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Nuisance tripping of residual current circuit breakers: A practical case
Electric Power Systems Research 106 (2014) 180–187
Contents lists available at ScienceDirect
Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr
Nuisance tripping of residual current circuit breakers: A practical case Carlos Roldán-Porta, Guillermo Escrivá-Escrivá ∗ , Francisco-Javier Cárcel-Carrasco, Carlos Roldán-Blay Institute for Energy Engineering, Universitat Politècnica de València, Camino de Vera, s/n, edificio 8E, escalera F, 2a planta, 46022 Valencia, Spain
a r t i c l e
i n f o
Article history: Received 10 October 2012 Received in revised form 1 July 2013 Accepted 30 July 2013 Available online 19 September 2013 Keywords: Residual current circuit breaker Nuisance tripping Power quality faults
a b s t r a c t Residual current circuit breakers (RCCBs) are often used to provide protection against indirect contacts in a grounded electrical installation. However, there are situations where the use of RCCBs presents certain problems. In some installations, tripping may occur for no apparent reason. Furthermore, it is very laborious and difficult to find the cause of such nuisance tripping in RCCBs. This article presents a case studied by the authors in the La Fe Hospital in Valencia (Spain), where the nuisance tripping of RCCBs became a serious problem. The methodology followed to find the causes and the solutions adopted are described. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Sometimes random tripping of residual current circuit breakers (RCCB) occurs without apparent cause, and after the reconnection of the equipment the RCCB runs properly. There are references to buildings where tripping events have become common during a period of time and then ceased, without the cause being discovered [1]. In some buildings, a common solution is to replace the devices for immunized residual current circuit breakers (SI RCCBs, commercial acronym for an improved type A RCCB of brands as Schneider Electric), but SI RCCBs are much more expensive than type A or AC standard residual current circuit breakers (A RCCBs or AC RCCBs). Among the possible causes of random tripping, the presence of harmonic currents is often cited [2,3]. Some researchers have attempted to establish the relationship between current harmonics and RCCB sensitivity [4–7]. Recently, an interesting work [1] analyzed the influence of harmonics on the value of the current that produces RCCB tripping. The influences of the time constant of an aperiodic current following an earth fault and the response of RCCBs to current pulses were also analyzed [2,7]. In currents with the presence of low-order harmonics, the minimum tripping current varies with harmonic content as well as the phase angle of the harmonic component [1]. RCCB tripping is primarily determined by the peak value of the current. Low-order harmonic components with angle that increase the peak value of the current facilitate the RCCB tripping. In contrast,
∗ Corresponding author. Tel.: +34 963 879 240; fax: +34 963 877 272. E-mail address: [email protected] (G. Escrivá-Escrivá). 0378-7796/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsr.2013.07.020
a desensitization of the RCCB is produced with the presence of high-order harmonics and the minimum tripping current generally increases with increasing harmonic frequency [1]. This article presents an investigation of the causes of false tripping in the La Fe Hospital in Valencia. Section 2 summarizes the basic checks carried out and the laboratory analysis using distorted waveforms. Transients detected in the connection and the disconnection of some receptors is discussed in Section 3. Section 4 details the design of a test system installed at the site that finally enabled the detection of the origin of the problem. Section 5 presents the measurements and results. Finally, some conclusions are drawn in Section 6. 2. Problem statement and basic checks The tripping occurred in a large hospital which became operational in February 2011. During the first days of use there were many incidents with RCCBs. On 21 February some 42 events occurred, 31 events occurred the following day, another 31 occurred the day after that, and 23 events occurred on 24 February. Given the need to find an urgent solution, on the 24 February maintenance staff began replacing AC RCCBs by SI RCCBs and the number of trips was reduced to a normal number of one or two a day. More than 2000 RCCBs were replaced at a significant cost. After the devices were replaced, the authors were asked to investigate the cause of the problem. The first checks were carried out to measure the leakage currents in receivers and various circuits where the trips had occurred. The measured values for leakage currents (between 0.5 mA and 1 mA) were very low and did not explain the tripping of the RCCBs with a nominal leakage current In = 30 mA.
