Electrical installations must be designed to make sure that, in the event of a fault, the resulting short circuit current (prospective short circuit current or PSCC) does not exceed the maximum breaking capacity of the circuit breaker. Otherwise the fault current might destroy the circuit breaker and could lead to catastrophic failure of the installation resulting in damage, injury or even death of personnel.
In addition, even a well-designed installation must be properly implemented with, for example, correctly rated protection devices, conductors of the right cross-sectional area and so on. And it is essential that all of the important parameters that could affect the safety and performance of the installation are verified before it is put into service.
The amplitude of a short circuit current in a circuit is dictated by the vector sum of the internal impedance of the external voltage source VS (secondary winding of the power transformer) and the impedance of conductors: live ZL and protective earth ZPE. The impedance of the short circuit itself is assumed to be zero.
The simplified circuit illustrating this relationship resembles a loop (Fig. 1) so the measurement used to determine the PSCC is referred to as “loop impedance testing” or, in electrician’s jargon, “loop testing”.
The no-load voltage can be easily measured by any voltmeter, but the impedance of the conductors and the voltage source are more difficult to measure. There are further complications resulting from the types of protection devices used in the network. For these reasons, a variety of different test methods are used, depending on requirements and the configuration of the installation.
Fig. 1. Concept of loop impedance
Two-wire high-current method
This method relies on connecting a sizeable current load, Iload, directly between the L and PE conductors (Fig. 2). Because the current is quite high (several amperes) the method is referred to as “high current”. The same test can be carried out between L-L or L-N conductors.
Fig. 2. Two-wire high-current method
This test is easy to use and it gives very reliable results. It should therefore always be used when possible. Instruments like MFT1700 and MFT1800 series provide this type of test with a nominal resolution of 0.01 Ω (Fig. 3). Instruments like MFT425 can measure with an even higher resolution of 0.001 Ω.
When using this test method, it is always a good idea to check the current load applied by the test instrument to ensure that the circuit breaker protecting the installation will not be tripped. For instance, some instruments apply a load exceeding 15 A, which in the worst case could trip a circuit breaker rated at 6 A.
On the one hand a higher current gives more stable readings, but on the other hand there is an increased risk of unwanted tripping of protective devices. Therefore it is not necessarily “better” to use testers with a higher load current.
Fig. 3. MFT1730 in two-wire high-current mode (a) and LTW425 in high-resolution mode (b)
Four-wire high-current method
The two-wire method just discussed can achieve the very high resolution of 0.001 Ω, but with very low values of loop impedance the connection between the probe tips and the components of the installation (for example, the screws in the terminals) becomes significant. The pressure exerted by the operator on the probes can change during the test and thus seemingly unreliable results can be produced. This is normal behaviour and it is not caused by shortcomings in the instrument, but rather by the limitations Mother Nature puts on the physics of surface contact between two metal parts.
Using four-contact voltage sensing instruments can eliminate this problem. The load current is applied in the usual way, but the effect of the variable resistance of the probe tips is eliminated (Fig. 4). This method should be used when the highest possible precision is needed in the measurement. Four-wire sensing instruments are, however, typically more complex and more expensive than their two-wire equivalents.
Fig. 4. NIM1000 utilises the four-wire method with load currents up to 1000 A
Three-wire low-current method
A high-current load cannot be used to carry out tests when an RCD (residual-current device) is present in the system. After all, it is the main job of an RCD to trip as soon as a significant current passes between the L and PE conductors. Therefore the test must be carried out in such a way that the test current is below about 50% of the RCD rating. A typical RCD is rated at 30 mA so, in order not to trip it, the test current must not exceed 15 mA. This is around 1000 times smaller than the load current used in the high-current two-wire test. This means that the measurement is around 1000 times more difficult for the instrument to perform, and averaging over longer test time needs to be used to minimise the influence of noise.
