Sensitivity of RCDs to disturbances

From Electrical Installation Guide

In certain cases, aspects of the environment can disturb the correct operation of RCDs:

  • “nuisance” tripping: Break in power supply without the situation being really hazardous. This type of tripping is often repetitive, causing major inconvenience and detrimental to the quality of the user’s electrical power supply.
  • non-tripping, in the event of a hazard. Less perceptible than nuisance tripping, these malfunctions must still be examined carefully since they undermine user safety.

Main electrical disturbance types

Permanent earth leakage currents

Every LV installation has a permanent leakage current to earth, which is either due to:

  • Unbalance of the intrinsic capacitance between live conductors and earth forthree-phase circuits or
  • Capacitance between live conductors and earth for single-phase circuits

The larger the installation the greater its capacitance with consequently increased leakage current.

The capacitive current to earth is sometimes increased significantly by filtering capacitors associated with electronic equipment (automation, IT and computer- based systems, etc.).

In the absence of more precise data, permanent leakage current in a given installation can be estimated from the following values, measured at 230 V 50 Hz:

  • Single-phase or three-phase line: 1.5 mA /100m
  • Heating floor: 1mA / kW
  • Fax terminal, printer: 1 mA
  • Microcomputer, workstation: 2 mA
  • Copy machine: 1.5 mA

Since RCDs complying with IEC and many national standards may trip between 0.5 IΔn and IΔn, for residual currents higher than 0. 5 IΔn it is recommended to divide the installation by sub-division of circuitsto avoid unwanted tripping.

For very particular cases, such as the extension, or partial renovation of extendedIT-earthed installations, the manufacturers must be consulted.

High frequency components (harmonics, transients, etc.), are generated by computer equipment power supplies, converters, motors with speed regulators, fluorescent lighting systems and in the vicinity of high power switching devices and reactive energy compensation banks.

Part of these high frequency currents may flow to earth through parasitic capacitances. Although not hazardous for the user, these currents can still cause the nuisance tripping of differential devices.

SI type RCDs have a specific frequency response curve designed to prevent nuisance tripping when non-dangerous high frequency residual currents are present.

Energization

The initial energization of the capacitances mentioned above gives rise to high frequency transient currents of very short duration, similar to that shown in Figure F53.

The sudden occurrence of a first-fault in an IT-earthed system also causes transient earth-leakage currents at high frequency, due to the sudden rise of the two healthy phases to phase/phase voltage above earth.

SI type RCDs present a small tripping delay, allowing to let pass this transient current without nuisance tripping.

Fig. F53 – Standardized 0.5 μs/100 KHz current transient wave

Common mode overvoltages

Electrical networks are subject to overvoltages due to lightning strikes or to abrupt changes of system operating conditions (faults, fuse operation, switching, etc.).

These sudden changes often cause large transient voltages and currents in inductive and capacitive circuits. Records have established that, on LV systems, overvoltages remain generally below 6 kV, and that they can be adequately represented by the conventional 1.2/50 μs impulse wave (see Figure F54).

Fig. F54 – Standardized 1.2/50 μs voltage transient wave

These overvoltages give rise to transient currents represented by a current impulse wave of the conventional 8/20 μs form, having a peak value of several tens of amperes (see Figure F55).

The transient currents flow to earth via the capacitances of the installation.

SI type RCDs offer a high surge current capability and can withstand a 8/20µs current impulse higher than 3kA.

Fig. F55 – Standardized current-impulse wave 8/20 μs

Main climatic disturbance types

Cold

In the cases of temperatures under - 5°C, very high sensitivity electromechanical relays in the RCD may be “welded” by the condensation – freezing action.

Type “SI” devices can operate at temperatures down to - 25 °C.

Atmospheres with high concentrations of chemicals or dust

The special alloys used to make the RCDs can be notably damaged by corrosion. Dust can also block the movement of mechanical parts.

See the measures to be taken according to the levels of severity defined by standards in Figure F56.

Regulations define the choice of earth leakage protection and its implementation.

The main reference text is IEC 60364-5-51:

  • It gives a classification (AFx) for external influences in the presence of corrosive or polluting substances,
  • It defines the choice of materials to be used according to extreme influences.
Fig. F56 – External influence classification according to IEC 60364-5-51 standard
Presence of corrosive or polluting substances (IEC 60364-5-51) Influence of the electrical network Characteristics required for selection and erection of equipment
Clean network Disturbed network
AF1 Negligible presence Standard immunized residual current protections

Type AC: DB431079.png

Super-immunized residual current protections

Type A SI: DB431080.png

Normal
AF2 Significant presence of atmospheric origin Super-immunized residual current protections

Type A SI: DB431080.png

According to the nature of substances (for example, compliance to salt mist test according to IEC 60068-2-11)
AF3 Intermittent or accidental subjection to corrosive or polluting chemical substances Super-immunized residual current protections

Type A SI: DB431080.png
+
Appropriate additional protection (sealed cabinet or unit)

Protection against corrosion according to equipment specification
AF4 Continuous subjection to corrosive or polluting chemical substances Super-immunized residual current protections

Type A SI: DB431080.png
+
Appropriate additional protection (sealed cabinet or unit + overpressure)

Equipment specially designed according to the nature of substances
Examples of exposed sites External influences
Iron and steel works. Presence of sulfur, sulfur vapor, hydrogen sulfide.
Marinas, trading ports, boats, sea edges, naval shipyards. Salt atmospheres, humid outside, low temperatures.
Swimming pools, hospitals, food & beverage. Chlorinated compounds.
Petrochemicals. Hydrogen, combustion gases, nitrogen oxides.
Breeding facilities, tips. Hydrogen sulfide.