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Some RCCBs were returned to the manufacturer so that their correct operation could be confirmed. It was confirmed that they were working correctly. The ground circuit was tested by measuring a neutral ground loop and a normal resistance value of between 1.5 and 2 was obtained. In conclusion, this preliminary analysis found no obvious cause for random tripping. 3. Effect of high frequency currents, transient receptor connection, and harmonic disturbances in RCCB tripping Assuming that any phenomenon that caused the tripping of the RCCBs should produce an appreciable effect on the RCCB tripping coil, the authors begin to analyze the effect in the tripping coil when the RCCB trips. Analysing the voltage and current in the circuits where the tripping had occurred, the first phenomenon that caught the attention of the authors was the permanent presence of a high frequency voltage component (about 30 kHz). After analyzing in detail the current wave of consumption, a high frequency component was also observed. The loads fed by the circuit were basically electronic lights, TVs, and some medical equipment. Authors assumed that the cause of the high frequency component was the operation of electronic lights in the analyzed circuits. However, this could not cause the RCCBs to trip because they are not sensitive to this amplitude and frequency [3]. The connection and disconnection of some receptors, notably lighting (4 downlights 2 × 26 W, 1 fluorescent 36 W lamp, and the electronic equipment fitted to the bed per room) caused much greater effect in the RCCB tripping coil. Installed RCCBs in the studied circuit were rated by a In = 30 mA, a current in the range of 15–30 mA could produce the trip and the trip must be assured with residual operating currents higher than 30 mA in accordance with IEC. The connections and disconnections of these loads produced high frequency voltage transients in the RCCB tripping coil of several volts in amplitude, with currents of more than 150 mA in amplitude (Fig. 1). Likewise, transient residual currents of more than 500 mA in amplitude were detected. Although these effects are clearly detectable, the high frequency of disturbances (between 30 and 50 kHz) makes RCCBs very insensitive and do not trip. So this phenomenon was not the cause of the tripping of the AC RCCBs. Although low-order harmonics can vary the value of the leakage current of the RCCBs that force the trip, this change is small, and can never explain the tripping of a 30 mA RCCB with 50 Hz residual currents of 1 mA (values detected in the analyzed circuits), regardless the presence of these harmonic components. A test was performed in the laboratory to analyze the influence of harmonics in the tripping of the RCCBs. An AC RCCB tetra-pole, 25 A and In = 30 mA (equal that those installed in the hospital) was tested. Fig. 2 shows the amplitude in function of the frequency of the fault current that caused the tripping of the analyzed AC RCCB. Note that at 50 Hz the tripping current amplitude is 40 mA (corresponding to 28 mA RMS). Authors stated that in currents with frequencies lower than 5 Hz the minimum tripping current amplitude increases. Also it is noted that the minimum tripping current rises gradually when the frequency is increased to 1 kHz and rises significantly after that frequency; conclusive results with those obtained in [1] section IV A. Fig. 2 Amplitude of the minimum current that forced the AC RCCB tripping in function of the frequency Other tests were performed to analyze the effect of the harmonic component addition to the fundamental current that forces RCCB tripping. A laboratory test was performed on the minimum fundamental component of the tripping current for different harmonic
Fig. 1. Transients detected by turning off the lights in a room. The voltage in the tripping coil (a) is over 200 mV peak (50 mV per division) and the residual current (b) exceeds 150 mA peak (50 mA per division).
contents. Two function generators were used to perform the tests. A function generator generated the harmonic component of a certain amplitude and frequency. Another function generator generated the fundamental component with variable amplitude until the tripping occurred. A sliding of the harmonic component on the fundamental was found by minimal inaccuracies in the frequencies generated by the sources. This enabled the authors to detect that the tripping occurred in a ‘critical’ specific phase angle between the fundamental and harmonic component for specific amplitudes of both. The trip occurred when the total amplitude was maximal (as the addition of fundamental and harmonic components). Table 1 shows results for the test performed in the laboratory. For example, a minimum value of 25 mA of amplitude for the fundamental
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Fig. 2. Amplitude of the minimum current that forced the AC RCCB tripping in function of the frequency.
component containing a 150 Hz harmonic component of 15 mA of amplitude is necessary to force the trip. Obtained results were consistent to [1] section IV C. Analysing the results, authors reached two conclusions. Firstly, it was note that the minimum tripping current generally increased with increasing the harmonic frequency. And secondly, it was stated that the tripping of the RCCB depended on the amplitude of the total current (considering fundamental and harmonic component) and the span of time (directly related on the harmonic frequency) that the instantaneous value of the total current exceeded a certain value. Summarizing, transient voltages of 50 s detected by turning off/on the lights (Fig. 1) or harmonics higher than 1.5 kHz do not trip the RCCBs (Fig. 2). So, in the light of all the information contained in [1] and tests performed in the laboratory, the authors concluded that any phenomenon that increases the peak value of the leakage current and has a certain length (about 500 s or more) facilitates the tripping of the RCCBs. However, the RCCBs become less sensitive during shorter disturbances (IEC 61008 states that the RCCB must be immune to a peak wave of 3 kA 8/20 s). As conclusion of this phase it could be stated that the magnitude of the harmonic currents in the building did not justify the tripping of the AC RCCBs that trip occasionally in the hospital. 4. Design of a test system at the site Given the difficulty of detecting the origin of the tripping, a test system was designed for installation at the site. The test system
produces a trip on one auxiliary RCCB to guarantee the service continuity. The test system also included instrumentation to record the supply disturbances associated with the RCCB tripping (Fig. 3). Fig. 3 Layout of the test system adopted to analyze the behaviour of the RCCBs without losses of service continuity. Two RCCBs were used for the test system:
- RCCB1 is tetra-pole, 25 A and In = 30mA. RCCB1 was installed in a line that feeds 9 rooms of the hospital. In this device, the magnetic circuit tripping coil was removed, so that it cannot trip even if activated. In this way disconnections to the loads were avoided. This is an AC RCCB equal as the ones under study that were replaced by SI RCCBs. The disturbances in the analyzed circuit tripped this model but disconnection was avoided by removing its magnetic circuit tripping coil that was installed in the RCCB2. - RCCB2 is bipolar, 25 A and In = 30mA. One pole was connected to a DC source of 9 V, which detected the opening of the contacts (a 10 k resistor was used to limit the circuit current). The other pole was connected to the voltage of phase B of the supply grid. The magnetic circuit tripping coil in this device was taken from the RCCB1, so that this device tripped if any disturbance capable of producing the disconnection occurred in the line feeding the 9 rooms (detected by the RCCB1). This was used to trip the oscilloscopes and record the events.
