Why disconnection time is a safety limit
When a fault connects a live conductor to earth, the protective device has to cut the supply before the resulting touch voltage becomes dangerous. The time allowed for that is not the designer's choice. IEC 60364 sets maximum disconnection times for each type of circuit, and the finished installation has to meet them under fault conditions, not just on paper.
The idea behind the limit is simple. A person touching a faulty appliance becomes part of a circuit for as long as the supply stays live. The longer that lasts, the greater the chance of a harmful current through the body, so the standard pairs a permitted touch voltage with a permitted time. Shorter times allow higher voltages; longer times demand lower ones.
The check turns the usual question around. Instead of asking whether a given loop impedance is low enough in the abstract, it asks what impedance will let the chosen device operate within its time limit. That target value is what the designer compares against the calculated or measured loop impedance.
Two numbers drive the whole exercise: the nominal voltage to earth, usually written U0, and the current the device needs in order to operate within the permitted time. Everything else follows from those two.
What the earth-fault loop impedance is
The loop impedance, Zs, is the impedance of the complete path an earth-fault current takes: from the source through the live conductor to the point of the fault, and back through the protective conductor, the main earthing terminal and the supply earth. It includes the supply, the transformer, the cables and every connection along the way.
The fault current is limited almost entirely by Zs. A lower impedance means a higher fault current and faster operation, which is why the check is written as a maximum allowed Zs rather than a minimum fault current. Two circuits of the same length can behave very differently if one has a larger protective conductor, because that single change lowers Zs.
Two values are used in practice. The calculated value comes from cable lengths and resistances; the measured value comes from an instrument on the completed installation. A design check uses the calculated figure with a temperature allowance, because conductors are hotter during a fault and their resistance rises. The measured figure confirms the design, and a cold measurement has to be corrected before it is compared with the design value.
How the device and the loop work together
The protective device is the other half of the pair. Its job is to turn the fault current into operation within the permitted time, and different devices need very different currents to do that. A miniature circuit breaker with a type B curve operates quickly at five times its rating; a type C curve needs ten times; a fuse follows its own time-current characteristic.
That is why Zs and the device cannot be chosen independently. A circuit that passes with a type B device of a given rating can fail with a type C device of the same rating, simply because more current is needed to trip it in time. The designer works from the device's operating current at the required time, not from its nominal rating.
The effect of conductor temperature
Conductor resistance rises with temperature, and during a fault the copper is hotter than during normal service. A calculation based on cold resistance will flatter the circuit. Applying a temperature allowance, commonly of the order of 1.2 on resistance for the usual conductor temperatures, keeps the design check on the safe side and produces a target Zs that the finished installation should still satisfy.
Where a residual current device is used as the means of fault protection, the required current is far smaller and the loop impedance is rarely the limiting factor. That is why TT installations, where the return path runs through the ground and Zs is high, depend on residual current devices rather than on overcurrent devices alone.
Maximum disconnection times by circuit type
IEC 60364-4-41 gives the times for TN systems. The table below lists the common cases for a supply with 230 V to earth.
| Circuit | Maximum disconnection time |
|---|---|
| Final circuit, 400 V, current up to 32 A | 0.2 s |
| Final circuit, 230 V, current up to 32 A | 0.4 s |
| Distribution circuit | 5 s |
For systems above 400 V the limit tightens to 0.1 s. Distribution circuits are allowed much longer, because the risk of a person being in contact during a fault is lower and the fault current there is usually large enough to operate the device quickly in any case.
If a residual current device provides the fault protection, the relevant time is the device's own operating time rather than the figures in this table, and the check becomes a question of confirming the device and its rating rather than a detailed impedance calculation.
Checking a circuit in four steps
The check is short once the circuit and its device are known.
- Identify the circuit type and the maximum disconnection time that applies to it.
- Find the operating current Ia, which is the current that makes the protective device operate within that time.
- Calculate the maximum loop impedance from Zs(max) = U0 / Ia.
- Compare it with the calculated or measured Zs, and record both figures in the project documentation.
If the actual Zs is higher than the maximum, the circuit fails the check and needs a shorter run, a larger protective conductor, a different device, or supplementary protection. Recording both values also gives the reviewer a baseline, so a later change to the circuit can be judged against the design it was approved on.
Worked example
A 230 V single-phase final circuit is protected by a 32 A type B circuit breaker. For a type B device, five times the rated current is used for the instantaneous check, so Ia = 5 x 32 = 160 A.
The maximum loop impedance is Zs(max) = 230 / 160 = 1.44 Ohm.
If the circuit is 25 m of 2.5 mm2 copper with the protective conductor in the same cable, the calculated Zs at operating temperature lands close to the limit. Shortening the run or increasing the cross-section brings the value down with margin.
A type C device operates at ten times its rating, so Ia = 320 A and Zs(max) = 0.72 Ohm. The same cable that passed with a type B device may now fail, which is why the device curve and the loop impedance are chosen together.
One further point is worth noting. If the circuit sits at the end of a long submain, the impedance of that submain adds to Zs at the far circuit. Checking the final circuit alone can hide a failure that the whole path would expose, so the check is best done on the complete route from the source.
Where the check usually fails
The same situations come up again and again.
- Long lighting or garden circuits at the end of a run, where cable impedance has accumulated.
- Devices chosen on current rating alone, without checking the trip curve against Zs.
- Mixed conductor sizes, where the protective conductor is much smaller than the live conductor.
- Measured values taken on a cold circuit and compared directly with a corrected design figure.
Recording the loop impedance and the device curve together makes the cause of a failure obvious instead of leaving it to guesswork, and it turns a vague complaint about a circuit into a specific fix.
Doing the check in GorkyCAD
In GorkyCAD the circuit, its protective device and the cable lengths come from the same electrical model. The short-circuit and cable calculations use that data, so the loop impedance behind the disconnection-time check is the one on the drawing rather than a figure copied into a separate sheet.
Because cables, devices and boards share a source, a change to one of them updates the others, and the check stays current as the design develops. You can open the browser-based editor, build a small circuit and run the calculation free of charge, with no card required.
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FAQ
What is the loop impedance in simple terms?
It is the total impedance of the path an earth-fault current follows, from the supply through the fault and back through the protective conductor. The lower it is, the more fault current flows and the faster the device operates.
Do I use the measured or the calculated value?
The design check uses the calculated value with a temperature allowance. The measured value confirms the finished installation, and a cold measurement should be corrected before it is compared with the design figure.
Why does the device curve change the limit?
Because the current needed to trip in time depends on the curve. A type B device operates at five times its rating and a type C at ten, so a type C device needs a lower loop impedance for the same rating.
What if the circuit fails the check?
Shorten the run, increase the cross-section of the live or the protective conductor, change the device, or add supplementary protection. The right answer depends on what is causing the high impedance.