Switching a 3KW (240V) immersion heater: Solid state or mechanical relays?

[mianchen]

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

[Psi]

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.

[Kjelt]

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.

[Psi]

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.

[SeanB]

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.

[Paul Price]

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.

[Psi]

10/15A? 
He's going to be putting 12.5A+ through it.
I say 20A min

[Paul Price]

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.

[Monkeh]

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.

Please do not advise people to cut corners with 6kA PFC. It is not worth it.

http://www.digikey.com/product-detail/en/T9AV1D12-12/T9AV1D12-12-ND/1128604

See how a little intelligent searching finds appropriate relays at appropriate prices?

This type may make 230V life easier: http://www.digikey.com/product-detail/en/T9AS1D22-12/PB304-ND/254521

[Paul Price]

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.

[Monkeh]

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.

[pickle9000]

30 amp relays are used in a bunch of appliance situations, air conditioners, washing machines and so on. Have a look at these. 

http://www.digikey.com/product-search/en/relays/power-relays-over-2-amps/1049447?k=relay%2030a

[Monkeh]

30 amp relays are used in a bunch of appliance situations, air conditioners, washing machines and so on. Have a look at these. 

http://www.digikey.com/product-search/en/relays/power-relays-over-2-amps/1049447?k=relay%2030a

Bearing in mind that you'll have to filter it down, as a massive number of those are automotive 14VDC or 28VDC relays.

[G7PSK]

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.

[Zero999]

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.

[bingo600]

I'm using Omron G7L-2A-TUB relays (240v/25A and 240v ac drive) for my waterheater
http://www.omron.com/ecb/products/pry/121/g7l.html
http://www.omron.com/ecb/products/pdf/en-g7l.pdf

For now i'm driving the power relay with a : Omron G6B-2214P-US-DC5 (5v relay) , but have considered a small SSR.
http://www.omron.com/ecb/products/pdf/en-g6b.pdf


Ohh i got a full G7L Box of relays cheap off *Bay , it uses Quick-Connect plugs (Like those used in cars) , i'd prob. have gotten screw connect if i had to pay full price.

/Bingo

[SeanB]

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.

[Kjelt]

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. 

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

[johansen]

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)

[5U4GB]

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?

[Zero999]

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. 

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.

[Gyro]

... 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.

[BeBuLamar]

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.

[themadhippy]

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.

[CaptDon]

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!!

[IanB]

... 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.

Why not use a contactor (designed for switching high power loads) rather than a relay?

[bdunham7]

My pool pumps--1HP and 1.5HP 240VAC--are switched by relays that are roughly the size of two sugar cubes and are mounted on the circuit board.  That's a commercial product (Intermatic) and it has been working fine for a decade.  I don't think you need a contactor for a resistive 12A AC load unless you need exceptionally high reliability for safety reasons.  Your overall design should include a thermal fuse if overheating or sticking on would pose any hazard beyond burning out your heating element.  I personally would prefer a relay at those power levels to be mounted separately, like this type:

https://www.mouser.com/datasheet/2/418/8/TE_Connectivity_10112022_ENG_DS_T9A_2-3051264.pdf

A commercial design would probably just use a PCB-mount relay like this:

https://www.mouser.com/datasheet/2/307/Omron_(ENG)G5PZ_E-1843616.pdf

IMO the only justification for screwing around with an SSR is if you have a really high switching rate or a very difficult load. 

[IanB]

A commercial design would probably just use a PCB-mount relay like this:

https://www.mouser.com/datasheet/2/307/Omron_(ENG)G5PZ_E-1843616.pdf

An issue I have observed with such commercial designs is the very small contact area available on the PCB mount pins to carry up to 20 A at mains voltage (either 120 or 240 V AC) through a solder joint. Over time (perhaps due to stresses from thermal cycling) this can become a weak joint, and a weak, high resistance joint subjected to mains voltage will tend to become a hotspot, which will accelerate the weakening until the joint fails.

Good practice says that mains connections should be made with spade lugs and crimped connectors, or with screw terminals. The trend towards putting high power mains loads on miniature PCB mount components was never, in my view, a good trend, unless early failure and planned obsolescence is a consideration.

[wraper]

It depends, if you want to switch it occasionally, say once in 30 minutes, use mechanical relay. If it's supposed to frequently cycle, then use SSR.

[BeBuLamar]

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.

A Crydom D2425 at 240VAC would have the off state leakage current of 10mA. It's not enough for you to feel the heat. Because it's 10mA at 0.2VAC (not the full 240VAC).

