protective device to open. This generally means you use a small 5A mains rated relay on the board and use that to drive a proper 16A or 25A mains motor rated contactor, with double break contacts so that it will still open even if the one contact has welded shut. The contactor is not a board mount component, it is mounted either on a DIN top hat rail or bolted into place in the enclosure. Driving a heater load is only a little worse than driving a motor, you just do not have a large power factor correction current to deal with, but have the same requirements for isolation distance when contacts are open, double break contacts and overload capacity. these are generally available with various coil ratings, 12VDC, 24 VDC, 24 VAC, 48VDC, 110VDC, 220VAC and 380VAC. Typically not available in a latching configuration, and all will typically draw 3-5W of coil current when energised.
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#5 Reply
Posted by
Paul Price
on 28 Sep, 2013 14:13
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A fairly small single relay rated at 10/15A /250VAC, like the types typically used in microwave ovens is all you need for this job.
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#6 Reply
Posted by
Psi
on 28 Sep, 2013 14:15
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10/15A?
He's going to be putting 12.5A+ through it.
I say 20A min
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#7 Reply
Posted by
Paul Price
on 28 Sep, 2013 14:30
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Yes, 20/25amp at 230VAC relay contact would be more robust, but a 15A might just do the job well enough and might be easier to find and having a lower price.
A check on prices at Mouser/Digikey/RS-Online shows 15A relays approx USD $3, while robust 25Amp relays $130 ea.
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#8 Reply
Posted by
Monkeh
on 28 Sep, 2013 20:23
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#9 Reply
Posted by
Paul Price
on 28 Sep, 2013 20:30
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Nice, smart Monkeh, for finding the relay at $3.50 US ea. I used the same search but did not spend the time to view all results. I don't know how contact current relates to Power Factor Correction?
It would also be a good idea to insert a snubber network (22-ohm 1W in series with .1uF to .33uF /250VAC high current polypropal plastic dielectric low ESR cap.) across the contacts to minimize contact arcing upon switch-off contact opening.
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#10 Reply
Posted by
Monkeh
on 28 Sep, 2013 20:41
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Nice, smart Monkeh, for finding the relay at $3.50 US ea. I used the same search but did not spend the time to view all results. I don't know how contact current relates to Power Factor Correction?
Potential Fault Current, not Power Factor Correction. One does not mess with a supply potentially capable of delivering 6kA+ into a fault. I believe some term it PSCC to avoid the confusion, I rely on context.
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#11 Reply
Posted by
pickle9000
on 28 Sep, 2013 21:06
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#12 Reply
Posted by
Monkeh
on 28 Sep, 2013 21:10
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#13 Reply
Posted by
G7PSK
on 29 Sep, 2013 08:19
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A fairly small single relay rated at 10/15A /250VAC, like the types typically used in microwave ovens is all you need for this job.
A micro wave is normally run for a few minuets at a time and has a fan in there as well. An immersion heater can be on for several hours at a time along with its own switching surges from the thermostat that is built into it.
A separate contactor is the way to go as that will be either next to the tank or in the main feed to the immersion tank heater, and remember if the contactor is fitted at the fuse box there are often two immersion heaters fitted to hot water tanks (top and bottom)so that is 7KW of power to be switched, also you don't want your internet gear in the fuse box or hot water tank enclosure so relay and contactor would be best using a contactor with 24 or 48 volt or even 240 volt coils in order to keep the loss low on a long signal wire.
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#14 Reply
Posted by
Zero999
on 01 Oct, 2013 15:17
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When the heating element is cold it is very low ohm, I wonder if a SSR will like that rush in current peak over time ?
Anyway from personal experience if you go for a SSR don't get them from China from Ebay cause a lot of them are fakes and can not handle a full load.
It's an immersion heater, presumably for heating water so the temperature rise will be under 60
oC which will mean the difference in resistance between hot and cold will be negligible.
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#15 Reply
Posted by
bingo600
on 01 Oct, 2013 17:55
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#16 Reply
Posted by
SeanB
on 01 Oct, 2013 18:36
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Just because the outer case of the immersion heater contacts water does not mean the inside element does not run at red heat. The outer sheath generally will reach over 400C as the water boils and forms steam pockets, this is what makes the kettle rumble during operation.
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#17 Reply
Posted by
Kjelt
on 01 Oct, 2013 20:43
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It's an immersion heater, presumably for heating water so the temperature rise will be under 60oC which will mean the difference in resistance between hot and cold will be negligible.
Measure the amps and you see what I mean.
Just because the outer case of the immersion heater contacts water does not mean the inside element does not run at red heat. The outer sheath generally will reach over 400C as the water boils and forms steam pockets, this is what makes the kettle rumble during operation.
