Loop impedance testing exists because a circuit can be perfectly continuous and still fail to protect anyone. Continuity confirms that a path exists. It does not tell you how much current that path would carry during a fault, and the protective device only operates quickly if the fault current is large enough. A high-resistance connection that beeps happily on a continuity tester can still leave a fault burning for far too long.
What the protective device actually needs
A circuit breaker or fuse clears a fault by carrying the fault current until its mechanism responds. That response is not a fixed time; it depends on the magnitude of the current. At a large multiple of the rating, a breaker’s magnetic element operates almost immediately. At a small multiple, only the thermal element responds, and that can take a long time. So the safety of an earth fault depends on whether the loop can deliver enough current to reach the fast part of the device’s characteristic. Loop impedance is what determines that. Too high an impedance, and a fault that should have been interrupted in a fraction of a second instead persists, holding exposed metalwork at a dangerous potential and heating the fault point.
What makes up the loop
The path a fault current takes is longer than people picture. It runs from the supply transformer, along the line conductor to the fault, through the protective conductor or earth path back toward the origin, and through the supply system to the transformer again. Every part contributes: the transformer winding, the service conductors, the branch circuit conductors, every termination, and the protective conductor itself. A loose lug, a corroded terminal, an undersized protective conductor, or a long run all add impedance. Because the total includes parts of the supply network you cannot see, the only practical way to know it is to measure it at the point of use rather than calculate it from the parts you happen to know about.
How the measurement is made
A loop impedance tester connects across the loop on a live circuit and briefly draws a defined current, measuring how far the voltage sags. Dividing the voltage change by the current gives the loop impedance. This is inherently a live test, which is why the instruments are purpose-built with substantial input protection, and why the work belongs to people trained for energized testing with the protective equipment the installation demands. On circuits protected by a residual-current device, a conventional high-current test will trip it, so instruments offer a low-current or no-trip mode that draws far less and takes longer to reach the same answer. Knowing which mode you are in matters, because the two do not always agree on marginal circuits.
Reading the result correctly
A measured impedance is compared against the maximum permitted for the protective device and the required disconnection time in the applicable rules. Those limits differ by device type, rating, system earthing arrangement, and jurisdiction, and they are tabulated in the wiring regulations that govern the installation rather than being a universal figure. Measurements should also be corrected for conductor temperature, because conductors warm up in service and their resistance rises, so a cold measurement understates the impedance during a real fault. Consult the standard that applies where the work is done, and treat any result close to a limit as a reason to investigate the connections rather than to sign off.
What a poor result is telling you
A high loop impedance is usually a symptom with a findable cause. Compare it against readings from other points on the same circuit and from the distribution board, since the increase between two points isolates the offending section. Common causes are loose or oxidized terminations, a protective conductor that is undersized or broken with a parallel path masking the break, long runs, and poor connections at the origin. Continuity testing of the protective conductor remains valuable alongside the impedance test, because it can identify a break that the parallel paths in a building would otherwise hide. The two tests answer different questions and neither replaces the other. Where a reading has drifted upward since the last inspection, the change itself is the finding, even if the absolute value still passes.
What to check before you act
- Treat loop impedance testing as live work requiring training and protection.
- Select the no-trip mode on circuits protected by a residual-current device.
- Compare readings along the circuit to locate where the impedance rises.
- Check results against the limits in the rules that apply to the installation.
Common questions
Does a good continuity reading mean the earth path is adequate?
No. Continuity shows a path exists. Only an impedance measurement shows whether that path would carry enough fault current to operate the protection quickly.
Why did the tester trip the RCD?
The standard test injects enough current to look like an earth fault. Use the instrument’s low-current mode on protected circuits.
The practical takeaway
Continuity and loop impedance answer different questions, and only the second one relates to whether protection will operate in time. Measure at the point of use, use the no-trip mode where residual protection is present, and compare results against the limits that govern the installation.
References and further reading
- OSHA: wiring design and protection, including grounding and overcurrent devices
- NFPA: workplace electrical safety standard covering energized testing
- CDC/NIOSH: how shock and arc hazards arise during electrical work
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