[Gyro]

... 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.

Why not use a contactor (designed for switching high power loads) rather than a relay?

Sure, the relays I mentioned have the proper 3mm contact gap and 4kV isolation, so would do the job, but are chassis mount and spade terminal. A DIN rail mounted contactor might be be a better connection and form factor fit. I'm not sure exactly how 5U4GB intends to drive it, but both are available with 240VAC coils if needed.

[BeBuLamar]

... 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.

If the switching isn't frequently then a contactor is fine. For the heater I often have the temperature controller to turn it on and off every second thus a contactor isn't suited for that.

Why not use a contactor (designed for switching high power loads) rather than a relay?

Sure, the relays I mentioned have the proper 3mm contact gap and 4kV isolation, so would do the job, but are chassis mount and spade terminal. A DIN rail mounted contactor might be be a better connection and form factor fit. I'm not sure exactly how 5U4GB intends to drive it, but both are available with 240VAC coils if needed.

[IanB]

Sure, the relays I mentioned have the proper 3mm contact gap and 4kV isolation, so would do the job, but are chassis mount and spade terminal. A DIN rail mounted contactor might be be a better connection and form factor fit. I'm not sure exactly how 5U4GB intends to drive it, but both are available with 240VAC coils if needed.

I see, so it may be a question of physical mounting arrangements? Given that the overall apparatus needs to be in a box or an enclosure with cables coming in and out, and the hardware mounted inside it.

[Gyro]

Pretty much, yes. Once you get to the point of it being a decent sized relay with reasonable (non-PCB) terminations and the same double break 3mm clearance contacts as a contactor, then it does really come down to preferred mounting and termination (screw for a contactor). The main difference with a contactor is flexibility, ability to DIN rail mount in a cabinet with wiring looms, and typically having rather more 'ways', aux contacts etc. [Edit: Coil power is correspondingly higher for a contactor that is closing more contacts of course.]

[BeBuLamar]

I use many DIN rail mounted SSR with integrated heatsink and the form factor is about the same as a contactor. I prefer SSR for heaters.

[rteodor]

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?

SSR's work very well for boiler control. I have a boiler with 2 heaters 800W and 1200W in which I changed the mechanical thermostat with SSR's. And they get a bit warm, maybe 40...50 degC but that is mostly because they do not make proper thermal contact with the boiler plate they are mounted on. For safety I kept the mechanical protection thermostat in place and I recommend the same to anyone regardless if they use a SSR or contactor.

Recently I used the same SSR's and relays to control a 3kW load. That load has two IEC connectors (computer mains power) in parallel probably because those connectors have a 10A rating. At full load they barely get warm. The Recom module is actually warmer that the SSR's.



Now I think maybe I jumped over the horse with two SSR's and two relays in series.

[rteodor]

Found the thermal pics too:

[Zero999]

I don't think anyone has said a solid state relay won't work, just that it will result in poorer efficiency, compared to a mechanical relay.

Example:
The current is 13A, for a 3kW load, at a mains voltage of 230V. Calculate the power loss in a solid state vs mechanical relay:

Solid state relay:
https://www.farnell.com/datasheets/3307636.pdf
On state voltage 1.25V, which doesn't change much with current.

P = VI = 1.25*13 = 16.25W
Coil power  = negligible

Mechanical relay:
https://docs.rs-online.com/8672/0900766b81587c7b.pdf
Maximum contact resistance = 50mΩ. Note this is the maximum, it's likely to be much lower.

P = I²R = 13²*0.05 = 169*0.05 = 8.45W
Coil power = 720mW

Total power  = 8.45 + 0.72 = 9.17W

In reality, the mechanical relay will waste much less power than the above calculation shows. Probably under half, as the contact resistance will be much lower than 50mΩ given on the data sheet.

[Gyro]

The Finder  65.31.xxx  https://docs.rs-online.com/ac53/0900766b816d5220.pdf

Power lost to the environment: Coil + contacts at full rated current (30A): 3.1W

[JustMeHere]

In general.   Resistive loads are easily switched by SSRs and Inductive loads are best switched by mechanical relays. 

[5U4GB]

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.

Good point, thanks!  One thing about the Finder 65's specifically (65.31, 65.61), Digikey is listing them as obsolete and unavailable while element14 gives no indication of problems, is that just a glitch with Digikey?

And a general question about SSR heat dissipation, just for my own education, the data sheets for the Panasonic SSRs I looked at as examples had fairly modest heatsinks even for 30-40A loads and had curves at 10A for no-heatsink use, do they really dissipate that much heat?

