Sensitivity of RCDs to disturbances
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 F54.
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.
Common mode overvoltages
Electrical networks are subject to overvoltages due to lightning strikes or to abruptchanges 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 F55).
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 F56).
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.
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 F57.
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.
| Influence of the electrical network | Disturbed network | Super-immunized residual current protections
Type A SI: |
Super-immunized residual current protections
SI: |
Super-immunized residual current protections
SI: |
Super-immunized residual current protections
SI: |
|---|---|---|---|---|---|
| Clean network | Standard immunized residual current protections
Type AC | ||||
| Presence of corrosive or polluting substances(IEC 60364-5-51) | Negligible presence | Significant presence of atmospheric origin | Intermittent or accidental subjection to corrosive or polluting chemical substances | Continuous subjection to corrosive or polluting chemical substances | |
| Severity level | AF1 | AF2 | AF3 | AF4 | |
| Characteristics required for selection and erection of equipment | Normal. | According to the nature of substances (for example, compliance to salt mist test according to IEC 60068-2-11) | Protection against corrosion according to equipment specification | 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. |
