Made 2 Measure: what’s the reason for high Zs readings? | Megger

The technical team at Megger answer another commonly asked question.

You ask:

When I was carrying loop impedance tests on a recent job, I got high Z_S readings on RCD protected circuits, even though the Z_E and R₁+R₂ readings were as expected. What’s the reason for this?

We answer:

RCDs and RCBOs usually incorporate a tiny transformer with a winding connected in series with the live conductor, and another winding in series with the neutral conductor. This transformer detects the difference between the currents in the live and neutral conductors, which indicates the presence of earth leakage. As the transformer windings are in series with the supply, most loop impedance testers can sometimes include this impedance of the windings in the loop impedance result. This is called “RCD uplift” and with a 30mA RCD or RCBO, it can increase your Z_S reading by as much as 0.5 Ω compared with the expected or calculated value. Fortunately, there’s now an easy solution to this problem. Megger’s latest MFT1741 multifunction installation tester uses a new loop test technique that is immune to the effects of RCD uplift. The MFT1741 also has a unique new confidence display that helps to reduce the time needed for loop tests on electrically quiet circuits, as well as making life much easier when you’re carrying out loop tests on noisy circuits.

Find out more about Megger’s equipment by clicking here