# What Should Zs Reading Be?

The BS 7671 tables state that the highest measured Zs should not be more than 0.8.

## What does a high Zs reading mean?

The fault current can be measured from the transformer to the building. The whole electrical cycle is covered. If Zs are too high, fault current can’t flow and the MCB won’t trip. RCDs are more common after the 17th edition.

## What is the ZS value?

The measured value of resistance is R1 + R2.

## What is the difference between Zs and Ze?

What is the name of the thing? The total impedance of the complete circuit is known as Zs. R1 is the circuit line conductor.

## What happens if earth fault loop impedance is too high?

The longer it takes for the circuit protection to work, the lower the fault current is. The impedance of the loop starts and ends at the point of the fault.

## What does a ZS test prove?

Earth continuity is confirmed by a Zs test and by lighting circuits. The schedule of test results should not have calculated values in it. The R1 and R2 confirm the same thing.

## Why does RCD trip when doing Zs?

I’m pretty sure that you still have a load connected somewhere on the RCD protected circuits. If you want to try again, turn off all of the other circuits. Try a ramp test at the same point to see what the RCD is capable of.

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## What is the maximum Zs for an RCBO?

The maximum values of earth fault loop impedance for RCDs can be found within Table 41.5, with the maximum Zs value for a 30mA device being 1667.

## What does a ZS test prove?

Earth continuity is confirmed by a Zs test and by lighting circuits. The schedule of test results should not have calculated values entered. The R1 and R2 confirm the same thing.

## What is a good earth loop impedance?

The value of external earth loop impedance may be different from the average maximum value declared by the electricity distributor. For a TN-C-S system, 0.35 is what it is.

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