How do differential circuit breakers work?

What is an earth leakage circuit breaker?

The earth leakage circuit breaker (ELCB) is a protective device that combines two distinct functions: differential protection and overcurrent protection. It should not be confused with the differential switch, which only provides differential protection without protecting against overloads and short circuits.

This safety device is installed in the electrical panel and constantly monitors the electrical current flowing in a circuit. Its main purpose is to detect current leaks to earth and automatically cut off the power supply when an anomaly is detected, thus protecting people and property.

Residual-current circuit breakers (RCDs) are mandatory in all new electrical installations, in accordance with standard NF C 15-100, which governs low-voltage electrical installations in France. This standard imposes precise criteria concerning the number, type and characteristics of differential devices to be installed.

Differential circuit breaker operating principle

Current leakage detection

The operation of the differential circuit breaker is based on a simple but ingenious physical principle: the permanent comparison of the current flowing into and out of an electrical circuit.

In a normal electrical installation, the current flowing into the phase wire must be strictly equal to the current flowing out of the neutral wire. This fundamental rule derives from the law of conservation of energy: any current that flows in must flow out. The differential circuit breaker constantly monitors this balance.

When a person accidentally touches a live device with an insulation fault, or comes into direct contact with an electrical conductor, part of the current no longer flows back through the neutral wire, but flows through the human body to earth. This current leakage, however small, creates an imbalance between incoming and outgoing current.

The magnetic core: the heart of the system

At the heart of the differential circuit breaker is an essential component known as the magnetic toroid or toroidal transformer. The phase and neutral conductors pass through this toroid, each creating a magnetic field.

In a normal situation, these two magnetic fields are equal and opposite in direction, so they cancel each other out. The resulting magnetic field in the torus is therefore zero. However, as soon as a current leakage occurs, the imbalance between phase and neutral current creates a residual magnetic field in the toroid.

This residual magnetic field induces a current in a secondary winding placed around the toroid. This induced current is proportional to the current leakage detected. It is then amplified by an electronic circuit which controls a trigger relay.

The trigger mechanism

When the leakage current exceeds the sensitivity threshold of the earth leakage circuit breaker, the relay releases the cut-off mechanism. A mechanical spring system rapidly opens the electrical contacts, instantly interrupting the power supply to the faulty circuit.

To be effective, this cut-off must be extremely rapid. Standards require a triggering time of less than a few tens of milliseconds to guarantee personal safety. For example, a 30 mA earth leakage circuit breaker must trip in less than 300 milliseconds when it detects a 30 mA leakage current, and even faster for higher leakage currents.

Technical characteristics of a differential circuit breaker

Differential sensitivity

Differential sensitivity, expressed in milliamperes, represents the maximum leakage current the circuit-breaker will tolerate before tripping. This is the most important feature for personal protection.

Earth leakage circuit breakers are available in several sensitivities:

30 mA: This is the standard sensitivity for protecting people in residential premises. A current of 30 mA passing through the human body for a limited time is considered the acceptable danger threshold. This sensitivity is mandatory for all terminal circuits in a home.

300 mA: This sensitivity is used at the head of the installation for protection against electrical fires. It does not directly protect people, but detects major insulation faults that could lead to dangerous overheating.

10 mA : Enhanced sensitivity used for circuits supplying equipment located in very humid areas or for certain medical installations. It offers a higher level of protection, but can lead to nuisance tripping if not correctly dimensioned.

Current rating

Like all circuit breakers, earth leakage circuit breakers have a rating that determines the maximum current they can withstand in normal operation. This rating is expressed in amperes and must be adapted to the power of the equipment supplied by the protected circuit.

Common ratings are 10A, 16A, 20A, 25A, 32A and 40A. The choice of gauge depends on the cross-section of the cables used and the nature of the devices connected. For example, a conventional socket-outlet circuit will generally be protected by a 16A or 20A circuit breaker, while an electric heating circuit will require a higher rating, often 20A or 32A depending on the power installed.

Types of differential circuit breakers

Earth leakage circuit breakers are classified into different types according to their ability to detect various forms of leakage current:

Type AC: This is the most basic type, designed to detect sinusoidal alternating leakage currents. It was very common in the past, but is now gradually being replaced by more efficient types. It is still suitable for lighting circuits and sockets supplying simple appliances.

Type A: This type detects AC leakage currents and pulsating DC leakage currents. It is mandatory for circuits supplying appliances with electronic components likely to generate DC leakage currents, such as cooker hobs, washing machines, heating appliances with dimmers, or charging stations for electric vehicles.

