The branch circuit breaker is a control and protection device that serves as a connection point between the ERDF network and a customer’s electrical installation, called a delivery point.
Everything above the circuit breaker is governed by standard NF C 14-100, which is exclusively the responsibility of ERDF operators and subcontractors.
These companies carry out work on ERDF-owned electricity networks after responding to a call for proposals and obtaining the contract. They are specialized and are chosen for their qualifications and skills. In short, an installer, although an electrician, can under no circumstance intervene in the upstream of the general circuit breaker. Any intervention must go through ERDF.
Physically the upstream corresponds to the upper part of the circuit breaker and is sealed.
The downstream (lower part) is within the domain of NF C 15-100 and is accessible to the customer.
The branch circuit breaker has a current threshold setting that allows the distributor to limit the power absorbed by the installation (subscribed power), and at the same time provides overall protection against short circuits and overcurrents (magnetothermal protection, depending on the subscription – 15, 30, 45 amperes…), and against insulation faults (differential protection, 500mA for a maximum resistance of 100 ohms).
In cases of insulation faults greater than 500mA, the branch circuit breaker could cut off the entire installation before one of the protections downstream has had time to act to cut only the faulty part (30mA differentials). Then comes the notion of selectivity. To be selective, the branch circuit breakers must be marked “type S”, indicating that they are delayed on tripping.
Therefore, if only one circuit in the house is faulty, the differential switch located directly above it will cut off all the circuits it serves before the branch circuit breaker has had time to cut off the entire installation.
The branch circuit breaker also allows the installation to be isolated from the electrical network by manually switching it over, which adds the emergency shutdown function when it is installed in the house.
Together with the electricity meter, they form what is called the control panel, or counting board.
Reminder of the primary functions of the branch circuit breaker:
- It is the connection point between the distribution network and the customer’s installation. It is, like everything below, the property of the customer, above it the property of ERDF.
- Allows the emergency shutdown of the entire installation if placed in the house, or simply to isolate it for intervention. Otherwise, another device must perform this function within the house, circuit breaker or switch disconnector.
- General protection against overcurrents, short circuits and insulation faults in the installation.
Types of circuit-breakers
Number of poles
There are bipolar (phase and neutral) and four-pole (three phases and one neutral) branch circuit breakers, depending on whether the contract is single-phase or three-phase.
The vast majority of domestic installations are supplied with single-phase power, for an operating voltage of 230V.
Three-phase is used for farms, industries, shops,… or places with electrical equipment that themselves operate in three-phase mode. These can be rotating machines (sorting motors), water heaters, heat pumps, etc.
The voltage is 400 volts between phases and 230 volts between each phase and the neutral.
It is possible to supply single-phase equipment from a three-phase network by taking their power supply between one of the phases and the neutral.
A sorting installation can be brought back to mono in order to minimize its subscription costs, but it is then necessary to replace the meter, circuit breaker and power cables. The distribution table will also have to be modified accordingly, and of course no longer have three-phase equipment to supply.
The opposite is also possible but more rare.
The rating refers to the maximum intensity threshold that the circuit breaker will tolerate. Since the intensity depends on the power absorbed by the installation and the voltage, limiting the intensity also means limiting the power.
The power of the contracts is expressed in voltamperes (VA), a unit of measurement for “apparent power”. This takes into account a phenomenon called “phase shifts” generated by certain devices such as motors, computers, fluorescent lamps, etc. To put it simply, they cause additional heating in the distribution cables, which forces them to oversize.
The active power, expressed in Watts (W), corresponds to the actual power consumed. It is equal to the apparent power for purely resistive devices (no phase shift) such as incandescent bulbs, electric heating, water heaters, etc.
As you might have guessed, the branch circuit breakers are adjustable, but in certain dimensions.
Circuit breakers have limited adjustment ranges because they must be able to withstand more or less high current flows, as well as for power supply cables, which must be sized taking into account the most unfavorable case, namely the highest adjustment threshold of the same circuit breaker.
Most frequent circuit breaker models by contract type
As mentioned above, the cross-section of the power cables must take into account the largest size of the circuit breaker in the event of a subsequent change of subscription.
We will discuss the cross-section of conductors later on, since it depends on their length and therefore on the location of the circuit breaker in relation to the distribution board.
Location of the circuit breaker
The counting panel when it is in the house must be placed in the HTD (see our article on Housing Technology Ducting).
Two scenarios: ERDF grid connection
It will actually be placed in the HTD if it is within 33 yards of the ERDF distribution box (containing a disconnecting device or fuses). It will also perform its emergency shutdown function as it is accessible from inside the unit.
The passage of the power cable and its connection will be provided and invoiced by ERDF.
ERDF grid connection If this is not the case (more than 33 yards), the counter/circuit breaker assembly will be placed in a box on the property boundary.
In this case, an emergency shutdown device must be added within the housing, which may be another branch circuit breaker (this time non-selective) or a simple switch disconnector that will not provide any other protection than manual isolation of the installation.
The customer or his electrician must be responsible for the passage of the power cable and the connection on the housing side.
Some ERDF centres tolerate greater distances for the integration of the counting panel into the HTD. To ensure this, it is preferable to ask them for a preliminary study.
Power cable cross-sections
The cross-section of the conductors depends primarily on the intensity of the current flowing through them. The higher the intensity, the more important the section will have to be as well.
It also depends on the length of the cables. Copper and aluminum are excellent conductors but are not perfect. Their internal resistance increases with the length of the conductors, resulting in losses due to the joule effect (heating) and a voltage drop.
A tool to calculate the voltage drops in a cable is available: voltage drop calculator
This tool is particularly useful for installations with one or more distribution boards. NFC 15-100 limits voltage drops to 3% of the nominal voltage (2% from the circuit breaker to the main panel, 1% for circuits downstream of the panel). The power supply to a distribution board taken from the main board will accumulate the losses of the two power cables, and in some cases lead us to be out of tolerance (on the distribution board side).
It is therefore important to consider the entire installation before choosing the general power cable, as its cross-section may be insufficient.
Further explanations and an example of a study are provided following the calculation tool.
It is indeed possible to use cables with aluminum conductors for the downstream connection of the circuit breaker, which are less expensive, but with larger cross-sections for the same intensity, given their higher resistance.