Table 1 Minimum values of the residual current that forces RCCB tripping for different harmonic component amplitudes. Harmonic amplitude
Harmonic 150 Hz Harmonic 250 Hz Harmonic 350 Hz Harmonic 500 Hz
10 (mA)
15 (mA)
20 (mA)
50 Ipeak (mA)
Ipeak (mA)
IRMS (mA)
50 Ipeak (mA)
Ipeak (mA)
IRMS (mA)
50 Ipeak (mA)
Ipeak (mA)
IRMS (mA)
34 34 38 43
44 44 48 53
25 25 28 31
25 30 35 38
40 45 50 53
21 24 27 29
20 22 32 37
40 42 52 57
20 21 27 30
50 Ipeak is the amplitude of the fundamental component. Ipeak is the amplitude of the total current. IRMS is the RMS value of the total current.
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Fig. 3. Layout of the test system adopted to analyze the behaviour of the RCCBs without losses of service continuity.
Three digital oscilloscopes were used to record the variables related to the trip:
- Oscilloscope 1: measured the residual current in the line feeding the rooms. An AC/DC clamp, with a range of frequencies up to 20 kHz, 5 mA minimum - 20 A maximum and output of 100 mV/1 A with 1 mA of resolution was used. Currents over 300 mA were detected and after measurement it was found that a range of 20 mV/div was adequate (corresponding to 200 mA/div). - Oscilloscope 2: measured the current flowing through magnetic circuit tripping coil of the RCCB1. A 1 ohm resistor was inserted in the circuit of the coil. To measure currents of about 20 mA, a range of 20 mV/div was selected in the device (corresponding to 20 mA/div). - Oscilloscope 3: measured the voltage of phase B of the supply circuit for detecting disturbances in the voltages. The voltage scale was set to 100 V/div.
All three oscilloscopes were tripped with a 9 V DC voltage signal that occurred when the RCCB2 contacts opened. The mode of operation for the oscilloscopes was a single trace pre-trigger of 10 ms, because it was found in the laboratory that there is a delay of about 4 ms from the beginning of the trip until the contacts separate.
5. Obtained measurements and results Several trips in RCCB2 were captured with this method. The time between two consecutive trips was about a week and rarely exceeded 10 days. Results are shown in Figs. 4–6. The curves indicate the presence of a residual current of appreciable value and duration of between 1 ms and 2 ms which trips the circuit. Fig. 7 shows all the voltage waveforms measured in the tripping coil at the same time scale to facilitate a comparison of these three signals when a AC RCCB trips. The length of the voltage acquired of the tripping current (wave 2) was clearly different from a defect that could result in 50 Hz event of 10 ms which corresponds to half
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Fig. 4. Capture of waveforms when oscilloscope 1 trips (x axis: 500 s/div). Channel A (y axis: 5 V DC/div): Voltage signal used to trip oscilloscope 1. Channel B (y axis: 20 mV AC/div): Residual current in the analyzed circuit.
of the wave period (wave 1). The length was also clearly different from the transients produced by switching on/off the room lamps (wave 3). However, the origin of the tripping remained unclear. In a following step, a power quality analyzer (PQA) was added to the test system (Fig. 3) because these phenomena suggested the presence of some disturbance in the supply voltages. The presence of strong disturbances in the grid voltage waveforms was finally detected after programming the PQA to record transients. Peak values increments (of up to 30% of the nominal amplitude) and deformations (of 0.5–2 ms of duration) in voltage waveforms in the three phases in the feeder were detected. It appeared that these disturbances caused the high current discharges that sometimes caused the RCCBs to trip.
Fig. 6. Capture of waveforms when oscilloscope 3 trips (x axis: 1 ms/div). Channel A (y axis: 5 V DC/div): voltage signal used to trip oscilloscope 3. Channel B (y axis: 100 V AC/div): Phase-neutral voltage. A disturbance at the time of the tripping event is detected.
Fig. 8 shows voltages and currents recorded using the PQA during a RCCB trip. Voltage in phase B (L2 in Fig. 8) turns to zero when RCCB2 trips (as shown in Fig. 3 one pole of RCCB2 is connected to phase B voltage). Current peaks in the three phases were also detected. Fig. 9 shows voltage waveforms and Fig. 10 shows currents obtained during a RCCB tripping event using the PQA. When the RCCB2 disconnects the voltage of phase B falls to zero. The fault causes a peak in the currents (Fig. 10). Phenomena detected after recording and analyzing several RCCB tripping events using the PQA were: - Whenever an RCCB trip occurred, high peak voltages were detected in some phases (amplitude values close to 400 V were
Fig. 5. Capture of waveforms when oscilloscope 2 trips (x axis: 500 s/div). Channel A (y axis: 5 V DC/div): Voltage signal used to trip oscilloscope 2. Channel B (y axis: 20 mV AC/div): Voltage in 1 resistor used to measure the current in the tripping coil of the RCCB1. The downward spike corresponds to the tripping of the RCCB.
