Sensing A Heating Element Failure

Hello All, 
I am looking for suggestion on the best way to sense a heating element failure in a consumer appliance product. There are numerous open and closed loop sensors, but they all have signification coast and design complexity issue.  Our heaters are 240VAC  50Amp.  They open when failed so current drops to zero.  I don't need precise measurements.. just would like to know when current is close to 0 Amps when it should be near 50 Amps.  

Thanks, Eric

Hi Eric,
I'm assuming these heaters run continuously and are not really controlled otherwise a thermostatically controlled stop would look like a failed heater aswell but anyway, how about just a simple ammeter, analog or digital. Analog-https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F183189074476Digital- https://www.amazon.com.au/Current-Amperage-Voltmeter-Multimeter-Transformer/dp/B07JB9B2QL/ref=asc_df_B07JB9B2QL/?tag=googleshopmob-22&linkCode=df0&hvadid=341793722567&hvpos=1o5&hvnetw=g&hvrand=3372015645855421561&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9068996&hvtargid=pla-593618260022&psc=1

Eric,I'm assuming that you are trying to build a low cost sensor into the heater.  (otherwise Alan's suggestion is good). The way I would approach this is to use a current transformer in line with the heating element. The primary can be one turn (one wire through a toriod). The secondary should have a large number of turns. This will reduce the current on the secondary. Then you  put a resistor across the secondary to measure voltage.To complicate matters, since you are using AC, the resulting voltage will be AC so you need to do an RMS conversion. Then you just it run to a comparator

I also thought about a Current Transformer Elizabeth, but RMS conversion is overkill... As Eric only wants a go/no go indication all you have to do is sense the AC - just rectify it and feed to a logic gate, or maybe use a missing pulse detector (one 555 timer IC)?

Yes, this seems like a good solution. The Elements will be controlled with a SSR or Contactor.. They are connected with 6 AWG wire, and I really don't want to bring that voltage and power onto my PCB.
Looks like there are PCB mounted Current Sense Transformers where the Supply Wire just needs to pass through the loop. CST206-1A - CURR SENSE XFMR 110A T/H.

yep, Just feeding an input on my MCU. MCU also controls Heater On/Off, so it will know what state the heater is in. 

A current transformer cheaply provides essential safety isolation between the heater circuit and the indicator circuit. The secondary of a current transformer must always have a load.A simple idea: Use a current transformer with turns ratio calculated to give 10 mA of secondary current with a good heater. Connect a pair of LEDs in opposite-polarity parallel across the secondary. LEDs on: heater on. LEDs off: heater off or open.A magnetic-field sensor close to the heater wire might also work.

Eric the  CST206-1A  would be just about ideal.  Bear in mind that you MUST put a resistance across the output of your CT, in this case the datasheet says "Maximum recommended terminating resistance of 1 ohm per turn."  It has 100 turns so 100 ohms max, I would start off with 47 ohms which is a very common value for this resistance, or "Burden" as it is known.   Try it with the rated current through it and see what voltage you get - if you can rectify it and limit it with a resistor / Zener and feed it straight to your MCU input, all good.  If it is too much you can reduce the burden, though a resistor/Zener should be ok.  If it is too small a voltage, try a larger burden up to 100 ohms.  You may just have to amplify it or use a comparator to get a logic signal for your MCU.

Current Transformers are a good way to go, but If you are going to create the interface on a PCB you may want to consider a Hall effect transducer like this range from Allegrohttps://www.allegromicro.com/en/Products/Sense/Current-Sensor-ICs/Zero-To-Fifty-Amp-Integrated-Conductor-Sensor-ICs
Or LEM https://www.lem.com/en/product-list?measurement=52&nominal_val=45-90
There are others. All are inherently isolated. But if you want to avoid the hazards and certification of making your own PCB you may want to consider ready made products like Phoenix’shttps://www.phoenixcontact.com/online/portal/ca?1dmy&urile=wcm:path:/caen/web/main/products/subcategory_pages/Current_transducers_AC_DC_P-14-08-01/51a5e3ce-a9b3-4b45-9526-837bbb8ec32a
Or Weidmuller’s
https://www.weidmuller.com/us/products/electr.../analogue-signal-conditioning/news/act20p-cmt.
Again there are many competitors. All of them are an;org outputs, but there are limit detectors like this one made by my employer
http://emphatec.com/Products/InstaLink/InstaL.../Product.aspx?ProductID=15&DeptID=28&
It closes a contact for currents greater than 1.5Amp- IIRC the CT will work beyond 50A. I could check if you are interested. Again there are competitors.

Eric
As Pete and David have mentioned "Bear in mind that you MUST put a resistance across the output of your CT, "There are products that will do this for you automatically. My employer makes one called CTSM (330123). I believe there are competitors.

Another Hall effect device is the AsahiKasei CZ-3700
And then there is a very similar approach using a technology called Anisotropic Magneto Resistive (try saying that fast 20 times) see the Aceinna MCA1101

Transformer:
https://www.amazon.ca/UEETEK-SCT-013-000-Non-...;sr=8-53orhttp://tinyurl.com/ua4qj9cDepending on what you want to do with the information, feed it to a Raspberry Pi or one of the smaller Arduinos.