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

[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']