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Fig. 7. Comparison of 50 Hz voltage waveform (1); perturbation measured during a tripping event (2); and transient perturbation detected by lighting switching (3).
measured versus 325 V corresponding to a nominal voltage of 230 V). - The peak voltage between neutral and ground was also high (over 30 V compared to normal values of 2 V). - Currents in all three phases reached higher peak values (more than 20 A, over normal values of less than 8 A). The conclusion of the research led to the detection of transient problems in power supply voltages which proved to be the cause of the RCCB nuisance tripping. These disturbances produce leakage currents (by capacitive effect and varistor discharges in electronic equipment). Leakage currents produce the tripping of AC RCCBs. Since these are
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transients of short durations, they do not cause the SI RCCBs (an improved type A RCCB) to trip. The type of loads present in the hospital, such as electronic loads, TV sets, electronic lights, and medical equipment with electronic power suppliers and batteries contribute to increasing the transient currents which, in turn, raise the neutral-earth voltages and leakage currents, and so increase the frequency of the tripping events. The short length of the supply line of the hospital transformers from the substation (200 m 20 kV HEPR underground cable) and the electrical situation of the hospital at the end of the line contribute to increasing the values of the surges [8–10]. Operation tasks on the high voltage network can also be the cause of the disturbances detected. Replacement of AC RCCBs by other SI RCCBs solves the problem of nuisance tripping, but also masks the frequent presence of voltage transients with peaks of more than 400 V in nominal voltage grids of 230 V rms, which will likely shorten the life of the receivers and increase the number of failures in these loads. This was the case at the La Fe Hospital in Valencia, where the problems of frequent RCCBs tripping events caused by voltage disturbances were solved by replacing more than 2000 AC RCCBs by SI RCCBs. Authors also propose that an alternative solution would have been ask the utility to improve the power quality of the supply voltage at the feeder of the hospital or install uninterrupted power suppliers (UPS) in the sensible circuits were problems of nuisance tripping occurred. This research enable to suggest the advisability of a preliminary analysis of the quality of the supply voltages to decide the installation of SI RCCB from the design stage in similar facilities thus avoiding the extra cost of the replacement.
Fig. 8. Voltage and current recording for the period 4/10–9/10. On 5/10 at 1.01 p.m. the power failed and at 6.44 p.m. the RCCB tripped (voltage in L2 turns to zero).
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Fig. 9. Disturbance in the voltages produced during an RCCB tripping event on the 8/11/2011 at 7.21 a.m.
Fig. 10. Disturbance in the currents produced during an RCCB tripping event on the 8/11/2011 at 7.21 a.m.
6. Conclusion In this study it has been stated that harmonics components are not directly the cause of the tripping of the RCCBs. RCCB tripping is primarily determined by the peak value of the current. Low-order harmonic components with angle that increase the peak value of the current facilitate the RCCB tripping. In contrast, a desensitization of the RCCB is produced with the presence of high-order harmonics (frequency up 1 kHz), and the minimum tripping current generally increases with increasing harmonic frequency. Also it has been determined that the connections and disconnections of loads produced high frequency voltage transients in
the RCCB tripping coil of several volts in amplitude, with currents of more than 150 mA in amplitude. Likewise, transient residual currents of more than 500 mA in amplitude were detected. Although these effects are clearly detectable, the high frequency of disturbances (between 30 and 50 kHz) makes RCCBs very insensitive and do not trip. There are often strong perturbations in the supply voltages in a building. These disturbances produce leakage currents (by capacitive effect and varistor discharges in electronic equipment). These leakage currents may cause RCCBs type AC and type A to trip–but short duration transients do not cause immunized RCCBs (SI RCCBs) to trip. So, the type of loads present in a facility should be considered because the loads may
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increase the transient currents, raise the neutral-earth voltages and leakage currents, and so increase the frequency of tripping events. Replacement of AC RCCBs by SI RCCBs considerably reduces the problem of nuisance tripping, but also masks the frequent presence of voltage transients and this will likely cause a shortening of life in the receivers, and may increase the number of failures in these loads. This was the case at the La Fe Hospital in Valencia, where the problems of frequent RCCBs tripping events caused by voltage disturbances were solved by replacing AC RCCBs by SI RCCBs. This research enable to suggest the advisability of a preliminary analysis of the quality of the supply voltages to decide the installation of SI RCCB from the design stage in similar facilities thus avoiding the extra cost of the replacement. Acknowledgments This research work has been possible with the support of La Fe Hospital and the Universitat Politècnica de València.
