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I would try to make it as clear as possible. I have a relay, and it has five pins( NC, NO, C, and coil) like any basic relay.

Now, I want to apply a logic - when the input (I would say it BMS from now) is 230V ( the mains supply is on) the alarm system should not buzz ( the alarm work at 230V mains supply) and when the input(BMS) is cut off (0V) the alarm system should start buzzing. In short, the alarm should start when the input is 0V and should turn off when the input is 230V mains. I realized the best way I can achieve is by using a relay and 2 different mains supply. The BMS will control the coil of the relay. The second power supply mains will be connected to the NC pin of the relay. And the alarm (hooter) will be connected to the C or common pin of the relay. Now, we consider two cases.

Case 1: The second power supply is On and BMS is also on.

When this is the case, the Coil of relay is high as we have connected BMS to the relay coil. So, NO and C of the relay will be connected to each other. We have nothing connected in the NO pin so the C pin would be low and the alarm will not start. Exactly what we want.

Case 2: The second power supply is On and BMS is cut-off

In this case, the coil of the relay will be cutoff so NC and C of the relay will be connected to each other. The second power supply is connected to the NC pin and as it is shorted to C of the relay, it will start the alarm as it will receive the supply voltage from the second power supply. So, when the BMS is cut-off, the alarm starts. Still, exactly the case we want.

The problem is that the BMS will always be on usually and the relay will always be in the switched state( NO and C of the relay will be connected). It will be only when the BMS is cut-off the relay will come to normal state( NC and C of the relay are connected).

Now, coming to my question. As the relay will be in the switched state forever ( NO AND C is always connected), will it damage my relay? Will the self-life of the relay decrease in that case? I hope you got my question. If it will damage my relay, please read further. If not, I would not look for alternatives.

Please help if you have any alternative for achieving the same thing.

If there is any other way of the same functioning ( the alarm should start when the BMS is cut-off) without using relay or using relay but with different connections, please do let me know. I am open for suggestions and please do help. Thanks for reading as I know I wrote a whole story here. Thank you.


marked as duplicate by Dmitry Grigoryev, Finbarr, winny, Dave Tweed Mar 5 at 12:17

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    \$\begingroup\$ Often a picture is better than a thousand words, so include a schematic of what you intent to make. The text only makes my brain hurt :-( \$\endgroup\$ – Bimpelrekkie Mar 5 at 8:18
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    \$\begingroup\$ As @DmitryGrigoryev mentioned your question seems to be exact duplicate of another question on the site. Instead of posting a whole story, you should ask the question in a general way and first do some research on the problem \$\endgroup\$ – Electric_90 Mar 5 at 9:15
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    \$\begingroup\$ You might want to use 'energised' rather than 'switched' because 'switched continuously' implies it is changing state all the time, which will wear it out. At least the terminology I'm familiar with, switching a relay or breaker means changing it from one state to the other rather than leaving it an energised or de-energised state. \$\endgroup\$ – Pete Kirkham Mar 5 at 12:09

There is no abrasion to the relay, as long as it is not switching. If it is switched or not switched for a long time, does not make any difference to the relay. Only the switching transition itself will reduce its lifetime.


A relay is an electromechanical device, its contacts wear out quickly if the relay is switched on/off rapidly and repeatedly.

In your case, since the relay is constantly on therefore no mechanical movement is there and hence no wearing out of contacts. So, you need not worry about its life.

If you keep the relay in a constant on state (i.e. the coil is always powered) there might be a problem of coil heating up and getting damaged. But this is extremely unlikely and will only happen if you have a low quality relay or apply excess current.

Take a look at your relay's datasheet, it will provide you with much more details on the ideal usage scenario.

Also take a look here.


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