This LearnElectrics video looks at Zs measurements that we might make when on site and considers the socalled 80% rule. Something that we should all know and use.

For some, including new learners, there can be concern over which numbers to use when testing and making out certificates and there can be some confusion over what the different figures actually mean.

This is an update to an earlier video on this subject, from a few years ago, and all references made are now to the brown Wiring Regulations book BS7671 : 2018 Amendment 2 : 2022

What is the 80 percent rule?
It is a method used to calculate the measured Zs values for different sizes of circuit breakers and fuses.
The measured values are those that we will measure when onsite and when testing circuits with our test meters and multifunction testers.

So, what does it actually mean? Show me the 80% rule?

And we start with covering a few basics, to clear up any confusion that there may be, and then look at the what, why and how of Zs and the 80% rule.

This is video number 233 and to view all our videos type in LearnElectrics all one word into the YouTube search bar.

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