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References [1] I.X. Luo, Y. Du, X.H. Wang, M.L. Chen, Tripping characteristics of residual current devices under nonsinusoidal currents, IEEE Transactions on Industry Applications 47 (2011) 1515–1521. [2] A.C. Liew, Nuisance trippings of residual-current circuit breakers or ground fault protectors of power sources connected to computer and electronic loads, Journal: Electric Power Systems Research 20 (1990) 23–30. [3] Schneider Electric S.A., Specification for residual current-operated circuitbreakers, Tecfoto, Madrid, 1983. [4] Schneider Electric S.A., Residual current operated circuit-breakers without integrated over-current protection for household and similar uses (RCCBs) Part 1: General Rules, Tecfoto, Madrid, 2006. [5] T.M. Lee, T.W. Chan, The effects of harmonics on the operational characteristics of residual-current circuit breakers, in: Proc. of EMPD ‘95, 2, 1995, pp. 548–553. [6] A.M. Featherstone, A.S. Sastrosubroto, An examination of RCD performance during system disturbances (and other relays), in: IEE Colloquium on Safeguarding Industrial Plant During Power System Disturbances, 3, 1989, pp. 1–6. [7] B. Colina, C. Chillet, A. Kedous-Lebouc, P. Mas, Effects of magnetic core geometry on false detection in residual current sensor, Journal of Magnetism and Magnetic Materials 304 (2006) 804–806. [8] S. Czapp, The effect of earth fault current harmonics on tripping of residual current devices, Electrical Review 1 (2009) 196–201. [9] A.G. Expósito, Analysis and Operation of Electric Power Systems, McGraw-Hill, Madrid, 2002. [10] J.A. Martinez, F. Gonzalez-Molina, Surge protection of underground distribution cables, IEEE Transactions on Power Delivery 15 (2000) 54–60.
Contents lists available at ScienceDirect
Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr
Nuisance tripping of residual current circuit breakers: A practical case Carlos Roldán-Porta, Guillermo Escrivá-Escrivá ∗ , Francisco-Javier Cárcel-Carrasco, Carlos Roldán-Blay Institute for Energy Engineering, Universitat Politècnica de València, Camino de Vera, s/n, edificio 8E, escalera F, 2a planta, 46022 Valencia, Spain
a r t i c l e
i n f o
Article history: Received 10 October 2012 Received in revised form 1 July 2013 Accepted 30 July 2013 Available online 19 September 2013 Keywords: Residual current circuit breaker Nuisance tripping Power quality faults
a b s t r a c t Residual current circuit breakers (RCCBs) are often used to provide protection against indirect contacts in a grounded electrical installation. However, there are situations where the use of RCCBs presents certain problems. In some installations, tripping may occur for no apparent reason. Furthermore, it is very laborious and difficult to find the cause of such nuisance tripping in RCCBs. This article presents a case studied by the authors in the La Fe Hospital in Valencia (Spain), where the nuisance tripping of RCCBs became a serious problem. The methodology followed to find the causes and the solutions adopted are described. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Sometimes random tripping of residual current circuit breakers (RCCB) occurs without apparent cause, and after the reconnection of the equipment the RCCB runs properly. There are references to buildings where tripping events have become common during a period of time and then ceased, without the cause being discovered [1]. In some buildings, a common solution is to replace the devices for immunized residual current circuit breakers (SI RCCBs, commercial acronym for an improved type A RCCB of brands as Schneider Electric), but SI RCCBs are much more expensive than type A or AC standard residual current circuit breakers (A RCCBs or AC RCCBs). Among the possible causes of random tripping, the presence of harmonic currents is often cited [2,3]. Some researchers have attempted to establish the relationship between current harmonics and RCCB sensitivity [4–7]. Recently, an interesting work [1] analyzed the influence of harmonics on the value of the current that produces RCCB tripping. The influences of the time constant of an aperiodic current following an earth fault and the response of RCCBs to current pulses were also analyzed [2,7]. In currents with the presence of low-order harmonics, the minimum tripping current varies with harmonic content as well as the phase angle of the harmonic component [1]. RCCB tripping is primarily determined by the peak value of the current. Low-order harmonic components with angle that increase the peak value of the current facilitate the RCCB tripping. In contrast,
∗ Corresponding author. Tel.: +34 963 879 240; fax: +34 963 877 272. E-mail address: [email protected] (G. Escrivá-Escrivá). 0378-7796/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsr.2013.07.020
a desensitization of the RCCB is produced with the presence of high-order harmonics and the minimum tripping current generally increases with increasing harmonic frequency [1]. This article presents an investigation of the causes of false tripping in the La Fe Hospital in Valencia. Section 2 summarizes the basic checks carried out and the laboratory analysis using distorted waveforms. Transients detected in the connection and the disconnection of some receptors is discussed in Section 3. Section 4 details the design of a test system installed at the site that finally enabled the detection of the origin of the problem. Section 5 presents the measurements and results. Finally, some conclusions are drawn in Section 6. 2. Problem statement and basic checks The tripping occurred in a large hospital which became operational in February 2011. During the first days of use there were many incidents with RCCBs. On 21 February some 42 events occurred, 31 events occurred the following day, another 31 occurred the day after that, and 23 events occurred on 24 February. Given the need to find an urgent solution, on the 24 February maintenance staff began replacing AC RCCBs by SI RCCBs and the number of trips was reduced to a normal number of one or two a day. More than 2000 RCCBs were replaced at a significant cost. After the devices were replaced, the authors were asked to investigate the cause of the problem. The first checks were carried out to measure the leakage currents in receivers and various circuits where the trips had occurred. The measured values for leakage currents (between 0.5 mA and 1 mA) were very low and did not explain the tripping of the RCCBs with a nominal leakage current In = 30 mA.
