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NCEES PE Practice Exam Q. 527 - Relays under DC load


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Hey guys,

I have a question about relays under DC load, after doing NCEES PE practice question 527.

Between a higher DC voltage and a higher DC current, which parameter would affect a relay life cycle more? Would it be higher DC current due to a more sustained arc shortening the relay life?

Thanks!

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@akyip I haven't yet opened the NCEES practice tests I have, but I took a look at the question. I guess you can compare it to the lifespan of DC battery based systems (just talking about electrical components, not the batteries). In my experience certain components of DC UPS battery systems have a lifespan of about 5-20 years. Whereas I have seen hundreds of panels, timeclocks, relays, etc. that are well over 50 years old and still operating.

The solution states this is typically due to the dc current constantly flowing only in one direction which would increase wear on components.

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So I guess it's higher DC current (moreso than DC voltage) that contributes more to decreasing life cycle of relays and other such components, then?

If I'm wrong, feel free to correct me. I just want to try to better understand this, since this is not a common question in the practice exams I have so far...

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8 minutes ago, akyip said:

So I guess it's higher DC current (moreso than DC voltage) that contributes more to decreasing life cycle of relays and other such components, then?

If I'm wrong, feel free to correct me. I just want to try to better understand this, since this is not a common question in the practice exams I have so far...

I believe that is correct. I could be wrong in my explanation as well, but this definitely seems like one of those questions that are reasoning based and based on the correct answer the reasoning is the DC current.

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I would say higher DC current because of the increased heat. I deal with a lot of DC relays that are 50-60 years old and, and usually you're replacing the contact or the coil. Coils like to fail because they get too hot - we see more coils fail in the summer than winter, the ones energized more often fail more often. Contacts need to be replaced because they start to wear out and the contact surface will start pitting or wearing away. Some of this is due to the arc, and sometimes, it's just that its been used so much that the contact is worn out.

Current is what generates the heat in an electric circuit. Voltage doesn't. If you ever do any infrared camera stuff. You can actually see the temperature difference between wires that may be carrying the same voltage, but have totally different currents on them - the wire with higher current will be much hotter.

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  • 1 month later...

A very good article that is worth reviewing before the exam: https://www.electronics-tutorials.ws/io/io_5.html

Here's a relevant snippet:

Quote

 

When the contacts are closed the contact resistance should be zero, a short circuit, but this is not always the case. All relay contacts have a certain amount of “contact resistance” when they are closed and this is called the “On-Resistance”, similar to FET’s.

With a new relay and contacts this ON-resistance will be very small, generally less than 0.2Ωbecause the tips are new and clean, but over time the tip resistance will increase.

For example. If the contacts are passing a load current of say 10A, then the voltage drop across the contacts using Ohms Law is 0.2 x 10 = 2 volts, which if the supply voltage is say 12 volts then the load voltage will be only 10 volts (12 – 2). As the contact tips begin to wear, and if they are not properly protected from high inductive or capacitive loads, they will start to show signs of arcing damage as the circuit current still wants to flow as the contacts begin to open when the relay coil is de-energized.

This arcing or sparking across the contacts will cause the contact resistance of the tips to increase further as the contact tips become damaged. If allowed to continue the contact tips may become so burnt and damaged to the point were they are physically closed but do not pass any or very little current.

If this arcing damage becomes to severe the contacts will eventually “weld” together producing a short circuit condition and possible damage to the circuit they are controlling. If now the contact resistance has increased due to arcing to say 1Ω the volt drop across the contacts for the same load current increases to 1 x 10 = 10 volts dc. This high voltage drop across the contacts may be unacceptable for the load circuit especially if operating at 12 or even 24 volts, then the faulty relay will have to be replaced.

 

 

Question for you guys: what sort of factors (Surface area, contact pressure, etc) do you think affect measured contact resistance?

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8 hours ago, Chattaneer PE said:

A very good article that is worth reviewing before the exam: https://www.electronics-tutorials.ws/io/io_5.html

Here's a relevant snippet:

 

Question for you guys: what sort of factors (Surface area, contact pressure, etc) do you think affect measured contact resistance?

Wouldn't surface area be one of the most contributing factors?  I don't think contact pressure would be (at the bottom of the list I would think).

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On 9/15/2020 at 4:13 PM, DuranDuran said:

Wouldn't surface area be one of the most contributing factors?  I don't think contact pressure would be (at the bottom of the list I would think).

Probably. Surprisingly, I haven't been able to find any equations describing the resistance of contacts. 

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