[5U4GB]

It depends, if you want to switch it occasionally, say once in 30 minutes, use mechanical relay. If it's supposed to frequently cycle, then use SSR.

For the specific use I have in mind, i.e. controlling when the HWC element runs, the switching will be very occasional, however at least for the 'on' cycle it'll probably have to carry the full load.  I haven't pulled the thermostat apart but given its age I'm assuming it's the bimetallic-disc style, which means the cylinder will mostly likely have cooled to the point where the thermostat contact is engaged so the relay contacts will carry the full load once it's energised.  For the 'off' cycle it's less likely, the heating element would have to be energised when the relay switches which is only the case a small percentage of the time.

[5U4GB]

SSR's work very well for boiler control. I have a boiler with 2 heaters 800W and 1200W in which I changed the mechanical thermostat with SSR's. And they get a bit warm, maybe 40...50 degC but that is mostly because they do not make proper thermal contact with the boiler plate they are mounted on.

There's a thought, mount the heatsink plates of the SSRs against the metal sides of the boiler so even if they do get hot they're helping the boiler along :-).

[Gyro]

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.

Good point, thanks!  One thing about the Finder 65's specifically (65.31, 65.61), Digikey is listing them as obsolete and unavailable while element14 gives no indication of problems, is that just a glitch with Digikey?

And a general question about SSR heat dissipation, just for my own education, the data sheets for the Panasonic SSRs I looked at as examples had fairly modest heatsinks even for 30-40A loads and had curves at 10A for no-heatsink use, do they really dissipate that much heat?

I don't know. RS in the UK have stock of some coil voltages, others are back order (June). Farnell (element 14), UK of course, have most voltages in stock. Presumably you only need one, maybe a spare. I don't know what coil voltage you're looking for.

Regarding SSR dissipation. Zero999 did the sums (reply #37), they look correct to me. Heatsinking requirements depend entirely on thermal resistance to ambient, ambient temperature, and maximum permissible / desirable operating temperature. In the UK, hot water tanks live in the airing cupboard (for airing -not drying- laundry). Inside temperature is significantly above the rest of the house. I don't know your situation there.

The AC1 rating of that particular relay is 7500VA. De-rating by ~50% for a heater load is my sweet spot for a long and happy life. I probably wouldn't be tempted to go for a lower contact current. The cost saving doesn't really make it worthwhile and you typically lose the desirable 3mm contact separation.

Edit: As I indicated in Reply#38, the total power loss of the finder relay is 3.1W at full 30A contact load. Given that the coil contributes 1W, running the contacts at <15A probably takes the total dissipation to <2W. Omron used to do a similar spec relay in the same for factor, I don't know if they still do.

[rteodor]

SSR's work very well for boiler control. I have a boiler with 2 heaters 800W and 1200W in which I changed the mechanical thermostat with SSR's. And they get a bit warm, maybe 40...50 degC but that is mostly because they do not make proper thermal contact with the boiler plate they are mounted on.

There's a thought, mount the heatsink plates of the SSRs against the metal sides of the boiler so even if they do get hot they're helping the boiler along :-).

Oh, but they are already mounted on the boiler .
The first thermal image is the underside of the boiler after about 30 minutes with both heaters on. The SSR's are mounted on the inside so they are not visible.

Anyway its the valve coils that switch the water circuit that, thermal wise are much more problematic than SSR's. Those suckers use about 25W for half of the day and get to about 85 degC.

[5U4GB]

Regarding SSR dissipation. Zero999 did the sums (reply #37), they look correct to me. Heatsinking requirements depend entirely on thermal resistance to ambient, ambient temperature, and maximum permissible / desirable operating temperature. In the UK, hot water tanks live in the airing cupboard (for airing -not drying- laundry). Inside temperature is significantly above the rest of the house. I don't know your situation there.

Yep, saw those, just trying to reconcile them with what the data sheets were implying based on the smallish heatsinks.

In terms of what's called a hot water cupboard here and which I realised as I typed it probably sounds odd to people from other countries, it depends on the cylinder, older/cheaper cylinders would bleed heat like crazy while newer/better ones don't, so the temperature in there will only be marginally higher than ambient.  In fact in some parts of the country, which are located about 50km from the surface of the sun with people audibly crackling and in some cases bursting into flames as they walk down the street, the hot water cupboard may be quite a bit cooler than the outside air (seriously!).