The low-current method most often used requires three connections, because a high current can be still drawn between the live and neutral conductors, so some parts of the impedance can be measured with better resolution, the low current being used only where it is essential. The neutral conductor can be thought of as an auxiliary aid during the three-wire test (Fig. 5). The neutral is required for the test, but its impedance is not included in the final value.
Fig. 5. Three-wire low-current method
The test sequence is performed automatically, and it is normal to hear relays clicking inside the tester when a three-wire test is being performed. Typically, the tester first applies a high-current load between the L and N conductors and measures their combined impedance ZL+N. Then it applies another high-current load between L and N, but senses the voltage drop between the N and the PE conductor. This allows it to measure the impedance of the N conductor alone, ZN.
In the final part of the test, a low-current load (typically below 15 mA) is applied either from L to PE or from N to PE, and the voltage drop is sensed between N and PE, which allows the instrument to measure either ZPE or ZN+PE. Since it now knows ZL+N, ZN and ZPE or ZN+PE) the instrument can calculate ZL+PE, which is the value it displays at the end of the test. This whole process is automatic and various manufacturers use different test sequences. The operator does not have to worry about the details, however, because the final displayed value is always ZL+PE.
It can be seen that the instrument has to work quite hard to derive these values automatically and, because the test has multiple stages, there is considerable scope for noise to affect the measurement, especially when a very low load current is used.
An additional problem is caused by some RCDs, the construction of which introduces a small, but noticeable additional impedance in the loop. In some circumstances, the instrument includes this in the loop impedance measurement. This is in fact correct, because the extra impedance is genuinely present when only a small current load is applied. However, because the extra impedance causes an unwanted increase in the total measured loop impedance value, which should not include the RCD impedance, users often consider the measurement to be incorrect and question the performance of the instrument they are using.
This conundrum is solved in Megger’s new MFT1741 multifunction installation tester, which adopts an innovative measuring technique that is insensitive to the additional impedance of RCDs.
It should be always borne in mind that no “non-tripping” test technique can guarantee that an RCD will not be tripped. Tripping is always a possibility and persons carrying out loop tests must always ensure before proceeding that accidental tripping will not create safety hazards or other problems. For example, when testing installations supplying IT equipment, unexpected loss of power caused by an RCD tripping could lead to unrecoverable data loss.
Two-wire low-current method
Unfortunately, at some points in electrical installations the neutral conductor is not easily accessible. Therefore the three-wire non-trip method cannot be used and, of course, the two-wire high-current method cannot be used either if the circuit is protected by an RCD. Megger is currently the only manufacturer to provide a solution for this problem, with instruments in both the MFT and LTW ranges offering a two-wire, low-current non-trip test option.
Fig. 6. Two-wire low-current method
For this to work, all parts of the test must be carried out with very low current loads, which makes the measurement extremely difficult to perform. The smallest measureable value using this test is 0.01Ω. The test current is typically around 15 mA, which produces a voltage drop of 0.15 mV. This voltage drop has to be measured in the presence of the 230 V mains supply, which means that the difficulty of making this measurement is comparable to measuring the distance from London to New York (which is 5585 km, by the way) with a precision of 4mm. And that really is millimetres not metres!This is the main reason why no competitors offer this method. With two-wire low-current testing, the measured results are acceptable in most circumstances, unless excessive noise is present.
Accuracy and stability
The four different methods described in this article employ different techniques and it should be expected that, in practice, there would be differences in the final results produced by each method. In particular, noise affects each method differently, as shown in the table on page 6. As a rule of thumb, it can be assumed that each “level” of method differs from the next by around an order of magnitude in terms of absolute accuracy and stability.
As can be seen, the four methods do not compete with each other – they are complementary. The test method chosen in any particular instance will depend on the facilities offered by the instrument being used and the configuration of the circuit under test. In general, however, modern instruments such as those in Megger’s MFT and LTW ranges offer facilities to suit all requirements in domestic and small commercial applications, while the NIM1000 will provide reliable results even in high power industrial and distribution systems.