Test & measurement
The end of uplift
An uplifting experience is generally considered to be something positive, but not when it comes to loop impedance testing. Here uplift is often a source of annoyance and confusion. But what exactly is uplift, why is it a problem, and what can be done about it? Peter Wade of Megger has the answers
Peter Wade, Megger
overcurrent protection, but also earth leakage protection. For this reason, it is now common to find circuits that include an RCBO or an RCD connected downstream of an MCB. These arrangements can, however, give rise to problems when making non-trip loop impedance measurements. In simple terms, the instrument used to measure the loop impedance may include the internal impedance of the RCD or RCBO in the result. This is usually known as RCD uplift. But does uplift really matter? Let us look at a practical example. The loop impedance of a circuit must be low enough to ensure that the overcurrent protective device operates in the required time. The details are provided in the IEC 60364 standard and its UK equivalent. Taking just one example: for a normal 32 A Type B circuit breaker the loop impedance should be below 1.37Ω. To provide a margin for error, the
R 52
ecent editions of the IET Wiring Regulations have required almost all circuits to include not only
IET Wiring Regulations multiply this by 0.8, giving a maximum acceptable loop impedance of 1.1Ω. The uplift produced by a typical 30 mA RCD can be 0.5Ω or more, which means that if the basic circuit resistance, as would be measured by a continuity test, is 0.7Ω, a measurement carried out with a loop impedance tester would give a result of 0.7Ω (the true circuit impedance) + 0.5Ω (the uplift) = 1.2Ω. On this basis, the circuit would fail the loop impedance test, even though its true impedance is only 0.7Ω, which is well within the prescribed limit. To put it another way – yes, RCD uplift does matter! It is important to note that uplift does not occur with all types of RCD and RCBO. In fact, there is no easy and reliable way of deciding which devices will produce uplift and which will not, and when uplift does occur, the actual amount of uplift can vary greatly between devices that are apparently the same. When they experience RCD uplift,
many users of electrical test equipment suspect that there is a problem with the
Figure 1: A typical RCD circuit
instrument they are using, but this is an incorrect conclusion. In fact, the instrument is correctly measuring the impedance in the circuit under test, under the conditions used in the test. Let us explain. Figure 1 shows a typical circuit for an RCD. Coils A and B create a magnetic flux
in the ferrite core of the RCD. If the currents in the coils are equal, the fluxes produced by the coils cancel out. If the currents are different, however, the fluxes do not cancel, and the sense coil C detects the difference. When the difference becomes large enough, the leakage current sense electronics operates the relay and disconnects the supply.
August 2018 Instrumentation
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102