Thanks for the thoughts on the relays, will go for a DIN-rail-mount one unless the electrician says otherwise.

[jonpaul]

For safety and life, use Properly rated industrial contactor.

Most relays are not robust for tat app

The SSR are not as reliable.

j

[Zero999]

For safety and life, use Properly rated industrial contactor.

An immersion heater is not  a safety critical application. If the switch fails short then it'll get too hot and the internal thermal fuse will trip.

[Gyro]

... and a thermostat, normally set to a water temperature of around 60'C. It is normal to leave them permanently powered in some households (just the mandated manual switched isolator on the wall).

[agehall]

Related question - I want to switch a power strip in my virtual pinball cabinet using 5V so that it powers on when the computer inside it powers up. I’m expecting less than 10A constant load on the power strip.

I’ve never felt confident estimating the heat output if I was to put a decent SSR to do this switching for me. Given the discussions here, I get the sense it would be perfectly OK to use a 16-25A SSR for this without a big heat sink. I would probably just screw it to the cabinet body (made out of wood) and cover it on the top to prevent anything from falling on it by accident.

Thoughts?

[Zero999]

Related question - I want to switch a power strip in my virtual pinball cabinet using 5V so that it powers on when the computer inside it powers up. I’m expecting less than 10A constant load on the power strip.

I’ve never felt confident estimating the heat output if I was to put a decent SSR to do this switching for me. Given the discussions here, I get the sense it would be perfectly OK to use a 16-25A SSR for this without a big heat sink. I would probably just screw it to the cabinet body (made out of wood) and cover it on the top to prevent anything from falling on it by accident.

Thoughts?

How about using a master-slave smart power strip, which turns the other sockets on, once the current drawn from the master socket exceeds a certain threshold?
https://www.belkin.com/au/support-article/?articleNum=5368
https://www.amazon.co.uk/Brennenstuhl-Secure-Tec-19-500A-surge-suppressor-Charcoal/dp/B00C24Y4LS?th=1

[BeBuLamar]

For safety and life, use Properly rated industrial contactor.

Most relays are not robust for tat app

The SSR are not as reliable.

j

I worked with heaters in industrial environment and we have SSR for most of them. What is not safe about the SSR? Also the SSR's last a lot longer than contactor. With the SSR I have the temperature controller switches them on and off every second. With contactor I have to extend it to 20 seconds but still the contactor doesn't last very long. Some of the machine I have to switch up to 30kW.

[agehall]

How about using a master-slave smart power strip, which turns the other sockets on, once the current drawn from the master socket exceeds a certain threshold?
https://www.belkin.com/au/support-article/?articleNum=5368
https://www.amazon.co.uk/Brennenstuhl-Secure-Tec-19-500A-surge-suppressor-Charcoal/dp/B00C24Y4LS?th=1

Yeah, that would be the easy and boring solution I guess

[5U4GB]

Thanks for the thoughts on the relays, will go for a DIN-rail-mount one unless the electrician says otherwise.

So I've looked into this a bit more (meaning Googled around while I should have been doing actual work) and for my case where Modbus control would be convenient you can get reputable Modbus-controlled relays like the Iskra BICOM425-40-MODBUS that seem to fit the bill.

Alternatively, use something like this Waveshare 8-channel Modbus relay module to drive standard DIN-rail contactors that do the actual switching.

[BeBuLamar]

Why use Modbus? I think the OP only needs to turn on and off 1 circuit? There will be actually more wiring involved and response time is slower (although it shouldn't matter in this case)

[5U4GB]

For my case I've already got Modbus control in the distribution board for a bunch of other things so using that to drive a contactor is the simplest solution.

My original plan was to use the Modbus relay module I'd linked to, which would in effect function as an interposer relay driving a higher-power contactor like the Finder 65's that people have recommended, with just something like a 1N4148 added as a snubber for the contactor, but it turns out that the same company makes Modbus digital I/O modules set up for driving relays directly, so I can just wire one of those onto the existing Modbus link and drive the contactor straight from that with no additional parts necessary.

[5U4GB]

driving a higher-power contactor like the Finder 65's that people have recommended,

A followup question about this, the usual-suspect electrical suppliers, meaning ones who provide the necessary SDOC, can only provide the 2xNO with a minimum order quantity of five while they have 1xNO, 1xNC in stock in single-unit quantities.  If I source it myself from Farnell or whatever it won't come with an SDOC.  Given that I'm only switching a pure resistive load that heats water rather than, say, turning limb-removing power machinery off, so even if contact welding occurs it's no big deal, is it worth going to the 2xNO supplied-by-user to switch active+neutral or is it good enough to use the 1xNO with SDOC to switch active only?

