Understanding Digital Logic


Understanding Digital Logic

Topic Progress:

Most relay testers stop when the element pick up and timing tests are complete. They’re only testing one-half of the relay, and it’s not even the most important half.

We’ve already shown you how you can incorporate trip logic into your element tests for faster and more productive tests. These videos will show you how to interpret more complicated logic schemes.



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Hi Chris,
In the Understanding Digital logic Part 2 video it is showing the 49T as a normally closed contact. This means that LT1 is always picked up. If the 49T operates it will drop out IN101, yes? If IN101 drops out how do you ultimately receive the DP_2 message? Please advise.

Chris Werstiuk (Administrator) December 4, 2020 at 9:02 am

I’m pretty sure that I answered this question in the videos. The 49T is a fail-safe contact, which means it will close if something goes wrong with the device itself, like a loss of power. Design engineers are supposed to show contact status in their de-energized position on drawings.

So the contact is closed when the 49T is powered down or it operates.

It is open under normal operating conditions.

Hi Chris, a small alignment here…

Chris Werstiuk (Administrator) October 28, 2021 at 7:56 am

Thanks for letting me know.

Do you know of any websites or content that have exercises to strengthen our understanding of logic?

Chris Werstiuk (Administrator) June 10, 2022 at 3:50 pm

Great question.

I wish I did. I have a basic chapter in the The Relay Testing Handbook: Principles and Practice and it will be an online course in the future, but that future gets further away every year.

You can try googling terms like “understand logic gates” or “understand digital logic” or “Understand Boolean Logic”

As I mention in all of my material about logic, DC schematics are just logic with different symbols, so print reading course might be more effective as it will teach you the power system schemes that are being replaced by relay logic, so the relay logic may be easier to understand.

I wish I had better news.

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