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Some RCCBs were returned to the manufacturer so that their correct operation could be confirmed. It was confirmed that they were working correctly. The ground circuit was tested by measuring a neutral ground loop and a normal resistance value of between 1.5 and 2 was obtained. In conclusion, this preliminary analysis found no obvious cause for random tripping. 3. Effect of high frequency currents, transient receptor connection, and harmonic disturbances in RCCB tripping Assuming that any phenomenon that caused the tripping of the RCCBs should produce an appreciable effect on the RCCB tripping coil, the authors begin to analyze the effect in the tripping coil when the RCCB trips. Analysing the voltage and current in the circuits where the tripping had occurred, the first phenomenon that caught the attention of the authors was the permanent presence of a high frequency voltage component (about 30 kHz). After analyzing in detail the current wave of consumption, a high frequency component was also observed. The loads fed by the circuit were basically electronic lights, TVs, and some medical equipment. Authors assumed that the cause of the high frequency component was the operation of electronic lights in the analyzed circuits. However, this could not cause the RCCBs to trip because they are not sensitive to this amplitude and frequency [3]. The connection and disconnection of some receptors, notably lighting (4 downlights 2 × 26 W, 1 fluorescent 36 W lamp, and the electronic equipment fitted to the bed per room) caused much greater effect in the RCCB tripping coil. Installed RCCBs in the studied circuit were rated by a In = 30 mA, a current in the range of 15–30 mA could produce the trip and the trip must be assured with residual operating currents higher than 30 mA in accordance with IEC. The connections and disconnections of these loads produced high frequency voltage transients in the RCCB tripping coil of several volts in amplitude, with currents of more than 150 mA in amplitude (Fig. 1). Likewise, transient residual currents of more than 500 mA in amplitude were detected. Although these effects are clearly detectable, the high frequency of disturbances (between 30 and 50 kHz) makes RCCBs very insensitive and do not trip. So this phenomenon was not the cause of the tripping of the AC RCCBs. Although low-order harmonics can vary the value of the leakage current of the RCCBs that force the trip, this change is small, and can never explain the tripping of a 30 mA RCCB with 50 Hz residual currents of 1 mA (values detected in the analyzed circuits), regardless the presence of these harmonic components. A test was performed in the laboratory to analyze the influence of harmonics in the tripping of the RCCBs. An AC RCCB tetra-pole, 25 A and In = 30 mA (equal that those installed in the hospital) was tested. Fig. 2 shows the amplitude in function of the frequency of the fault current that caused the tripping of the analyzed AC RCCB. Note that at 50 Hz the tripping current amplitude is 40 mA (corresponding to 28 mA RMS). Authors stated that in currents with frequencies lower than 5 Hz the minimum tripping current amplitude increases. Also it is noted that the minimum tripping current rises gradually when the frequency is increased to 1 kHz and rises significantly after that frequency; conclusive results with those obtained in [1] section IV A. Fig. 2 Amplitude of the minimum current that forced the AC RCCB tripping in function of the frequency Other tests were performed to analyze the effect of the harmonic component addition to the fundamental current that forces RCCB tripping. A laboratory test was performed on the minimum fundamental component of the tripping current for different harmonic
Fig. 1. Transients detected by turning off the lights in a room. The voltage in the tripping coil (a) is over 200 mV peak (50 mV per division) and the residual current (b) exceeds 150 mA peak (50 mA per division).
contents. Two function generators were used to perform the tests. A function generator generated the harmonic component of a certain amplitude and frequency. Another function generator generated the fundamental component with variable amplitude until the tripping occurred. A sliding of the harmonic component on the fundamental was found by minimal inaccuracies in the frequencies generated by the sources. This enabled the authors to detect that the tripping occurred in a ‘critical’ specific phase angle between the fundamental and harmonic component for specific amplitudes of both. The trip occurred when the total amplitude was maximal (as the addition of fundamental and harmonic components). Table 1 shows results for the test performed in the laboratory. For example, a minimum value of 25 mA of amplitude for the fundamental
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Fig. 2. Amplitude of the minimum current that forced the AC RCCB tripping in function of the frequency.
component containing a 150 Hz harmonic component of 15 mA of amplitude is necessary to force the trip. Obtained results were consistent to [1] section IV C. Analysing the results, authors reached two conclusions. Firstly, it was note that the minimum tripping current generally increased with increasing the harmonic frequency. And secondly, it was stated that the tripping of the RCCB depended on the amplitude of the total current (considering fundamental and harmonic component) and the span of time (directly related on the harmonic frequency) that the instantaneous value of the total current exceeded a certain value. Summarizing, transient voltages of 50 s detected by turning off/on the lights (Fig. 1) or harmonics higher than 1.5 kHz do not trip the RCCBs (Fig. 2). So, in the light of all the information contained in [1] and tests performed in the laboratory, the authors concluded that any phenomenon that increases the peak value of the leakage current and has a certain length (about 500 s or more) facilitates the tripping of the RCCBs. However, the RCCBs become less sensitive during shorter disturbances (IEC 61008 states that the RCCB must be immune to a peak wave of 3 kA 8/20 s). As conclusion of this phase it could be stated that the magnitude of the harmonic currents in the building did not justify the tripping of the AC RCCBs that trip occasionally in the hospital. 4. Design of a test system at the site Given the difficulty of detecting the origin of the tripping, a test system was designed for installation at the site. The test system
produces a trip on one auxiliary RCCB to guarantee the service continuity. The test system also included instrumentation to record the supply disturbances associated with the RCCB tripping (Fig. 3). Fig. 3 Layout of the test system adopted to analyze the behaviour of the RCCBs without losses of service continuity. Two RCCBs were used for the test system:
- RCCB1 is tetra-pole, 25 A and In = 30mA. RCCB1 was installed in a line that feeds 9 rooms of the hospital. In this device, the magnetic circuit tripping coil was removed, so that it cannot trip even if activated. In this way disconnections to the loads were avoided. This is an AC RCCB equal as the ones under study that were replaced by SI RCCBs. The disturbances in the analyzed circuit tripped this model but disconnection was avoided by removing its magnetic circuit tripping coil that was installed in the RCCB2. - RCCB2 is bipolar, 25 A and In = 30mA. One pole was connected to a DC source of 9 V, which detected the opening of the contacts (a 10 k resistor was used to limit the circuit current). The other pole was connected to the voltage of phase B of the supply grid. The magnetic circuit tripping coil in this device was taken from the RCCB1, so that this device tripped if any disturbance capable of producing the disconnection occurred in the line feeding the 9 rooms (detected by the RCCB1). This was used to trip the oscilloscopes and record the events.