Type F: An evolution of type A, it offers greater immunity to nuisance tripping caused by rapid current variations. It is particularly well-suited to circuits powering computers, freezers or any sensitive equipment whose untimely shutdown could have serious consequences.

Type B: This type detects all leakage currents, including smooth direct currents. It is mainly used in industrial plants, laboratories or for the protection of certain medical equipment. In the residential sector, it is becoming increasingly relevant with the development of photovoltaic installations and fast-charging systems for electric vehicles.

Installation and positioning in the electrical panel

Installation position

The earth leakage circuit breaker can occupy different positions in an electrical installation depending on its function:

At the head of the installation, a 300 mA residual current device can be installed immediately after the service circuit breaker to provide general fire protection. This configuration, although not mandatory in all situations, offers additional safety by detecting major insulation faults before they cause dangerous overheating.

30 mA differential circuit breakers are installed at the head of each circuit group, usually on a DIN rail in the electrical panel. The NF C 15-100 standard requires circuits to be distributed over several differential devices to ensure partial continuity of service in the event of tripping. A single residual current device must not protect more than eight circuits or points of use.

Circuit distribution rules

The distribution of circuits to the various differential circuit breakers must comply with certain rules to optimize the safety and availability of the installation:

It is recommended not to group all circuits in the same zone on a single differential. For example, if the differential protecting the kitchen trips, it's best to keep the lighting in this room functional so that the problem can be identified and resolved safely.

Circuits for critical appliances such as freezers must be distributed intelligently to minimize the risk of accidental tripping. The use of an F-type residual-current circuit breaker for this equipment considerably reduces unwanted tripping.

Outdoor circuits, which are particularly exposed to the elements, should ideally be grouped on a dedicated differential to prevent a fault on an outdoor installation from affecting indoor circuits.

The advantages of differential circuit breakers

Optimum personal protection

The main advantage of the earth leakage circuit breaker is its ability to protect people against electric shock. Direct or indirect contact with a live part can be fatal, but the 30 mA residual current circuit breaker cuts the power supply in milliseconds, considerably reducing the risk of serious electrocution.

This protection is particularly important in wet rooms such as bathrooms, kitchens or laundry rooms, where humidity significantly increases electrical risks. The wet human body has a much lower electrical resistance, which increases the current intensity when in contact with an electrical source.

Preventing electrical fires

Insulation faults are one of the main causes of house fires. When a leakage current flows for a long time, even at a relatively low current, it can cause localized heating that can ignite nearby combustible materials.

The 300 mA earth leakage circuit breaker (ELCB) at the head of the system detects these major insulation faults and cuts off the power supply before the situation becomes dangerous. Even 30 mA differentials contribute to this protection by detecting more localized faults.

Double protection in a single device

Unlike the residual-current switch, the residual-current circuit-breaker also provides protection against overloads and short-circuits, thanks to its magnetothermal tripping devices. This dual function simplifies installation and reduces the space required in the electrical panel.

In the event of prolonged overload, e.g. when too many devices are operating simultaneously on the same circuit, the circuit-breaker cuts off the power supply to prevent the cables from overheating. In the event of a short-circuit, it intervenes almost instantaneously to protect the conductors and prevent any risk of fire.

Maintenance and testing of differential circuit breakers

Mandatory monthly test

Each earth leakage circuit breaker is fitted with a test button, usually identified by the letter "T". This button is used to check that the device is operating correctly by simulating a current leak. When this button is pressed, the circuit-breaker should trip immediately, cutting off the power supply.

This check should be carried out at least once a month to ensure that the detection and release mechanism is working properly. Over time, mechanical wear, dust and oxidation can alter the device's operation. Regular testing can detect these problems before they compromise safety.

If the circuit-breaker does not trip when the test button is pressed, it must be replaced immediately, as it can no longer perform its protective function. This is a situation that requires the rapid intervention of a qualified electrician.

Spurious tripping

An untimely trip occurs when the differential circuit breaker cuts off the power supply for no apparent reason, in the absence of a real fault. There are several reasons for this phenomenon:

Moisture is one of the most common causes. It can cause micro-current leaks in sockets, switches or electrical appliances, enough to trip a sensitive differential. Even the slightest ingress of water into an external junction box can be enough to cause a trip.

The ageing of the plant is also an important factor. Over time, cable insulation can degrade, especially if exposed to mechanical stress, temperature variations or humidity. This damage creates leakage points that can cause the differential to trip.

The accumulation of electrostatic charges or natural leakage currents on very long circuits can also be the cause, particularly if the residual current circuit breaker is too sensitive for the installation configuration.