+1
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#18 Reply
Posted by
johansen
on 01 Oct, 2013 21:20
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I don't think i've ever noticed more than a 10% change from cold to red heat on any off the shelf nichrome heater, boiler, radiator, forced air, baseboard heater, etc.
Kanthal A1 for instance: 2e-05 ohm/ohm/°C (0-100°C)
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#19 Reply
Posted by
5U4GB
on 08 Apr, 2024 12:34
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Resurrecting an ancient thread that Google directed me to, since reputable-brand 25 and 40A SSRs from the likes of Digikey are now a few tens of dollars I've been looking at using one to turn power to a hot water cylinder off during peak-rate times, so it'll heat off cheap off-peak power rather than whenever the thermostat tells it to including at maximum rates, in effect emulating the old ripple control that hasn't been used for years. This means it'll only switch on and off once a day, and be carrying 2-3kW of load for brief periods while heating. Is an SSR still the best option for this sort of usage?
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#20 Reply
Posted by
Zero999
on 08 Apr, 2024 12:53
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It's an immersion heater, presumably for heating water so the temperature rise will be under 60oC which will mean the difference in resistance between hot and cold will be negligible.
Measure the amps and you see what I mean.
Just because the outer case of the immersion heater contacts water does not mean the inside element does not run at red heat. The outer sheath generally will reach over 400C as the water boils and forms steam pockets, this is what makes the kettle rumble during operation.
+1
I call BS.
The immersion heater won't get to boiling, let alone red hot. If it did, then the steam bubbles would form an insulating layer, which would drastically reduce the heat transfer.
Resurrecting an ancient thread that Google directed me to, since reputable-brand 25 and 40A SSRs from the likes of Digikey are now a few tens of dollars I've been looking at using one to turn power to a hot water cylinder off during peak-rate times, so it'll heat off cheap off-peak power rather than whenever the thermostat tells it to including at maximum rates, in effect emulating the old ripple control that hasn't been used for years. This means it'll only switch on and off once a day, and be carrying 2-3kW of load for brief periods while heating. Is an SSR still the best option for this sort of usage?
I would recommend a mechanical relay. It would also be more efficient, since a solid state relay will dissipate around 20W, at that power level. Life can be maximised by using a higher current rating, say 20A and connecting two contacts in series, which quenches the arc more effectively.
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#21 Reply
Posted by
Gyro
on 08 Apr, 2024 12:58
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... Is an SSR still the best option for this sort of usage?
I'm not sure an SSR was ever the best option. As Zero999 says, heat dissipation of an SSR will be significant. Go for something like the Finder 65 series mechanical relay, 30A rated with double break contacts. It is smaller, more efficient and
way cheaper than the equivalent SSR.
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#22 Reply
Posted by
BeBuLamar
on 08 Apr, 2024 13:00
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I have no problem switching heaters of various sizes with solid state relay. In rush current on the heater isn't any where as bad as an inductive load. Cold heater has lower resistance but not by much.
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Just because the outer case of the immersion heater contacts water does not mean the inside element does not run at red heat. The outer sheath generally will reach over 400C as the water boils and forms steam pockets,
If thats happening then time to change the thermostat in your immersion heater, 80c is about the maximum it should be reaching.As to the original question properer relay/contractor for me,if the water heater is suppoised to be off i want it off not being kept tepid by the bleed current in an ssr.
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#24 Reply
Posted by
CaptDon
on 08 Apr, 2024 13:31
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I stand with Johansen and Bebu, I had a small horsefarm with a lake that supplied my barn use water. I had an immersion heater in the spring fed pumphouse and an immersion heater in the water trough and the hot vs. cold current draw change is indeed 10% OR LESS across the range of being frozen in ice to the settling temperature of around 55F. I did the measurement at 20F turn on vs. running 20 minutes later. So huge inrush is a non-issue. Grounding is by far the Most Important issue!! Often you can find a 2 to 3 volt difference between local earth ground and the mains ground, mainly on systems where neutral and ground are tied together somewhere. Hooved animals standing on wet ground can sense that tiny voltage referenced to the damp / wet earth they are standing on and may refuse to drink from a heated water source!! A 25A rated solid state or mechanical relay will be fine. A.C. crosses zero so don't add a shitty snubber as suggested. A 22 ohm 1 watt resistor would nearly explode if the series capacitor shorted and we don't need any unexplained fires in the middle of the night!! Spade lug push on terminals are trash for currents above 8 amps and I don't care what the ratings say they can do. I have seen far to many burned connections at 10 amps!!! Those lugs develop resistance over time and fail in bad ways!! Get relays (either type) with screw down compression clamp terminals. DON'T GO CHEAP!!! Spend a few extra dollars for an installation that will last longer than you do!! Remember, EMP energy from local lightning strikes has a huge effect on the electrical system of remote out-buildings!! Install sensible fuses, breakers and ground fault devices!!