A Brewer mate of mine has a temperature controller which switches a bank of domestic immersion heaters in a hot water tank via solid state relays. One of the elements has just failed to earth (blowing its fuse). The failure seems to have taken out the temperature controller too. I suppose it could have been a spike on the incoming mains, or perhaps local spikes associated with the element failing to earth? This made me wonder whether it would be worth adding a suppressor to the supply to the controller (a basic West 6100 type). I see from ebay that there are plenty of suppressors for washing machines for about a tenner. Would it be worth adding one of these? Or are they more about keeping "machine" noise off the mains?
Sorry, I thought you were implying that the high current on the electronic "contacts" side of the relay was causing a high current or voltage on the "virtual coil" side, the part which is connected directly to the temperature controller. Ultimately the "mains" all comes from the same consumer unit, but there are separate MCBs for each heater and for the temperature controller. So if the heater element failure leads to the busbar voltage bouncing around, that gets fed to the controller via its MCB. Which is why I wondered whether a suppressor in that line might help.
Given that washing machines, dishwashers, tumble dryers are built down to a price in a very competitive market, I wonder why the (presumably competent) designers include them. (I could believe it is driven by RFI).
They mostly don't include them. When they are used they're part of a circuit that does something of use and works, very different to simply sticking one across the mains and being excessively optimistic.
To protect from a short you need a very fast switch, but often when elements start to faile the rise in current is not instant and it can cook devices and make them fail that way.The best thing is some kind of current monitor on each element I'd imagine.
As for suppressors. These are usually just inductive and capacitive filters to try to stop switch transients and in the more modern machines, digital and thyristor noises from going back into the mains. In my view such things should already be in the in any case.
I had a thought, with all the current rubbish powerline adaptors for running networks over mains cables, perhaps one could get a l plug in device to render them useless on your mains phase and put an end to the interference and make people use proper networks. Brian
My first thoughts were that mains front ends of these things are pretty robust and unlikely to pass transients to the electronics. Yes, some domestic mains thermostats use dodgy capacitive droppers that could pass transients but for something a bit more industrial, I'd expect a fully isolated supply of a simple transformer or switched mode type.
You also mention that they are on separately MCB'd spurs so I'd expect fully separate cables so no shared neutral path and earths only commoned at the MCB box. All good practice.
I'd expect the solid state relays to be opto isolated too no path there either but I hope that signal or driver cables are not bundled together with load cables.
Only thing left are the temperature sensors. Are these robust in stainless sleeves or some kind of surface mounted device. I suppose it is possible that the failure to earth on the heater found an alternate path for at least some of the fault current through the sensor, blowing the interface. Do you think that might be possible? Again, sensor cables should not be bundled with power.
Only now looking back at the West 6100 I see it is a pretty small device so they may have cut corners on the supply side but the wide supply range 100-265V AC does suggest a proper SMPS. I see from the manual that they do suggest using a mains isolating transformer to isolate the unit from transients and while I assume you have the mains unit (suitable for a 1:1 mains isolating transformer), the 24V AC unit may be less susceptible if fed from a 240/24V transformer.
Other than looking at the sensor path for the failure, I'd be more inclined to look at transformer isolation on the supply rather than use MOVs as they are pretty unreliable. I suspect that the washing machine suppressors you mention are EMC filters rather than transient suppressors and whilst they are effective in reducing RF interference on other electronics, they are not intended or designed to remove damaging transients. Post a link to one though and I will let you know what I think it is.
The manual has other noise/transient suppression suggestions.
Not bundled together, but I will have a look at the routing, there may be a couple of "crossovers".
MI thermocouple, I think it is in a pocket but not quite sure. Sensor cables not bundled with power. I will check for resistance to earth in case the pocket/clad has a water leak.
That's a very good idea. I have a cheap (< £30) West clone on order for delivery tomorrow for fitting over the weekend. I don't think I have an isolating transformer among my collection of useful stuff, but I'll order one in case it happens again.
Won't bother with them then. I had already speculated that they might be there for RFI (which I guess is part of the standards these days).
It's not otherwise an electrically noisy environment.
Crossovers aren't a problem, you need a significant close coupled path to induce noise (a trivial problem) and even more coupling to induce damage.
Thermocouple input is really low level so there will be a sensitive amplifier in the path which could have popped but you mentioned a total failure (display out) which makes me think again of a supply transient. In a compact can, space is at a premium so as I said, they may have cut corners on input components close to limits so that may be a place to look at again.
Once you are behind an isolating transformer (significant impedance to the mains) I'd be more inclined to stick a MOV transient suppressor on the device's supply terminals. Something like this perhaps:
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Noise & (damaging) transients couple via the same mechanisms so there may still be some useful advice there, just ignore the stuff that you don't think is applicable.
You're welcome, all you have said points to a well thought out installation so it may be a challenge to track down the failure mechanism. Next step might be to carry out an insulation test on the remaining elements to pre-empt the next failure (after disconnecting the relays and new controller of course :-)
Yes, I suppose they generally are used for more industrial type loads or power tools.
Neat and easy mounting but I'm afraid the load regulation is a bit shit (100-83.6 = 16.5%), meaning that on a 240V supply you could see 287V at light loads which is way too high, and that's before you get any tolerances or surges on the incoming supply. Sadly, it looks like we could be more likely to pop your unit than protect it.
Good news however is that your unit appears to have a true switch mode power supply wide input range of 100-240V ac, meaning that you could run the unit from a single 115V winding instead of connecting it to the series connected 230/240V ones. That would give you say 120-144V at the input which would be fine and if it was a supply transient that killed it then you have even more headroom to play with (plus any differential transient would be reduced in half by the transformer).
This is dependent of course on your cheapie replacement also having the same wide input range (100-240V) so do check that.
I'd make sure that the installation is labelled clearly to say that the unit is operating at 115V deliberately and that it must not be used with the windings series connected (240+ V).
The reduction in input voltage probably means that you wont need a MOV but if you decide to belt and brace it then put this lower voltage item on the 115V output of the isolating transformer:
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(150V ac nominal).
If your new unit is 240V only than sadly we should prob leave the iso transformer behind (there are tricks to reduce the supply voltage before the transformer but let's not get too tricky I think). I couldn't find a multitapped isolating transformer on a quick search.
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