Also, am I overthinking this? :-).

[BeBuLamar]

If one of the line is neutral then switching only the hot line is OK.

[Siwastaja]

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?

Just use mechanical relay. They are much cheaper, more resilient against grid overvoltage transients, and do not have massive leakage current like SSRs. Switching a few times per day, mechanical relay lasts forever. SSRs are useful for process control heating etc. where you have to switch hundreds if not thousands a time per day.

[Gyro]

A followup question about this, the usual-suspect electrical suppliers, meaning ones who provide the necessary SDOC, can only provide the 2xNO with a minimum order quantity of five while they have 1xNO, 1xNC in stock in single-unit quantities.  If I source it myself from Farnell or whatever it won't come with an SDOC.  Given that I'm only switching a pure resistive load that heats water rather than, say, turning limb-removing power machinery off, so even if contact welding occurs it's no big deal, is it worth going to the 2xNO supplied-by-user to switch active+neutral or is it good enough to use the 1xNO with SDOC to switch active only?

Also, am I overthinking this? :-).

A double pole wall mounted mechanical isolator switch/ cable outlet is required for immersion heaters (well it is in the UK anyway), that should already be in place. A single pole relay / contactor should be sufficient for this 'auxiliary' function. I believe the internal thermostat inside the immersion heater only uses a single pole contact.

[IanB]

ones who provide the necessary SDOC

Curious why the SDOC is required? Is this an Australian thing?

[5U4GB]

Also, am I overthinking this? :-).

It looks like I may have been underthinking it, since I didn't consider failure modes of the control system.  HWCs need to be run up to a certain temperature to kill off bacteria like legionella at least once a day, which means that if the control system for the relay fails in some way you want it to fail closed rather than failing open.  If it fails closed it'll just run as a normal HWC, if it fails open then it won't be brought up to the required bacteria-killing temperature.  So for the specific case of running a HWC you'd want a NC relay, not a NO one.

[5U4GB]

ones who provide the necessary SDOC

Curious why the SDOC is required? Is this an Australian thing?

I'm not an electrician so this is at best an approximation: Without an SDOC being available if required for whatever's being installed the electrician can't issue a COC, or possibly the COC will say "except for this bit" for the item without an SDOC.  I don't know what the implications of that are but they're probably not good if, say, there's an electrical fire.

This stuff and other restrictions on what you can and can't do were brought in to deal with people importing masses of dubious through to downright dangerous electrical items from (usually) China.  You can still as an individual buy suicide kits from Aliexpress and bodge them in yourself but it's become a lot harder for vendors and resellers to do.

[5U4GB]

To put that into context, feast your eyes on this delicacy:



It's an LED downlight with a 1m power cord attached to it.  So you install it into your ceiling and then... ahhh... WTF?

The regulations say that as a home owner you're not allowed to mess with house wiring, so to legally sell something like this it has to be able to be plugged into a standard power outlet.  Which this is, and it's not the seller's responsibility to check how it really ends up being used once you get it home.  The Russian term vranyo applies magnificently.

[themadhippy]

that luminare aint as weird as you'd initially think.Here in the uk a few years back there was a push to use rock roses for ceiling lighting fixtures,it didn't  take off domestically ,but is pretty common in comercial installations.Whats a rock rose? a plug able celling rose.

[IanB]

It's an LED downlight with a 1m power cord attached to it.  So you install it into your ceiling and then... ahhh... WTF?

My garage is fitted out with switched plug sockets distributed across the ceiling. I was therefore able to buy replacement light fixtures fitted with mains plugs, and just plug them into those sockets. Having switched sockets for lighting is a common thing in the USA.

[Gyro]

I've just purchased a Schneider / Drayton Wiser Immersion heater intelligent control switch... https://wiser.draytoncontrols.co.uk/electrical-heat-switch-and-remote-floor-temperature-sensor

It's rated for 16A resistive / heating element load. Just for reference, it uses a Schrack RT314005 relay... https://eu.mouser.com/ProductDetail/TE-Connectivity-PB/RT314005F?qs=8wHch9UpSvZWQ8Bw29FURw%3D%3D

Personally I would have gone for something beefier, like the one I referenced earlier in the thread, [Edit: although at 30k electrical operations at 240V 16A, I suppose at one or two spaced makes/breaks per day it could be considered 'cost effective']