Type SI RCDs immunity to nuisance tripping

Type SI RCDs have been designed to avoid nuisance tripping or non-tripping in case of polluted network, lightning effect, high frequency currents, RF waves, etc.

Figure F57 below indicates the levels of tests undergone by this type of RCDs.

Fig. F57 – Immunity to nuisance tripping tests undergone by Schneider Electric RCDs
Disturbance type Rated test wave Immunity

Acti 9, ID-RCCB, DPN Vigi, Vigi iC60, Vigi C120, Vigi NG125 SI type

Continuous disturbances
Harmonics 1 kHz Earth leakage current = 8 x I∆n
Transient disturbances
Lightning induced overvoltage 1.2 / 50 µs pulse

(IEC/EN 61000-4-5)

4.5 kV between conductors 5.5 kV / earth
Lightning induced current 8 / 20 µs pulse

(IEC/EN 61008)

5 kA peak
Switching transient, indirect lightning currents 0.5 µs / 100 kHz “ring wave”

(IEC/EN 61008)

400 A peak
Downstream surge arrester operation, capacitance loading 10 ms pulse 500 A
Electromagnetic compatibility
Inductive load switchings fluorescent lights, motors, etc.) Repeated bursts

(IEC 61000-4-4)

5 kV / 2.5 kHz

4 kV / 400 kHz

Fluorescent lights, thyristor controlled circuits, etc. RF conducted waves

(level 4 IEC 61000-4-6)
(level 4 IEC 61000-4-16)


30 V (150 kHz to 230 MHz)

250 mA (15 kHz to 150 kHz)

RF waves (TV & radio, broadcact, telecommunications,etc.) RF radiated waves 80 MHz to 1 GHz

(IEC 61000-4-3)

30 V / m

Recommendations concerning the installation of RCDs with separate toroidal current transformers

The detector of residual current is a closed magnetic circuit (usually circular) of very high magnetic permeability, on which is wound a coil of wire, the ensemble constituting a toroidal (or ring-type) current transformer.

Because of its high permeability, any small deviation from perfect symmetry of the conductors encompassed by the core, and the proximity of ferrous material (steel enclosure, chassis members, etc.) can affect the balance of magnetic forces sufficiently, at times of large load currents (motor-starting current, transformer energizing current surge, etc.) to cause unwanted tripping of the RCD.

Unless particular measures are taken, the ratio of operating current IΔn to maximum phase current Iph (max.) is generally less than 1/1000.

This limit can be increased substantially (i.e. the response can be desensitized) by adopting the measures shown in Figure F58, and summarized in Figure F59.

L = twice the diameter of the magnetic ring core
Fig. F58 – Means of reducing the ratio IΔn/Iph (max.)
Fig. F59 – Means of reducing the ratio IΔn/Iph (max.)
Measures Diameter (mm) Sensitivity diminution factor
Careful centralizing of cables through the ring core 3
Oversizing of the ring core ø 50 → ø 100 2
ø 80 → ø 200 2
ø 120 → ø 300 6
Use of a steel or soft-iron shielding sleeve ø 50 4
  • Of wall thickness 0.5 mm
ø 80 3
  • Of length 2 x inside diameter of ring core
ø 120 3
  • Completely surrounding the conductors and overlapping
    the circular core equally at both ends
ø 200 2
  • These measures can be combined. By carefully centralizing the cables in a ring core of 200 mm diameter, where a 50 mm core would be large enough, and using a sleeve, the ratio 1/1000 could become 1/30000.

Choice of characteristics of a residual-current circuit breaker (RCCB - IEC 61008)

Rated current

The rated current of a RCCB is chosen according to the maximum sustained load current it will carry.

  • If the RCCB is connected in series with, and downstream of a circuit breaker, the rated current of both items will be the same, i.e. In ≥ In1 (see (a) of Figure F60)
  • If the RCCB is located upstream of a group of circuits, protected by circuitbreakers, as shown in (b) of Figure F60, then the RCCB rated current will be given by: In ≥ ku x ks (In1 + In2 + In3 + In4)

Electrodynamic withstand requirements

Protection against short-circuits must be provided by an upstream SCPD (Short-Circuit Protective Device). Coordination between the RCCB and the SCPDs is necessary, and manufacturers generally provide tables associating RCCBs and circuit breakers or fuses.

Fig. F60 – Residual current circuit breakers (RCCBs)
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