Table 1 Minimum values of the residual current that forces RCCB tripping for different harmonic component amplitudes. Harmonic amplitude
Harmonic 150 Hz Harmonic 250 Hz Harmonic 350 Hz Harmonic 500 Hz
10 (mA)
15 (mA)
20 (mA)
50 Ipeak (mA)
Ipeak (mA)
IRMS (mA)
50 Ipeak (mA)
Ipeak (mA)
IRMS (mA)
50 Ipeak (mA)
Ipeak (mA)
IRMS (mA)
34 34 38 43
44 44 48 53
25 25 28 31
25 30 35 38
40 45 50 53
21 24 27 29
20 22 32 37
40 42 52 57
20 21 27 30
50 Ipeak is the amplitude of the fundamental component. Ipeak is the amplitude of the total current. IRMS is the RMS value of the total current.
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Fig. 3. Layout of the test system adopted to analyze the behaviour of the RCCBs without losses of service continuity.
Three digital oscilloscopes were used to record the variables related to the trip:
- Oscilloscope 1: measured the residual current in the line feeding the rooms. An AC/DC clamp, with a range of frequencies up to 20 kHz, 5 mA minimum - 20 A maximum and output of 100 mV/1 A with 1 mA of resolution was used. Currents over 300 mA were detected and after measurement it was found that a range of 20 mV/div was adequate (corresponding to 200 mA/div). - Oscilloscope 2: measured the current flowing through magnetic circuit tripping coil of the RCCB1. A 1 ohm resistor was inserted in the circuit of the coil. To measure currents of about 20 mA, a range of 20 mV/div was selected in the device (corresponding to 20 mA/div). - Oscilloscope 3: measured the voltage of phase B of the supply circuit for detecting disturbances in the voltages. The voltage scale was set to 100 V/div.
All three oscilloscopes were tripped with a 9 V DC voltage signal that occurred when the RCCB2 contacts opened. The mode of operation for the oscilloscopes was a single trace pre-trigger of 10 ms, because it was found in the laboratory that there is a delay of about 4 ms from the beginning of the trip until the contacts separate.
5. Obtained measurements and results Several trips in RCCB2 were captured with this method. The time between two consecutive trips was about a week and rarely exceeded 10 days. Results are shown in Figs. 4–6. The curves indicate the presence of a residual current of appreciable value and duration of between 1 ms and 2 ms which trips the circuit. Fig. 7 shows all the voltage waveforms measured in the tripping coil at the same time scale to facilitate a comparison of these three signals when a AC RCCB trips. The length of the voltage acquired of the tripping current (wave 2) was clearly different from a defect that could result in 50 Hz event of 10 ms which corresponds to half
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Fig. 4. Capture of waveforms when oscilloscope 1 trips (x axis: 500 s/div). Channel A (y axis: 5 V DC/div): Voltage signal used to trip oscilloscope 1. Channel B (y axis: 20 mV AC/div): Residual current in the analyzed circuit.
of the wave period (wave 1). The length was also clearly different from the transients produced by switching on/off the room lamps (wave 3). However, the origin of the tripping remained unclear. In a following step, a power quality analyzer (PQA) was added to the test system (Fig. 3) because these phenomena suggested the presence of some disturbance in the supply voltages. The presence of strong disturbances in the grid voltage waveforms was finally detected after programming the PQA to record transients. Peak values increments (of up to 30% of the nominal amplitude) and deformations (of 0.5–2 ms of duration) in voltage waveforms in the three phases in the feeder were detected. It appeared that these disturbances caused the high current discharges that sometimes caused the RCCBs to trip.
Fig. 6. Capture of waveforms when oscilloscope 3 trips (x axis: 1 ms/div). Channel A (y axis: 5 V DC/div): voltage signal used to trip oscilloscope 3. Channel B (y axis: 100 V AC/div): Phase-neutral voltage. A disturbance at the time of the tripping event is detected.
