How to choose the RCD in different applications

During the use of electrical appliances, how to ensure the safety of electricity use, especially to prevent the residual current from causing harm to life and property, is a matter of great concern.

What is RCD?

The use of residual current protective devices (RCD) is the main means of protecting residual current in daily life.

It is an electrical safety device that can quickly disconnect the leakage current grounded circuit, thereby avoiding or reducing the damage caused by electric shock. 

What is leakage current?

Residual current is the current in the low-voltage distribution line of each phase (including the neutral line) of the vector and is not zero current.

Generally speaking, when an accident occurs on the power side, the current flows from the charged body to the earth via the human body, so that the current in and out of the main circuit is of unequal size between phase I and phase I. At this time, the instantaneous vector integrated rms value of the current is called residual current, commonly known as leakage current, and leakage current is a residual current.

Fault residual current may appear in various forms according to the load characteristics:

1. Residual sinusoidal AC current

2. Pulsating DC residual current

3. Multi-frequency compound

4. Smooth DC residual current.

What are the dangers of earth leakage?

1. Personal safety

When the shell of electrical equipment is electrically charged due to insulation damage and workers come into contact with the shell, it can cause personal electrocution accidents.

2. Fire caused by damaged insulation

Burning electrical equipment can cause a fire.

Long-term leakage current, especially the two-phase leakage current through the over-resistance grounding, when passing through the damaged part of the equipment insulation, will release a lot of heat, so that the insulation is further damaged, and even make flammable materials on fire.

3. Cause short circuit

According to statistics, about 30% of single-phase grounding faults develop into short circuits.

Long-term leakage current and electrical sparks further damage the insulation of the leakage point, endangering the insulation between the phases, causing a short circuit.

4. Cause electrical fire accidents

When the grid is single-phase grounded or when equipment is single-phase contacted, an electric spark is generated at the point of contact. If the electrical spark has sufficient energy, it may ignite flammable and explosive items.

What are the different types of residual current breakers?

Classification according to sensitivity:

RCD sensitivity is expressed as the rated residual operating current, noted IΔn.

Preferred values have been defined by the IEC, thus making it possible to divide RCDs into three groups according to their IΔn value:

1.The leakage circuit breaker of ≤30mA provides protection for direct contact or life injury.

2.The 30~100mA leakage circuit breaker provides protection for electric shock caused by indirect contact.

3.The 100~300mA leakage circuit breaker provides protection for electrical fires and electrical equipment damage caused by grounding faults of electrical lines or electrical equipment.

Classification according to the detection leakage type:

IEC Standard 60755 (General requirements for residual current operated protective devices) defines three types of RCD depending on the waveforms and frequency of the fault current.

• Type AC RCD:

RCD developed and designed for sinusoidal leakage current, which ensures tripping for sudden or slowly rising residual sinusoidal alternating current.

• Type A RCD:

An RCD that includes AC-type characteristics and superimposes pulsating DC residual current and pulsating DC residual current ≤ 6mA smooth residual current to ensure tripping.

• Type B RCD (Type B RCDs detect the sum of the leakage currents):

It can reliably protect sinusoidal AC signals, pulsating DC signals and smooth signals, and has higher design requirements than A-type leakage circuit breakers.

It ensures tripping under the following conditions:

1. For residual sinusoidal alternating current up to 1 kHz.

2. For residual alternating current superimposed on smooth direct current.

3. For residual pulsating direct current superimposed on smooth direct current.

4. Residual pulsating rectified direct current generated by two or more phases.

5. For residual smooth direct current, whether applied suddenly or increased slowly, regardless of polarity.

Why does DC in the electrical system negatively impact on an AC RCD?

Some loads (or power supplies) may generate DC leakage currents in normal operation.

When the equipment generates DC in its construction, it connects to the rest of the electrical installation.

DC can saturate the core and effectively blind or lock the RCD.

This is called "blinding" and makes them insensitive to AC faults on the circuit they are protecting, so they will not be able to ensure their protective function, resulting in a dangerous situation.

Which devices have DC residual current?

1. Washing machines

2. coffee makers

3. boilers

4. LED lighting

5. dishwashers

6. tumble dryers

7. refrigerators

8. variable speed drives

Especially today, applications such as electric vehicles (EV Charger) and solar photovoltaics (PV) place special demands on the protection of DC residual currents, especially smooth DC currents.

Selecting the correct Type of RCD

6mA of DC is not actually dangerous due to the presence of residual DC current, especially a smooth DC current, but it can blind certain types of RCDs, inhibiting their proper operation.

Especially in scenarios where there is residual DC current such as electric vehicle charging (EV Charger) and photovoltaic power generation (PV) systems, RCD blinding is not allowed, so the correct selection of RCD becomes particularly important.

For details, please refer to the table below:

Note:

1. Type A RCD (See: EKL1-63, EKL6-100, EKL5-63) provide protection for AC residual currents and pulsating DC currents superimposed on < 6mA smooth DC. Type A RCDs do not detect smooth DC. Therefore, they cannot provide protection if there is a risk of the smooth DC component exceeding 6mA.
    
2. Type B RCD (See: EKL1-63B, EKL6-100B, EKL5-63B) detects DC residual currents and trips if the smooth DC current exceeds the trip threshold. Product standards for Type B RCDs stipulate that tripping must occur for a smooth DC residual current at between 0.5 and 2 times the value of the rated residual current( IΔn).
    
3. Type EV RCD (See: EKL6-63EV) is designed specifically for use in Mode 3 charging stations for electric vehicles. 

These RCDs may only be used for protection when charging electric vehicles in line. They may not be used for any other applications. Of course, it is also possible to use Type B RCD in EV charging applications.
EV Type RCD may not be used as a replacement for Type B under any circumstances. It only detects Type A residual currents (AC and pulsating residual currents at the rated frequency of 50 Hz), as well as smooth DC residual currents (switch-off at max. 6 mA) thanks to the integrated additional function.

Conclusion

Nowadays, the residual current protector is the main means to protect the residual current. Due to the rapid development of modern society, the application of different types of power equipment is accompanied by the emergence of different types of residual current. But everyone often neglects to choose the correct type of residual current protector, which leads to a series of life and property safety accidents.

Therefore, please be sure to choose a rated value higher than the maximum expected short-circuit current of the circuit it protects, and pay special attention to the scene where there is DC residual DC, and you can consult the equipment supplier for details. This will ensure that the RCD trips and fully protects you in the event of a malfunction.

ETEK Electric develops and produces different types of residual current protectors to meet the needs of different scenarios, all of which meet international standards and have obtained CE, TUV, VDE and other certificates. If you are interested, you can click RCD to get more product information, or you can contact us on info@etek-china.com.