To resolve recurring nuisance tripping, we recommend calling in an electrician who can :

- Measure circuit insulation to identify leakage points

- Check condition of connections and equipment

- Adapt the type of differential to the characteristics of the equipment installed

- Suggest a better distribution of circuits over several differentials

Service life and replacement

An earth leakage circuit breaker has a limited lifespan, generally estimated at between 10 and 15 years under normal conditions of use. Several factors can influence this lifespan:

The number of trips: each trip causes mechanical wear to the device. Systems subject to frequent tripping will see their circuit breakers wear out more quickly.

The environment: humidity, dust and temperature variations can accelerate the degradation of electronic and mechanical components.

Device quality: quality differences between manufacturers can be significant. Recognized brands generally offer greater reliability and longer life.

We recommend that you have your electrical installation checked by a professional every 10 years, in particular to ensure that protective devices are working properly and to assess the need to replace them.

Technological developments and prospects

Connected earth leakage circuit breakers

The advent of home automation and connected objects is also affecting the field of electrical protection. New-generation residual-current circuit-breakers now include intelligent features that improve safety and ease of use.

These connected devices can send smartphone notifications in the event of an alarm being triggered, enabling occupants to be informed immediately, even in their absence. This feature is particularly useful for monitoring freezers or safety systems that require a continuous power supply.

Some models also allow you to consult tripping history, analyze electricity consumption by circuit, or even reset the circuit-breaker remotely once the problem has been identified and resolved. These features greatly facilitate the maintenance and management of electrical installations.

Improving selectivity

Selectivity is the ability of an installation to isolate only the faulty circuit without affecting other circuits. Manufacturers are developing technologies to improve this selectivity, notably with devices featuring delayed triggering or adaptive trigger curves.

These innovations reduce the inconvenience of power cuts by limiting their impact to only those circuits actually affected by a fault. In complex installations, this selectivity becomes a major challenge to ensure continuity of service for critical equipment.

Adapting to new uses

Changes in electrical usage, with the proliferation of electronic devices, the development of domestic renewable energies and the boom in electric mobility, are placing new demands on protection systems.

Type B earth leakage circuit breakers become more relevant with the installation of photovoltaic panels or fast-charging stations, which can generate DC leakage currents. Manufacturers are also developing devices capable of discriminating true faults from the normal transients generated by some modern electronic equipment.

Standards and regulations

NF C 15-100 standard

Standard NF C 15-100 is the regulatory reference for all low-voltage electrical installations in France. It imposes strict requirements on differential devices:

All new or completely renovated installations must include at least two 30 mA type A differential switches or circuit breakers. One of these must protect kitchen and laundry circuits, as well as outlets for major appliances.

Each circuit must be protected by a high-sensitivity differential device. No terminal circuit can be left unprotected, including lighting, which represents a major step forward compared with previous versions of the standard.

Bathrooms are subject to special requirements, with the obligation to install 30 mA differential protection for all circuits, whatever their nature.

The obligation to comply

Although there is no systematic obligation to bring older installations up to standard, certain situations make compliance mandatory:

When selling a property, an electrical diagnosis must be carried out for installations over 15 years old. If serious anomalies are detected, such as the absence of differential devices, the purchaser is informed and can negotiate compliance work.

In the case of major renovations involving more than 50% of the living area or a substantial change to the electrical installation, full compliance with standards becomes mandatory.

In rental properties, the landlord must ensure a minimum level of safety, including the presence of differential protection devices, even if the installation is not fully compliant with the latest version of the standard.

Conclusion

The earth leakage circuit breaker (ELCB) is a fundamental element of modern electrical safety. Its operating principle, based on the detection of minute imbalances between incoming and outgoing current, makes it a vigilant guardian of your electrical installation, capable of reacting in milliseconds to protect people and property.

Understanding how these devices work helps you appreciate their importance and better manage your electrical installation. The type of residual current circuit breaker, its sensitivity, rating and location in the electrical panel must be carefully selected to ensure optimum protection for your home and the equipment you use.

Regular maintenance, including monthly testing of the test button, and periodic verification by a qualified professional, are essential to ensure that these safety devices fully fulfill their protective mission. Don't forget that your safety and that of your loved ones depends to a large extent on the proper functioning of this equipment.

As electrical applications and technologies continue to evolve, earth leakage circuit breakers (ELCBs) continue to improve, offering ever-higher levels of protection and intelligent features to make them easier to use on a day-to-day basis. Investing in quality equipment that complies with the latest standards is a wise choice to guarantee the longevity and safety of your electrical installation.