Fig. 8 shows voltages and currents recorded using the PQA during a RCCB trip. Voltage in phase B (L2 in Fig. 8) turns to zero when RCCB2 trips (as shown in Fig. 3 one pole of RCCB2 is connected to phase B voltage). Current peaks in the three phases were also detected. Fig. 9 shows voltage waveforms and Fig. 10 shows currents obtained during a RCCB tripping event using the PQA. When the RCCB2 disconnects the voltage of phase B falls to zero. The fault causes a peak in the currents (Fig. 10). Phenomena detected after recording and analyzing several RCCB tripping events using the PQA were: - Whenever an RCCB trip occurred, high peak voltages were detected in some phases (amplitude values close to 400 V were
Fig. 5. Capture of waveforms when oscilloscope 2 trips (x axis: 500 s/div). Channel A (y axis: 5 V DC/div): Voltage signal used to trip oscilloscope 2. Channel B (y axis: 20 mV AC/div): Voltage in 1 resistor used to measure the current in the tripping coil of the RCCB1. The downward spike corresponds to the tripping of the RCCB.
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Fig. 7. Comparison of 50 Hz voltage waveform (1); perturbation measured during a tripping event (2); and transient perturbation detected by lighting switching (3).
measured versus 325 V corresponding to a nominal voltage of 230 V). - The peak voltage between neutral and ground was also high (over 30 V compared to normal values of 2 V). - Currents in all three phases reached higher peak values (more than 20 A, over normal values of less than 8 A). The conclusion of the research led to the detection of transient problems in power supply voltages which proved to be the cause of the RCCB nuisance tripping. These disturbances produce leakage currents (by capacitive effect and varistor discharges in electronic equipment). Leakage currents produce the tripping of AC RCCBs. Since these are
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transients of short durations, they do not cause the SI RCCBs (an improved type A RCCB) to trip. The type of loads present in the hospital, such as electronic loads, TV sets, electronic lights, and medical equipment with electronic power suppliers and batteries contribute to increasing the transient currents which, in turn, raise the neutral-earth voltages and leakage currents, and so increase the frequency of the tripping events. The short length of the supply line of the hospital transformers from the substation (200 m 20 kV HEPR underground cable) and the electrical situation of the hospital at the end of the line contribute to increasing the values of the surges [8–10]. Operation tasks on the high voltage network can also be the cause of the disturbances detected. Replacement of AC RCCBs by other SI RCCBs solves the problem of nuisance tripping, but also masks the frequent presence of voltage transients with peaks of more than 400 V in nominal voltage grids of 230 V rms, which will likely shorten the life of the receivers and increase the number of failures in these loads. This was the case at the La Fe Hospital in Valencia, where the problems of frequent RCCBs tripping events caused by voltage disturbances were solved by replacing more than 2000 AC RCCBs by SI RCCBs. Authors also propose that an alternative solution would have been ask the utility to improve the power quality of the supply voltage at the feeder of the hospital or install uninterrupted power suppliers (UPS) in the sensible circuits were problems of nuisance tripping occurred. This research enable to suggest the advisability of a preliminary analysis of the quality of the supply voltages to decide the installation of SI RCCB from the design stage in similar facilities thus avoiding the extra cost of the replacement.
Fig. 8. Voltage and current recording for the period 4/10–9/10. On 5/10 at 1.01 p.m. the power failed and at 6.44 p.m. the RCCB tripped (voltage in L2 turns to zero).
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Fig. 9. Disturbance in the voltages produced during an RCCB tripping event on the 8/11/2011 at 7.21 a.m.
Fig. 10. Disturbance in the currents produced during an RCCB tripping event on the 8/11/2011 at 7.21 a.m.
6. Conclusion In this study it has been stated that harmonics components are not directly the cause of the tripping of the RCCBs. RCCB tripping is primarily determined by the peak value of the current. Low-order harmonic components with angle that increase the peak value of the current facilitate the RCCB tripping. In contrast, a desensitization of the RCCB is produced with the presence of high-order harmonics (frequency up 1 kHz), and the minimum tripping current generally increases with increasing harmonic frequency. Also it has been determined that the connections and disconnections of loads produced high frequency voltage transients in
the RCCB tripping coil of several volts in amplitude, with currents of more than 150 mA in amplitude. Likewise, transient residual currents of more than 500 mA in amplitude were detected. Although these effects are clearly detectable, the high frequency of disturbances (between 30 and 50 kHz) makes RCCBs very insensitive and do not trip. There are often strong perturbations in the supply voltages in a building. These disturbances produce leakage currents (by capacitive effect and varistor discharges in electronic equipment). These leakage currents may cause RCCBs type AC and type A to trip–but short duration transients do not cause immunized RCCBs (SI RCCBs) to trip. So, the type of loads present in a facility should be considered because the loads may
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increase the transient currents, raise the neutral-earth voltages and leakage currents, and so increase the frequency of tripping events. Replacement of AC RCCBs by SI RCCBs considerably reduces the problem of nuisance tripping, but also masks the frequent presence of voltage transients and this will likely cause a shortening of life in the receivers, and may increase the number of failures in these loads. This was the case at the La Fe Hospital in Valencia, where the problems of frequent RCCBs tripping events caused by voltage disturbances were solved by replacing AC RCCBs by SI RCCBs. This research enable to suggest the advisability of a preliminary analysis of the quality of the supply voltages to decide the installation of SI RCCB from the design stage in similar facilities thus avoiding the extra cost of the replacement. Acknowledgments This research work has been possible with the support of La Fe Hospital and the Universitat Politècnica de València.
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